path: root/Documentation/trace/histogram.txt

Event Histograms

		    Documentation written by Tom Zanussi

1. Introduction
===============

  Histogram triggers are special event triggers that can be used to
  aggregate trace event data into histograms.  For information on
  trace events and event triggers, see Documentation/trace/events.txt.


2. Histogram Trigger Command
============================

  A histogram trigger command is an event trigger command that
  aggregates event hits into a hash table keyed on one or more trace
  event format fields (or stacktrace) and a set of running totals
  derived from one or more trace event format fields and/or event
  counts (hitcount).

  The format of a hist trigger is as follows:

        hist:keys=<field1[,field2,...]>[:values=<field1[,field2,...]>]
          [:sort=<field1[,field2,...]>][:size=#entries][:pause][:continue]
          [:clear][:name=histname1] [if <filter>]

  When a matching event is hit, an entry is added to a hash table
  using the key(s) and value(s) named.  Keys and values correspond to
  fields in the event's format description.  Values must correspond to
  numeric fields - on an event hit, the value(s) will be added to a
  sum kept for that field.  The special string 'hitcount' can be used
  in place of an explicit value field - this is simply a count of
  event hits.  If 'values' isn't specified, an implicit 'hitcount'
  value will be automatically created and used as the only value.
  Keys can be any field, or the special string 'stacktrace', which
  will use the event's kernel stacktrace as the key.  The keywords
  'keys' or 'key' can be used to specify keys, and the keywords
  'values', 'vals', or 'val' can be used to specify values.  Compound
  keys consisting of up to two fields can be specified by the 'keys'
  keyword.  Hashing a compound key produces a unique entry in the
  table for each unique combination of component keys, and can be
  useful for providing more fine-grained summaries of event data.
  Additionally, sort keys consisting of up to two fields can be
  specified by the 'sort' keyword.  If more than one field is
  specified, the result will be a 'sort within a sort': the first key
  is taken to be the primary sort key and the second the secondary
  key.  If a hist trigger is given a name using the 'name' parameter,
  its histogram data will be shared with other triggers of the same
  name, and trigger hits will update this common data.  Only triggers
  with 'compatible' fields can be combined in this way; triggers are
  'compatible' if the fields named in the trigger share the same
  number and type of fields and those fields also have the same names.
  Note that any two events always share the compatible 'hitcount' and
  'stacktrace' fields and can therefore be combined using those
  fields, however pointless that may be.

  'hist' triggers add a 'hist' file to each event's subdirectory.
  Reading the 'hist' file for the event will dump the hash table in
  its entirety to stdout.  If there are multiple hist triggers
  attached to an event, there will be a table for each trigger in the
  output.  The table displayed for a named trigger will be the same as
  any other instance having the same name. Each printed hash table
  entry is a simple list of the keys and values comprising the entry;
  keys are printed first and are delineated by curly braces, and are
  followed by the set of value fields for the entry.  By default,
  numeric fields are displayed as base-10 integers.  This can be
  modified by appending any of the following modifiers to the field
  name:

        .hex        display a number as a hex value
	.sym        display an address as a symbol
	.sym-offset display an address as a symbol and offset
	.syscall    display a syscall id as a system call name
	.execname   display a common_pid as a program name
	.log2       display log2 value rather than raw number
	.usecs      display a common_timestamp in microseconds

  Note that in general the semantics of a given field aren't
  interpreted when applying a modifier to it, but there are some
  restrictions to be aware of in this regard:

    - only the 'hex' modifier can be used for values (because values
      are essentially sums, and the other modifiers don't make sense
      in that context).
    - the 'execname' modifier can only be used on a 'common_pid'.  The
      reason for this is that the execname is simply the 'comm' value
      saved for the 'current' process when an event was triggered,
      which is the same as the common_pid value saved by the event
      tracing code.  Trying to apply that comm value to other pid
      values wouldn't be correct, and typically events that care save
      pid-specific comm fields in the event itself.

  A typical usage scenario would be the following to enable a hist
  trigger, read its current contents, and then turn it off:

  # echo 'hist:keys=skbaddr.hex:vals=len' > \
    /sys/kernel/debug/tracing/events/net/netif_rx/trigger

  # cat /sys/kernel/debug/tracing/events/net/netif_rx/hist

  # echo '!hist:keys=skbaddr.hex:vals=len' > \
    /sys/kernel/debug/tracing/events/net/netif_rx/trigger

  The trigger file itself can be read to show the details of the
  currently attached hist trigger.  This information is also displayed
  at the top of the 'hist' file when read.

  By default, the size of the hash table is 2048 entries.  The 'size'
  parameter can be used to specify more or fewer than that.  The units
  are in terms of hashtable entries - if a run uses more entries than
  specified, the results will show the number of 'drops', the number
  of hits that were ignored.  The size should be a power of 2 between
  128 and 131072 (any non- power-of-2 number specified will be rounded
  up).

  The 'sort' parameter can be used to specify a value field to sort
  on.  The default if unspecified is 'hitcount' and the default sort
  order is 'ascending'.  To sort in the opposite direction, append
  .descending' to the sort key.

  The 'pause' parameter can be used to pause an existing hist trigger
  or to start a hist trigger but not log any events until told to do
  so.  'continue' or 'cont' can be used to start or restart a paused
  hist trigger.

  The 'clear' parameter will clear the contents of a running hist
  trigger and leave its current paused/active state.

  Note that the 'pause', 'cont', and 'clear' parameters should be
  applied using 'append' shell operator ('>>') if applied to an
  existing trigger, rather than via the '>' operator, which will cause
  the trigger to be removed through truncation.

- enable_hist/disable_hist

  The enable_hist and disable_hist triggers can be used to have one
  event conditionally start and stop another event's already-attached
  hist trigger.  Any number of enable_hist and disable_hist triggers
  can be attached to a given event, allowing that event to kick off
  and stop aggregations on a host of other events.

  The format is very similar to the enable/disable_event triggers:

      enable_hist:<system>:<event>[:count]
      disable_hist:<system>:<event>[:count]

  Instead of enabling or disabling the tracing of the target event
  into the trace buffer as the enable/disable_event triggers do, the
  enable/disable_hist triggers enable or disable the aggregation of
  the target event into a hash table.

  A typical usage scenario for the enable_hist/disable_hist triggers
  would be to first set up a paused hist trigger on some event,
  followed by an enable_hist/disable_hist pair that turns the hist
  aggregation on and off when conditions of interest are hit:

  # echo 'hist:keys=skbaddr.hex:vals=len:pause' > \
    /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger

  # echo 'enable_hist:net:netif_receive_skb if filename==/usr/bin/wget' > \
    /sys/kernel/debug/tracing/events/sched/sched_process_exec/trigger

  # echo 'disable_hist:net:netif_receive_skb if comm==wget' > \
    /sys/kernel/debug/tracing/events/sched/sched_process_exit/trigger

  The above sets up an initially paused hist trigger which is unpaused
  and starts aggregating events when a given program is executed, and
  which stops aggregating when the process exits and the hist trigger
  is paused again.

  The examples below provide a more concrete illustration of the
  concepts and typical usage patterns discussed above.

  'special' event fields
  ------------------------

  There are a number of 'special event fields' available for use as
  keys or values in a hist trigger.  These look like and behave as if
  they were actual event fields, but aren't really part of the event's
  field definition or format file.  They are however available for any
  event, and can be used anywhere an actual event field could be.
  They are:

    common_timestamp       u64 - timestamp (from ring buffer) associated
                                 with the event, in nanoseconds.  May be
				 modified by .usecs to have timestamps
				 interpreted as microseconds.
    cpu                    int - the cpu on which the event occurred.

6.2 'hist' trigger examples
---------------------------

  The first set of examples creates aggregations using the kmalloc
  event.  The fields that can be used for the hist trigger are listed
  in the kmalloc event's format file:

    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/format
    name: kmalloc
    ID: 374
    format:
	field:unsigned short common_type;	offset:0;	size:2;	signed:0;
	field:unsigned char common_flags;	offset:2;	size:1;	signed:0;
	field:unsigned char common_preempt_count;		offset:3;	size:1;	signed:0;
	field:int common_pid;					offset:4;	size:4;	signed:1;

	field:unsigned long call_site;				offset:8;	size:8;	signed:0;
	field:const void * ptr;					offset:16;	size:8;	signed:0;
	field:size_t bytes_req;					offset:24;	size:8;	signed:0;
	field:size_t bytes_alloc;				offset:32;	size:8;	signed:0;
	field:gfp_t gfp_flags;					offset:40;	size:4;	signed:0;

  We'll start by creating a hist trigger that generates a simple table
  that lists the total number of bytes requested for each function in
  the kernel that made one or more calls to kmalloc:

    # echo 'hist:key=call_site:val=bytes_req' > \
            /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger

  This tells the tracing system to create a 'hist' trigger using the
  call_site field of the kmalloc event as the key for the table, which
  just means that each unique call_site address will have an entry
  created for it in the table.  The 'val=bytes_req' parameter tells
  the hist trigger that for each unique entry (call_site) in the
  table, it should keep a running total of the number of bytes
  requested by that call_site.

  We'll let it run for awhile and then dump the contents of the 'hist'
  file in the kmalloc event's subdirectory (for readability, a number
  of entries have been omitted):

    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
    # trigger info: hist:keys=call_site:vals=bytes_req:sort=hitcount:size=2048 [active]

    { call_site: 18446744072106379007 } hitcount:          1  bytes_req:        176
    { call_site: 18446744071579557049 } hitcount:          1  bytes_req:       1024
    { call_site: 18446744071580608289 } hitcount:          1  bytes_req:      16384
    { call_site: 18446744071581827654 } hitcount:          1  bytes_req:         24
    { call_site: 18446744071580700980 } hitcount:          1  bytes_req:          8
    { call_site: 18446744071579359876 } hitcount:          1  bytes_req:        152
    { call_site: 18446744071580795365 } hitcount:          3  bytes_req:        144
    { call_site: 18446744071581303129 } hitcount:          3  bytes_req:        144
    { call_site: 18446744071580713234 } hitcount:          4  bytes_req:       2560
    { call_site: 18446744071580933750 } hitcount:          4  bytes_req:        736
    .
    .
    .
    { call_site: 18446744072106047046 } hitcount:         69  bytes_req:       5576
    { call_site: 18446744071582116407 } hitcount:         73  bytes_req:       2336
    { call_site: 18446744072106054684 } hitcount:        136  bytes_req:     140504
    { call_site: 18446744072106224230 } hitcount:        136  bytes_req:      19584
    { call_site: 18446744072106078074 } hitcount:        153  bytes_req:       2448
    { call_site: 18446744072106062406 } hitcount:        153  bytes_req:      36720
    { call_site: 18446744071582507929 } hitcount:        153  bytes_req:      37088
    { call_site: 18446744072102520590 } hitcount:        273  bytes_req:      10920
    { call_site: 18446744071582143559 } hitcount:        358  bytes_req:        716
    { call_site: 18446744072106465852 } hitcount:        417  bytes_req:      56712
    { call_site: 18446744072102523378 } hitcount:        485  bytes_req:      27160
    { call_site: 18446744072099568646 } hitcount:       1676  bytes_req:      33520

    Totals:
        Hits: 4610
        Entries: 45
        Dropped: 0

  The output displays a line for each entry, beginning with the key
  specified in the trigger, followed by the value(s) also specified in
  the trigger.  At the beginning of the output is a line that displays
  the trigger info, which can also be displayed by reading the
  'trigger' file:

    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
    hist:keys=call_site:vals=bytes_req:sort=hitcount:size=2048 [active]

  At the end of the output are a few lines that display the overall
  totals for the run.  The 'Hits' field shows the total number of
  times the event trigger was hit, the 'Entries' field shows the total
  number of used entries in the hash table, and the 'Dropped' field
  shows the number of hits that were dropped because the number of
  used entries for the run exceeded the maximum number of entries
  allowed for the table (normally 0, but if not a hint that you may
  want to increase the size of the table using the 'size' parameter).

  Notice in the above output that there's an extra field, 'hitcount',
  which wasn't specified in the trigger.  Also notice that in the
  trigger info output, there's a parameter, 'sort=hitcount', which
  wasn't specified in the trigger either.  The reason for that is that
  every trigger implicitly keeps a count of the total number of hits
  attributed to a given entry, called the 'hitcount'.  That hitcount
  information is explicitly displayed in the output, and in the
  absence of a user-specified sort parameter, is used as the default
  sort field.

  The value 'hitcount' can be used in place of an explicit value in
  the 'values' parameter if you don't really need to have any
  particular field summed and are mainly interested in hit
  frequencies.

  To turn the hist trigger off, simply call up the trigger in the
  command history and re-execute it with a '!' prepended:

    # echo '!hist:key=call_site:val=bytes_req' > \
           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger

  Finally, notice that the call_site as displayed in the output above
  isn't really very useful.  It's an address, but normally addresses
  are displayed in hex.  To have a numeric field displayed as a hex
  value, simply append '.hex' to the field name in the trigger:

    # echo 'hist:key=call_site.hex:val=bytes_req' > \
           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger

    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
    # trigger info: hist:keys=call_site.hex:vals=bytes_req:sort=hitcount:size=2048 [active]

    { call_site: ffffffffa026b291 } hitcount:          1  bytes_req:        433
    { call_site: ffffffffa07186ff } hitcount:          1  bytes_req:        176
    { call_site: ffffffff811ae721 } hitcount:          1  bytes_req:      16384
    { call_site: ffffffff811c5134 } hitcount:          1  bytes_req:          8
    { call_site: ffffffffa04a9ebb } hitcount:          1  bytes_req:        511
    { call_site: ffffffff8122e0a6 } hitcount:          1  bytes_req:         12
    { call_site: ffffffff8107da84 } hitcount:          1  bytes_req:        152
    { call_site: ffffffff812d8246 } hitcount:          1  bytes_req:         24
    { call_site: ffffffff811dc1e5 } hitcount:          3  bytes_req:        144
    { call_site: ffffffffa02515e8 } hitcount:          3  bytes_req:        648
    { call_site: ffffffff81258159 } hitcount:          3  bytes_req:        144
    { call_site: ffffffff811c80f4 } hitcount:          4  bytes_req:        544
    .
    .
    .
    { call_site: ffffffffa06c7646 } hitcount:        106  bytes_req:       8024
    { call_site: ffffffffa06cb246 } hitcount:        132  bytes_req:      31680
    { call_site: ffffffffa06cef7a } hitcount:        132  bytes_req:       2112
    { call_site: ffffffff8137e399 } hitcount:        132  bytes_req:      23232
    { call_site: ffffffffa06c941c } hitcount:        185  bytes_req:     171360
    { call_site: ffffffffa06f2a66 } hitcount:        185  bytes_req:      26640
    { call_site: ffffffffa036a70e } hitcount:        265  bytes_req:      10600
    { call_site: ffffffff81325447 } hitcount:        292  bytes_req:        584
    { call_site: ffffffffa072da3c } hitcount:        446  bytes_req:      60656
    { call_site: ffffffffa036b1f2 } hitcount:        526  bytes_req:      29456
    { call_site: ffffffffa0099c06 } hitcount:       1780  bytes_req:      35600

    Totals:
        Hits: 4775
        Entries: 46
        Dropped: 0

  Even that's only marginally more useful - while hex values do look
  more like addresses, what users are typically more interested in
  when looking at text addresses are the corresponding symbols
  instead.  To have an address displayed as symbolic value instead,
  simply append '.sym' or '.sym-offset' to the field name in the
  trigger:

    # echo 'hist:key=call_site.sym:val=bytes_req' > \
           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger

    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
    # trigger info: hist:keys=call_site.sym:vals=bytes_req:sort=hitcount:size=2048 [active]

    { call_site: [ffffffff810adcb9] syslog_print_all                              } hitcount:          1  bytes_req:       1024
    { call_site: [ffffffff8154bc62] usb_control_msg                               } hitcount:          1  bytes_req:          8
    { call_site: [ffffffffa00bf6fe] hidraw_send_report [hid]                      } hitcount:          1  bytes_req:          7
    { call_site: [ffffffff8154acbe] usb_alloc_urb                                 } hitcount:          1  bytes_req:        192
    { call_site: [ffffffffa00bf1ca] hidraw_report_event [hid]                     } hitcount:          1  bytes_req:          7
    { call_site: [ffffffff811e3a25] __seq_open_private                            } hitcount:          1  bytes_req:         40
    { call_site: [ffffffff8109524a] alloc_fair_sched_group                        } hitcount:          2  bytes_req:        128
    { call_site: [ffffffff811febd5] fsnotify_alloc_group                          } hitcount:          2  bytes_req:        528
    { call_site: [ffffffff81440f58] __tty_buffer_request_room                     } hitcount:          2  bytes_req:       2624
    { call_site: [ffffffff81200ba6] inotify_new_group                             } hitcount:          2  bytes_req:         96
    { call_site: [ffffffffa05e19af] ieee80211_start_tx_ba_session [mac80211]      } hitcount:          2  bytes_req:        464
    { call_site: [ffffffff81672406] tcp_get_metrics                               } hitcount:          2  bytes_req:        304
    { call_site: [ffffffff81097ec2] alloc_rt_sched_group                          } hitcount:          2  bytes_req:        128
    { call_site: [ffffffff81089b05] sched_create_group                            } hitcount:          2  bytes_req:       1424
    .
    .
    .
    { call_site: [ffffffffa04a580c] intel_crtc_page_flip [i915]                   } hitcount:       1185  bytes_req:     123240
    { call_site: [ffffffffa0287592] drm_mode_page_flip_ioctl [drm]                } hitcount:       1185  bytes_req:     104280
    { call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state [i915]            } hitcount:       1402  bytes_req:     190672
    { call_site: [ffffffff812891ca] ext4_find_extent                              } hitcount:       1518  bytes_req:     146208
    { call_site: [ffffffffa029070e] drm_vma_node_allow [drm]                      } hitcount:       1746  bytes_req:      69840
    { call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23 [i915]         } hitcount:       2021  bytes_req:     792312
    { call_site: [ffffffffa02911f2] drm_modeset_lock_crtc [drm]                   } hitcount:       2592  bytes_req:     145152
    { call_site: [ffffffffa0489a66] intel_ring_begin [i915]                       } hitcount:       2629  bytes_req:     378576
    { call_site: [ffffffffa046041c] i915_gem_execbuffer2 [i915]                   } hitcount:       2629  bytes_req:    3783248
    { call_site: [ffffffff81325607] apparmor_file_alloc_security                  } hitcount:       5192  bytes_req:      10384
    { call_site: [ffffffffa00b7c06] hid_report_raw_event [hid]                    } hitcount:       5529  bytes_req:     110584
    { call_site: [ffffffff8131ebf7] aa_alloc_task_context                         } hitcount:      21943  bytes_req:     702176
    { call_site: [ffffffff8125847d] ext4_htree_store_dirent                       } hitcount:      55759  bytes_req:    5074265

    Totals:
        Hits: 109928
        Entries: 71
        Dropped: 0

  Because the default sort key above is 'hitcount', the above shows a
  the list of call_sites by increasing hitcount, so that at the bottom
  we see the functions that made the most kmalloc calls during the
  run.  If instead we we wanted to see the top kmalloc callers in
  terms of the number of bytes requested rather than the number of
  calls, and we wanted the top caller to appear at the top, we can use
  the 'sort' parameter, along with the 'descending' modifier:

    # echo 'hist:key=call_site.sym:val=bytes_req:sort=bytes_req.descending' > \
           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger

    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
    # trigger info: hist:keys=call_site.sym:vals=bytes_req:sort=bytes_req.descending:size=2048 [active]

    { call_site: [ffffffffa046041c] i915_gem_execbuffer2 [i915]                   } hitcount:       2186  bytes_req:    3397464
    { call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23 [i915]         } hitcount:       1790  bytes_req:     712176
    { call_site: [ffffffff8125847d] ext4_htree_store_dirent                       } hitcount:       8132  bytes_req:     513135
    { call_site: [ffffffff811e2a1b] seq_buf_alloc                                 } hitcount:        106  bytes_req:     440128
    { call_site: [ffffffffa0489a66] intel_ring_begin [i915]                       } hitcount:       2186  bytes_req:     314784
    { call_site: [ffffffff812891ca] ext4_find_extent                              } hitcount:       2174  bytes_req:     208992
    { call_site: [ffffffff811ae8e1] __kmalloc                                     } hitcount:          8  bytes_req:     131072
    { call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state [i915]            } hitcount:        859  bytes_req:     116824
    { call_site: [ffffffffa02911f2] drm_modeset_lock_crtc [drm]                   } hitcount:       1834  bytes_req:     102704
    { call_site: [ffffffffa04a580c] intel_crtc_page_flip [i915]                   } hitcount:        972  bytes_req:     101088
    { call_site: [ffffffffa0287592] drm_mode_page_flip_ioctl [drm]                } hitcount:        972  bytes_req:      85536
    { call_site: [ffffffffa00b7c06] hid_report_raw_event [hid]                    } hitcount:       3333  bytes_req:      66664
    { call_site: [ffffffff8137e559] sg_kmalloc                                    } hitcount:        209  bytes_req:      61632
    .
    .
    .
    { call_site: [ffffffff81095225] alloc_fair_sched_group                        } hitcount:          2  bytes_req:        128
    { call_site: [ffffffff81097ec2] alloc_rt_sched_group                          } hitcount:          2  bytes_req:        128
    { call_site: [ffffffff812d8406] copy_semundo                                  } hitcount:          2  bytes_req:         48
    { call_site: [ffffffff81200ba6] inotify_new_group                             } hitcount:          1  bytes_req:         48
    { call_site: [ffffffffa027121a] drm_getmagic [drm]                            } hitcount:          1  bytes_req:         48
    { call_site: [ffffffff811e3a25] __seq_open_private                            } hitcount:          1  bytes_req:         40
    { call_site: [ffffffff811c52f4] bprm_change_interp                            } hitcount:          2  bytes_req:         16
    { call_site: [ffffffff8154bc62] usb_control_msg                               } hitcount:          1  bytes_req:          8
    { call_site: [ffffffffa00bf1ca] hidraw_report_event [hid]                     } hitcount:          1  bytes_req:          7
    { call_site: [ffffffffa00bf6fe] hidraw_send_report [hid]                      } hitcount:          1  bytes_req:          7

    Totals:
        Hits: 32133
        Entries: 81
        Dropped: 0

  To display the offset and size information in addition to the symbol
  name, just use 'sym-offset' instead:

    # echo 'hist:key=call_site.sym-offset:val=bytes_req:sort=bytes_req.descending' > \
           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger

    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
    # trigger info: hist:keys=call_site.sym-offset:vals=bytes_req:sort=bytes_req.descending:size=2048 [active]

    { call_site: [ffffffffa046041c] i915_gem_execbuffer2+0x6c/0x2c0 [i915]                  } hitcount:       4569  bytes_req:    3163720
    { call_site: [ffffffffa0489a66] intel_ring_begin+0xc6/0x1f0 [i915]                      } hitcount:       4569  bytes_req:     657936
    { call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23+0x694/0x1020 [i915]      } hitcount:       1519  bytes_req:     472936
    { call_site: [ffffffffa045e646] i915_gem_do_execbuffer.isra.23+0x516/0x1020 [i915]      } hitcount:       3050  bytes_req:     211832
    { call_site: [ffffffff811e2a1b] seq_buf_alloc+0x1b/0x50                                 } hitcount:         34  bytes_req:     148384
    { call_site: [ffffffffa04a580c] intel_crtc_page_flip+0xbc/0x870 [i915]                  } hitcount:       1385  bytes_req:     144040
    { call_site: [ffffffff811ae8e1] __kmalloc+0x191/0x1b0                                   } hitcount:          8  bytes_req:     131072
    { call_site: [ffffffffa0287592] drm_mode_page_flip_ioctl+0x282/0x360 [drm]              } hitcount:       1385  bytes_req:     121880
    { call_site: [ffffffffa02911f2] drm_modeset_lock_crtc+0x32/0x100 [drm]                  } hitcount:       1848  bytes_req:     103488
    { call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state+0x2c/0xa0 [i915]            } hitcount:        461  bytes_req:      62696
    { call_site: [ffffffffa029070e] drm_vma_node_allow+0x2e/0xd0 [drm]                      } hitcount:       1541  bytes_req:      61640
    { call_site: [ffffffff815f8d7b] sk_prot_alloc+0xcb/0x1b0                                } hitcount:         57  bytes_req:      57456
    .
    .
    .
    { call_site: [ffffffff8109524a] alloc_fair_sched_group+0x5a/0x1a0                       } hitcount:          2  bytes_req:        128
    { call_site: [ffffffffa027b921] drm_vm_open_locked+0x31/0xa0 [drm]                      } hitcount:          3  bytes_req:         96
    { call_site: [ffffffff8122e266] proc_self_follow_link+0x76/0xb0                         } hitcount:          8  bytes_req:         96
    { call_site: [ffffffff81213e80] load_elf_binary+0x240/0x1650                            } hitcount:          3  bytes_req:         84
    { call_site: [ffffffff8154bc62] usb_control_msg+0x42/0x110                              } hitcount:          1  bytes_req:          8
    { call_site: [ffffffffa00bf6fe] hidraw_send_report+0x7e/0x1a0 [hid]                     } hitcount:          1  bytes_req:          7
    { call_site: [ffffffffa00bf1ca] hidraw_report_event+0x8a/0x120 [hid]                    } hitcount:          1  bytes_req:          7

    Totals:
        Hits: 26098
        Entries: 64
        Dropped: 0

  We can also add multiple fields to the 'values' parameter.  For
  example, we might want to see the total number of bytes allocated
  alongside bytes requested, and display the result sorted by bytes
  allocated in a descending order:

    # echo 'hist:keys=call_site.sym:values=bytes_req,bytes_alloc:sort=bytes_alloc.descending' > \
           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger

    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
    # trigger info: hist:keys=call_site.sym:vals=bytes_req,bytes_alloc:sort=bytes_alloc.descending:size=2048 [active]

    { call_site: [ffffffffa046041c] i915_gem_execbuffer2 [i915]                   } hitcount:       7403  bytes_req:    4084360  bytes_alloc:    5958016
    { call_site: [ffffffff811e2a1b] seq_buf_alloc                                 } hitcount:        541  bytes_req:    2213968  bytes_alloc:    2228224
    { call_site: [ffffffffa0489a66] intel_ring_begin [i915]                       } hitcount:       7404  bytes_req:    1066176  bytes_alloc:    1421568
    { call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23 [i915]         } hitcount:       1565  bytes_req:     557368  bytes_alloc:    1037760
    { call_site: [ffffffff8125847d] ext4_htree_store_dirent                       } hitcount:       9557  bytes_req:     595778  bytes_alloc:     695744
    { call_site: [ffffffffa045e646] i915_gem_do_execbuffer.isra.23 [i915]         } hitcount:       5839  bytes_req:     430680  bytes_alloc:     470400
    { call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state [i915]            } hitcount:       2388  bytes_req:     324768  bytes_alloc:     458496
    { call_site: [ffffffffa02911f2] drm_modeset_lock_crtc [drm]                   } hitcount:       3911  bytes_req:     219016  bytes_alloc:     250304
    { call_site: [ffffffff815f8d7b] sk_prot_alloc                                 } hitcount:        235  bytes_req:     236880  bytes_alloc:     240640
    { call_site: [ffffffff8137e559] sg_kmalloc                                    } hitcount:        557  bytes_req:     169024  bytes_alloc:     221760
    { call_site: [ffffffffa00b7c06] hid_report_raw_event [hid]                    } hitcount:       9378  bytes_req:     187548  bytes_alloc:     206312
    { call_site: [ffffffffa04a580c] intel_crtc_page_flip [i915]                   } hitcount:       1519  bytes_req:     157976  bytes_alloc:     194432
    .
    .
    .
    { call_site: [ffffffff8109bd3b] sched_autogroup_create_attach                 } hitcount:          2  bytes_req:        144  bytes_alloc:        192
    { call_site: [ffffffff81097ee8] alloc_rt_sched_group                          } hitcount:          2  bytes_req:        128  bytes_alloc:        128
    { call_site: [ffffffff8109524a] alloc_fair_sched_group                        } hitcount:          2  bytes_req:        128  bytes_alloc:        128
    { call_site: [ffffffff81095225] alloc_fair_sched_group                        } hitcount:          2  bytes_req:        128  bytes_alloc:        128
    { call_site: [ffffffff81097ec2] alloc_rt_sched_group                          } hitcount:          2  bytes_req:        128  bytes_alloc:        128
    { call_site: [ffffffff81213e80] load_elf_binary                               } hitcount:          3  bytes_req:         84  bytes_alloc:         96
    { call_site: [ffffffff81079a2e] kthread_create_on_node                        } hitcount:          1  bytes_req:         56  bytes_alloc:         64
    { call_site: [ffffffffa00bf6fe] hidraw_send_report [hid]                      } hitcount:          1  bytes_req:          7  bytes_alloc:          8
    { call_site: [ffffffff8154bc62] usb_control_msg                               } hitcount:          1  bytes_req:          8  bytes_alloc:          8
    { call_site: [ffffffffa00bf1ca] hidraw_report_event [hid]                     } hitcount:          1  bytes_req:          7  bytes_alloc:          8

    Totals:
        Hits: 66598
        Entries: 65
        Dropped: 0

  Finally, to finish off our kmalloc example, instead of simply having
  the hist trigger display symbolic call_sites, we can have the hist
  trigger additionally display the complete set of kernel stack traces
  that led to each call_site.  To do that, we simply use the special
  value 'stacktrace' for the key parameter:

    # echo 'hist:keys=stacktrace:values=bytes_req,bytes_alloc:sort=bytes_alloc' > \
           /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger

  The above trigger will use the kernel stack trace in effect when an
  event is triggered as the key for the hash table.  This allows the
  enumeration of every kernel callpath that led up to a particular
  event, along with a running total of any of the event fields for
  that event.  Here we tally bytes requested and bytes allocated for
  every callpath in the system that led up to a kmalloc (in this case
  every callpath to a kmalloc for a kernel compile):

    # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
    # trigger info: hist:keys=stacktrace:vals=bytes_req,bytes_alloc:sort=bytes_alloc:size=2048 [active]

    { stacktrace:
         __kmalloc_track_caller+0x10b/0x1a0
         kmemdup+0x20/0x50
         hidraw_report_event+0x8a/0x120 [hid]
         hid_report_raw_event+0x3ea/0x440 [hid]
         hid_input_report+0x112/0x190 [hid]
         hid_irq_in+0xc2/0x260 [usbhid]
         __usb_hcd_giveback_urb+0x72/0x120
         usb_giveback_urb_bh+0x9e/0xe0
         tasklet_hi_action+0xf8/0x100
         __do_softirq+0x114/0x2c0
         irq_exit+0xa5/0xb0
         do_IRQ+0x5a/0xf0
         ret_from_intr+0x0/0x30
         cpuidle_enter+0x17/0x20
         cpu_startup_entry+0x315/0x3e0
         rest_init+0x7c/0x80
    } hitcount:          3  bytes_req:         21  bytes_alloc:         24
    { stacktrace:
         __kmalloc_track_caller+0x10b/0x1a0
         kmemdup+0x20/0x50
         hidraw_report_event+0x8a/0x120 [hid]
         hid_report_raw_event+0x3ea/0x440 [hid]
         hid_input_report+0x112/0x190 [hid]
         hid_irq_in+0xc2/0x260 [usbhid]
         __usb_hcd_giveback_urb+0x72/0x120
         usb_giveback_urb_bh+0x9e/0xe0
         tasklet_hi_action+0xf8/0x100
         __do_softirq+0x114/0x2c0
         irq_exit+0xa5/0xb0
         do_IRQ+0x5a/0xf0
         ret_from_intr+0x0/0x30
    } hitcount:          3  bytes_req:         21  bytes_alloc:         24
    { stacktrace:
         kmem_cache_alloc_trace+0xeb/0x150
         aa_alloc_task_context+0x27/0x40
         apparmor_cred_prepare+0x1f/0x50
         security_prepare_creds+0x16/0x20
         prepare_creds+0xdf/0x1a0
         SyS_capset+0xb5/0x200
         system_call_fastpath+0x12/0x6a
    } hitcount:          1  bytes_req:         32  bytes_alloc:         32
    .
    .
    .
    { stacktrace:
         __kmalloc+0x11b/0x1b0
         i915_gem_execbuffer2+0x6c/0x2c0 [i915]
         drm_ioctl+0x349/0x670 [drm]
         do_vfs_ioctl+0x2f0/0x4f0
         SyS_ioctl+0x81/0xa0
         system_call_fastpath+0x12/0x6a
    } hitcount:      17726  bytes_req:   13944120  bytes_alloc:   19593808
    { stacktrace:
         __kmalloc+0x11b/0x1b0
         load_elf_phdrs+0x76/0xa0
         load_elf_binary+0x102/0x1650
         search_binary_handler+0x97/0x1d0
         do_execveat_common.isra.34+0x551/0x6e0
         SyS_execve+0x3a/0x50
         return_from_execve+0x0/0x23
    } hitcount:      33348  bytes_req:   17152128  bytes_alloc:   20226048
    { stacktrace:
         kmem_cache_alloc_trace+0xeb/0x150
         apparmor_file_alloc_security+0x27/0x40
         security_file_alloc+0x16/0x20
         get_empty_filp+0x93/0x1c0
         path_openat+0x31/0x5f0
         do_filp_open+0x3a/0x90
         do_sys_open+0x128/0x220
         SyS_open+0x1e/0x20
         system_call_fastpath+0x12/0x6a
    } hitcount:    4766422  bytes_req:    9532844  bytes_alloc:   38131376
    { stacktrace:
         __kmalloc+0x11b/0x1b0
         seq_buf_alloc+0x1b/0x50
         seq_read+0x2cc/0x370
         proc_reg_read+0x3d/0x80
         __vfs_read+0x28/0xe0
         vfs_read+0x86/0x140
         SyS_read+0x46/0xb0
         system_call_fastpath+0x12/0x6a
    } hitcount:      19133  bytes_req:   78368768  bytes_alloc:   78368768

    Totals:
        Hits: 6085872
        Entries: 253
        Dropped: 0

  If you key a hist trigger on common_pid, in order for example to
  gather and display sorted totals for each process, you can use the
  special .execname modifier to display the executable names for the
  processes in the table rather than raw pids.  The example below
  keeps a per-process sum of total bytes read:

    # echo 'hist:key=common_pid.execname:val=count:sort=count.descending' > \
           /sys/kernel/debug/tracing/events/syscalls/sys_enter_read/trigger

    # cat /sys/kernel/debug/tracing/events/syscalls/sys_enter_read/hist
    # trigger info: hist:keys=common_pid.execname:vals=count:sort=count.descending:size=2048 [active]

    { common_pid: gnome-terminal  [      3196] } hitcount:        280  count:    1093512
    { common_pid: Xorg            [      1309] } hitcount:        525  count:     256640
    { common_pid: compiz          [      2889] } hitcount:         59  count:     254400
    { common_pid: bash            [      8710] } hitcount:          3  count:      66369
    { common_pid: dbus-daemon-lau [      8703] } hitcount:         49  count:      47739
    { common_pid: irqbalance      [      1252] } hitcount:         27  count:      27648
    { common_pid: 01ifupdown      [      8705] } hitcount:          3  count:      17216
    { common_pid: dbus-daemon     [       772] } hitcount:         10  count:      12396
    { common_pid: Socket Thread   [      8342] } hitcount:         11  count:      11264
    { common_pid: nm-dhcp-client. [      8701] } hitcount:          6  count:       7424
    { common_pid: gmain           [      1315] } hitcount:         18  count:       6336
    .
    .
    .
    { common_pid: postgres        [      1892] } hitcount:          2  count:         32
    { common_pid: postgres        [      1891] } hitcount:          2  count:         32
    { common_pid: gmain           [      8704] } hitcount:          2  count:         32
    { common_pid: upstart-dbus-br [      2740] } hitcount:         21  count:         21
    { common_pid: nm-dispatcher.a [      8696] } hitcount:          1  count:         16
    { common_pid: indicator-datet [      2904] } hitcount:          1  count:         16
    { common_pid: gdbus           [      2998] } hitcount:          1  count:         16
    { common_pid: rtkit-daemon    [      2052] } hitcount:          1  count:          8
    { common_pid: init            [         1] } hitcount:          2  count:          2

    Totals:
        Hits: 2116
        Entries: 51
        Dropped: 0

  Similarly, if you key a hist trigger on syscall id, for example to
  gather and display a list of systemwide syscall hits, you can use
  the special .syscall modifier to display the syscall names rather
  than raw ids.  The example below keeps a running total of syscall
  counts for the system during the run:

    # echo 'hist:key=id.syscall:val=hitcount' > \
           /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/trigger

    # cat /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/hist
    # trigger info: hist:keys=id.syscall:vals=hitcount:sort=hitcount:size=2048 [active]

    { id: sys_fsync                     [ 74] } hitcount:          1
    { id: sys_newuname                  [ 63] } hitcount:          1
    { id: sys_prctl                     [157] } hitcount:          1
    { id: sys_statfs                    [137] } hitcount:          1
    { id: sys_symlink                   [ 88] } hitcount:          1
    { id: sys_sendmmsg                  [307] } hitcount:          1
    { id: sys_semctl                    [ 66] } hitcount:          1
    { id: sys_readlink                  [ 89] } hitcount:          3
    { id: sys_bind                      [ 49] } hitcount:          3
    { id: sys_getsockname               [ 51] } hitcount:          3
    { id: sys_unlink                    [ 87] } hitcount:          3
    { id: sys_rename                    [ 82] } hitcount:          4
    { id: unknown_syscall               [ 58] } hitcount:          4
    { id: sys_connect                   [ 42] } hitcount:          4
    { id: sys_getpid                    [ 39] } hitcount:          4
    .
    .
    .
    { id: sys_rt_sigprocmask            [ 14] } hitcount:        952
    { id: sys_futex                     [202] } hitcount:       1534
    { id: sys_write                     [  1] } hitcount:       2689
    { id: sys_setitimer                 [ 38] } hitcount:       2797
    { id: sys_read                      [  0] } hitcount:       3202
    { id: sys_select                    [ 23] } hitcount:       3773
    { id: sys_writev                    [ 20] } hitcount:       4531
    { id: sys_poll                      [  7] } hitcount:       8314
    { id: sys_recvmsg                   [ 47] } hitcount:      13738
    { id: sys_ioctl                     [ 16] } hitcount:      21843

    Totals:
        Hits: 67612
        Entries: 72
        Dropped: 0

    The syscall counts above provide a rough overall picture of system
    call activity on the system; we can see for example that the most
    popular system call on this system was the 'sys_ioctl' system call.

    We can use 'compound' keys to refine that number and provide some
    further insight as to which processes exactly contribute to the
    overall ioctl count.

    The command below keeps a hitcount for every unique combination of
    system call id and pid - the end result is essentially a table
    that keeps a per-pid sum of system call hits.  The results are
    sorted using the system call id as the primary key, and the
    hitcount sum as the secondary key:

    # echo 'hist:key=id.syscall,common_pid.execname:val=hitcount:sort=id,hitcount' > \
           /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/trigger

    # cat /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/hist
    # trigger info: hist:keys=id.syscall,common_pid.execname:vals=hitcount:sort=id.syscall,hitcount:size=2048 [active]

    { id: sys_read                      [  0], common_pid: rtkit-daemon    [      1877] } hitcount:          1
    { id: sys_read                      [  0], common_pid: gdbus           [      2976] } hitcount:          1
    { id: sys_read                      [  0], common_pid: console-kit-dae [      3400] } hitcount:          1
    { id: sys_read                      [  0], common_pid: postgres        [      1865] } hitcount:          1
    { id: sys_read                      [  0], common_pid: deja-dup-monito [      3543] } hitcount:          2
    { id: sys_read                      [  0], common_pid: NetworkManager  [       890] } hitcount:          2
    { id: sys_read                      [  0], common_pid: evolution-calen [      3048] } hitcount:          2
    { id: sys_read                      [  0], common_pid: postgres        [      1864] } hitcount:          2
    { id: sys_read                      [  0], common_pid: nm-applet       [      3022] } hitcount:          2
    { id: sys_read                      [  0], common_pid: whoopsie        [      1212] } hitcount:          2
    .
    .
    .
    { id: sys_ioctl                     [ 16], common_pid: bash            [      8479] } hitcount:          1
    { id: sys_ioctl                     [ 16], common_pid: bash            [      3472] } hitcount:         12
    { id: sys_ioctl                     [ 16], common_pid: gnome-terminal  [      3199] } hitcount:         16
    { id: sys_ioctl                     [ 16], common_pid: Xorg            [      1267] } hitcount:       1808
    { id: sys_ioctl                     [ 16], common_pid: compiz          [      2994] } hitcount:       5580
    .
    .
    .
    { id: sys_waitid                    [247], common_pid: upstart-dbus-br [      2690] } hitcount:          3
    { id: sys_waitid                    [247], common_pid: upstart-dbus-br [      2688] } hitcount:         16
    { id: sys_inotify_add_watch         [254], common_pid: gmain           [       975] } hitcount:          2
    { id: sys_inotify_add_watch         [254], common_pid: gmain           [      3204] } hitcount:          4
    { id: sys_inotify_add_watch         [254], common_pid: gmain           [      2888] } hitcount:          4
    { id: sys_inotify_add_watch         [254], common_pid: gmain           [      3003] } hitcount:          4
    { id: sys_inotify_add_watch         [254], common_pid: gmain           [      2873] } hitcount:          4
    { id: sys_inotify_add_watch         [254], common_pid: gmain           [      3196] } hitcount:          6
    { id: sys_openat                    [257], common_pid: java            [      2623] } hitcount:          2
    { id: sys_eventfd2                  [290], common_pid: ibus-ui-gtk3    [      2760] } hitcount:          4
    { id: sys_eventfd2                  [290], common_pid: compiz          [      2994] } hitcount:          6

    Totals:
        Hits: 31536
        Entries: 323
        Dropped: 0

    The above list does give us a breakdown of the ioctl syscall by
    pid, but it also gives us quite a bit more than that, which we
    don't really care about at the moment.  Since we know the syscall
    id for sys_ioctl (16, displayed next to the sys_ioctl name), we
    can use that to filter out all the other syscalls:

    # echo 'hist:key=id.syscall,common_pid.execname:val=hitcount:sort=id,hitcount if id == 16' > \
           /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/trigger

    # cat /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/hist
    # trigger info: hist:keys=id.syscall,common_pid.execname:vals=hitcount:sort=id.syscall,hitcount:size=2048 if id == 16 [active]

    { id: sys_ioctl                     [ 16], common_pid: gmain           [      2769] } hitcount:          1
    { id: sys_ioctl                     [ 16], common_pid: evolution-addre [      8571] } hitcount:          1
    { id: sys_ioctl                     [ 16], common_pid: gmain           [      3003] } hitcount:          1
    { id: sys_ioctl                     [ 16], common_pid: gmain           [      2781] } hitcount:          1
    { id: sys_ioctl                     [ 16], common_pid: gmain           [      2829] } hitcount:          1
    { id: sys_ioctl                     [ 16], common_pid: bash            [      8726] } hitcount:          1
    { id: sys_ioctl                     [ 16], common_pid: bash            [      8508] } hitcount:          1
    { id: sys_ioctl                     [ 16], common_pid: gmain           [      2970] } hitcount:          1
    { id: sys_ioctl                     [ 16], common_pid: gmain           [      2768] } hitcount:          1
    .
    .
    .
    { id: sys_ioctl                     [ 16], common_pid: pool            [      8559] } hitcount:         45
    { id: sys_ioctl                     [ 16], common_pid: pool            [      8555] } hitcount:         48
    { id: sys_ioctl                     [ 16], common_pid: pool            [      8551] } hitcount:         48
    { id: sys_ioctl                     [ 16], common_pid: avahi-daemon    [       896] } hitcount:         66
    { id: sys_ioctl                     [ 16], common_pid: Xorg            [      1267] } hitcount:      26674
    { id: sys_ioctl                     [ 16], common_pid: compiz          [      2994] } hitcount:      73443

    Totals:
        Hits: 101162
        Entries: 103
        Dropped: 0

    The above output shows that 'compiz' and 'Xorg' are far and away
    the heaviest ioctl callers (which might lead to questions about
    whether they really need to be making all those calls and to
    possible avenues for further investigation.)

    The compound key examples used a key and a sum value (hitcount) to
    sort the output, but we can just as easily use two keys instead.
    Here's an example where we use a compound key composed of the the
    common_pid and size event fields.  Sorting with pid as the primary
    key and 'size' as the secondary key allows us to display an
    ordered summary of the recvfrom sizes, with counts, received by
    each process:

    # echo 'hist:key=common_pid.execname,size:val=hitcount:sort=common_pid,size' > \
           /sys/kernel/debug/tracing/events/syscalls/sys_enter_recvfrom/trigger

    # cat /sys/kernel/debug/tracing/events/syscalls/sys_enter_recvfrom/hist
    # trigger info: hist:keys=common_pid.execname,size:vals=hitcount:sort=common_pid.execname,size:size=2048 [active]

    { common_pid: smbd            [       784], size:          4 } hitcount:          1
    { common_pid: dnsmasq         [      1412], size:       4096 } hitcount:        672
    { common_pid: postgres        [      1796], size:       1000 } hitcount:          6
    { common_pid: postgres        [      1867], size:       1000 } hitcount:         10
    { common_pid: bamfdaemon      [      2787], size:         28 } hitcount:          2
    { common_pid: bamfdaemon      [      2787], size:      14360 } hitcount:          1
    { common_pid: compiz          [      2994], size:          8 } hitcount:          1
    { common_pid: compiz          [      2994], size:         20 } hitcount:         11
    { common_pid: gnome-terminal  [      3199], size:          4 } hitcount:          2
    { common_pid: firefox         [      8817], size:          4 } hitcount:          1
    { common_pid: firefox         [      8817], size:          8 } hitcount:          5
    { common_pid: firefox         [      8817], size:        588 } hitcount:          2
    { common_pid: firefox         [      8817], size:        628 } hitcount:          1
    { common_pid: firefox         [      8817], size:       6944 } hitcount:          1
    { common_pid: firefox         [      8817], size:     408880 } hitcount:          2
    { common_pid: firefox         [      8822], size:          8 } hitcount:          2
    { common_pid: firefox         [      8822], size:        160 } hitcount:          2
    { common_pid: firefox         [      8822], size:        320 } hitcount:          2
    { common_pid: firefox         [      8822], size:        352 } hitcount:          1
    .
    .
    .
    { common_pid: pool            [      8923], size:       1960 } hitcount:         10
    { common_pid: pool            [      8923], size:       2048 } hitcount:         10
    { common_pid: pool            [      8924], size:       1960 } hitcount:         10
    { common_pid: pool            [      8924], size:       2048 } hitcount:         10
    { common_pid: pool            [      8928], size:       1964 } hitcount:          4
    { common_pid: pool            [      8928], size:       1965 } hitcount:          2
    { common_pid: pool            [      8928], size:       2048 } hitcount:          6
    { common_pid: pool            [      8929], size:       1982 } hitcount:          1
    { common_pid: pool            [      8929], size:       2048 } hitcount:          1

    Totals:
        Hits: 2016
        Entries: 224
        Dropped: 0

  The above example also illustrates the fact that although a compound
  key is treated as a single entity for hashing purposes, the sub-keys
  it's composed of can be accessed independently.

  The next example uses a string field as the hash key and
  demonstrates how you can manually pause and continue a hist trigger.
  In this example, we'll aggregate fork counts and don't expect a
  large number of entries in the hash table, so we'll drop it to a
  much smaller number, say 256:

    # echo 'hist:key=child_comm:val=hitcount:size=256' > \
           /sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger

    # cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist
    # trigger info: hist:keys=child_comm:vals=hitcount:sort=hitcount:size=256 [active]

    { child_comm: dconf worker                        } hitcount:          1
    { child_comm: ibus-daemon                         } hitcount:          1
    { child_comm: whoopsie                            } hitcount:          1
    { child_comm: smbd                                } hitcount:          1
    { child_comm: gdbus                               } hitcount:          1
    { child_comm: kthreadd                            } hitcount:          1
    { child_comm: dconf worker                        } hitcount:          1
    { child_comm: evolution-alarm                     } hitcount:          2
    { child_comm: Socket Thread                       } hitcount:          2
    { child_comm: postgres                            } hitcount:          2
    { child_comm: bash                                } hitcount:          3
    { child_comm: compiz                              } hitcount:          3
    { child_comm: evolution-sourc                     } hitcount:          4
    { child_comm: dhclient                            } hitcount:          4
    { child_comm: pool                                } hitcount:          5
    { child_comm: nm-dispatcher.a                     } hitcount:          8
    { child_comm: firefox                             } hitcount:          8
    { child_comm: dbus-daemon                         } hitcount:          8
    { child_comm: glib-pacrunner                      } hitcount:         10
    { child_comm: evolution                           } hitcount:         23

    Totals:
        Hits: 89
        Entries: 20
        Dropped: 0

  If we want to pause the hist trigger, we can simply append :pause to
  the command that started the trigger.  Notice that the trigger info
  displays as [paused]:

    # echo 'hist:key=child_comm:val=hitcount:size=256:pause' >> \
           /sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger

    # cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist
    # trigger info: hist:keys=child_comm:vals=hitcount:sort=hitcount:size=256 [paused]

    { child_comm: dconf worker                        } hitcount:          1
    { child_comm: kthreadd                            } hitcount:          1
    { child_comm: dconf worker                        } hitcount:          1
    { child_comm: gdbus                               } hitcount:          1
    { child_comm: ibus-daemon                         } hitcount:          1
    { child_comm: Socket Thread                       } hitcount:          2
    { child_comm: evolution-alarm                     } hitcount:          2
    { child_comm: smbd                                } hitcount:          2
    { child_comm: bash                                } hitcount:          3
    { child_comm: whoopsie                            } hitcount:          3
    { child_comm: compiz                              } hitcount:          3
    { child_comm: evolution-sourc                     } hitcount:          4
    { child_comm: pool                                } hitcount:          5
    { child_comm: postgres                            } hitcount:          6
    { child_comm: firefox                             } hitcount:          8
    { child_comm: dhclient                            } hitcount:         10
    { child_comm: emacs                               } hitcount:         12
    { child_comm: dbus-daemon                         } hitcount:         20
    { child_comm: nm-dispatcher.a                     } hitcount:         20
    { child_comm: evolution                           } hitcount:         35
    { child_comm: glib-pacrunner                      } hitcount:         59

    Totals:
        Hits: 199
        Entries: 21
        Dropped: 0

  To manually continue having the trigger aggregate events, append
  :cont instead.  Notice that the trigger info displays as [active]
  again, and the data has changed:

    # echo 'hist:key=child_comm:val=hitcount:size=256:cont' >> \
           /sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger

    # cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist
    # trigger info: hist:keys=child_comm:vals=hitcount:sort=hitcount:size=256 [active]

    { child_comm: dconf worker                        } hitcount:          1
    { child_comm: dconf worker                        } hitcount:          1
    { child_comm: kthreadd                            } hitcount:          1
    { child_comm: gdbus                               } hitcount:          1
    { child_comm: ibus-daemon                         } hitcount:          1
    { child_comm: Socket Thread                       } hitcount:          2
    { child_comm: evolution-alarm                     } hitcount:          2
    { child_comm: smbd                                } hitcount:          2
    { child_comm: whoopsie                            } hitcount:          3
    { child_comm: compiz                              } hitcount:          3
    { child_comm: evolution-sourc                     } hitcount:          4
    { child_comm: bash                                } hitcount:          5
    { child_comm: pool                                } hitcount:          5
    { child_comm: postgres                            } hitcount:          6
    { child_comm: firefox                             } hitcount:          8
    { child_comm: dhclient                            } hitcount:         11
    { child_comm: emacs                               } hitcount:         12
    { child_comm: dbus-daemon                         } hitcount:         22
    { child_comm: nm-dispatcher.a                     } hitcount:         22
    { child_comm: evolution                           } hitcount:         35
    { child_comm: glib-pacrunner                      } hitcount:         59

    Totals:
        Hits: 206
        Entries: 21
        Dropped: 0

  The previous example showed how to start and stop a hist trigger by
  appending 'pause' and 'continue' to the hist trigger command.  A
  hist trigger can also be started in a paused state by initially
  starting the trigger with ':pause' appended.  This allows you to
  start the trigger only when you're ready to start collecting data
  and not before.  For example, you could start the trigger in a
  paused state, then unpause it and do something you want to measure,
  then pause the trigger again when done.

  Of course, doing this manually can be difficult and error-prone, but
  it is possible to automatically start and stop a hist trigger based
  on some condition, via the enable_hist and disable_hist triggers.

  For example, suppose we wanted to take a look at the relative
  weights in terms of skb length for each callpath that leads to a
  netif_receieve_skb event when downloading a decent-sized file using
  wget.

  First we set up an initially paused stacktrace trigger on the
  netif_receive_skb event:

    # echo 'hist:key=stacktrace:vals=len:pause' > \
           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger

  Next, we set up an 'enable_hist' trigger on the sched_process_exec
  event, with an 'if filename==/usr/bin/wget' filter.  The effect of
  this new trigger is that it will 'unpause' the hist trigger we just
  set up on netif_receive_skb if and only if it sees a
  sched_process_exec event with a filename of '/usr/bin/wget'.  When
  that happens, all netif_receive_skb events are aggregated into a
  hash table keyed on stacktrace:

    # echo 'enable_hist:net:netif_receive_skb if filename==/usr/bin/wget' > \
           /sys/kernel/debug/tracing/events/sched/sched_process_exec/trigger

  The aggregation continues until the netif_receive_skb is paused
  again, which is what the following disable_hist event does by
  creating a similar setup on the sched_process_exit event, using the
  filter 'comm==wget':

    # echo 'disable_hist:net:netif_receive_skb if comm==wget' > \
           /sys/kernel/debug/tracing/events/sched/sched_process_exit/trigger

  Whenever a process exits and the comm field of the disable_hist
  trigger filter matches 'comm==wget', the netif_receive_skb hist
  trigger is disabled.

  The overall effect is that netif_receive_skb events are aggregated
  into the hash table for only the duration of the wget.  Executing a
  wget command and then listing the 'hist' file will display the
  output generated by the wget command:

    $ wget https://www.kernel.org/pub/linux/kernel/v3.x/patch-3.19.xz

    # cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist
    # trigger info: hist:keys=stacktrace:vals=len:sort=hitcount:size=2048 [paused]

    { stacktrace:
         __netif_receive_skb_core+0x46d/0x990
         __netif_receive_skb+0x18/0x60
         netif_receive_skb_internal+0x23/0x90
         napi_gro_receive+0xc8/0x100
         ieee80211_deliver_skb+0xd6/0x270 [mac80211]
         ieee80211_rx_handlers+0xccf/0x22f0 [mac80211]
         ieee80211_prepare_and_rx_handle+0x4e7/0xc40 [mac80211]
         ieee80211_rx+0x31d/0x900 [mac80211]
         iwlagn_rx_reply_rx+0x3db/0x6f0 [iwldvm]
         iwl_rx_dispatch+0x8e/0xf0 [iwldvm]
         iwl_pcie_irq_handler+0xe3c/0x12f0 [iwlwifi]
         irq_thread_fn+0x20/0x50
         irq_thread+0x11f/0x150
         kthread+0xd2/0xf0
         ret_from_fork+0x42/0x70
    } hitcount:         85  len:      28884
    { stacktrace:
         __netif_receive_skb_core+0x46d/0x990
         __netif_receive_skb+0x18/0x60
         netif_receive_skb_internal+0x23/0x90
         napi_gro_complete+0xa4/0xe0
         dev_gro_receive+0x23a/0x360
         napi_gro_receive+0x30/0x100
         ieee80211_deliver_skb+0xd6/0x270 [mac80211]
         ieee80211_rx_handlers+0xccf/0x22f0 [mac80211]
         ieee80211_prepare_and_rx_handle+0x4e7/0xc40 [mac80211]
         ieee80211_rx+0x31d/0x900 [mac80211]
         iwlagn_rx_reply_rx+0x3db/0x6f0 [iwldvm]
         iwl_rx_dispatch+0x8e/0xf0 [iwldvm]
         iwl_pcie_irq_handler+0xe3c/0x12f0 [iwlwifi]
         irq_thread_fn+0x20/0x50
         irq_thread+0x11f/0x150
         kthread+0xd2/0xf0
    } hitcount:         98  len:     664329
    { stacktrace:
         __netif_receive_skb_core+0x46d/0x990
         __netif_receive_skb+0x18/0x60
         process_backlog+0xa8/0x150
         net_rx_action+0x15d/0x340
         __do_softirq+0x114/0x2c0
         do_softirq_own_stack+0x1c/0x30
         do_softirq+0x65/0x70
         __local_bh_enable_ip+0xb5/0xc0
         ip_finish_output+0x1f4/0x840
         ip_output+0x6b/0xc0
         ip_local_out_sk+0x31/0x40
         ip_send_skb+0x1a/0x50
         udp_send_skb+0x173/0x2a0
         udp_sendmsg+0x2bf/0x9f0
         inet_sendmsg+0x64/0xa0
         sock_sendmsg+0x3d/0x50
    } hitcount:        115  len:      13030
    { stacktrace:
         __netif_receive_skb_core+0x46d/0x990
         __netif_receive_skb+0x18/0x60
         netif_receive_skb_internal+0x23/0x90
         napi_gro_complete+0xa4/0xe0
         napi_gro_flush+0x6d/0x90
         iwl_pcie_irq_handler+0x92a/0x12f0 [iwlwifi]
         irq_thread_fn+0x20/0x50
         irq_thread+0x11f/0x150
         kthread+0xd2/0xf0
         ret_from_fork+0x42/0x70
    } hitcount:        934  len:    5512212

    Totals:
        Hits: 1232
        Entries: 4
        Dropped: 0

  The above shows all the netif_receive_skb callpaths and their total
  lengths for the duration of the wget command.

  The 'clear' hist trigger param can be used to clear the hash table.
  Suppose we wanted to try another run of the previous example but
  this time also wanted to see the complete list of events that went
  into the histogram.  In order to avoid having to set everything up
  again, we can just clear the histogram first:

    # echo 'hist:key=stacktrace:vals=len:clear' >> \
           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger

  Just to verify that it is in fact cleared, here's what we now see in
  the hist file:

    # cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist
    # trigger info: hist:keys=stacktrace:vals=len:sort=hitcount:size=2048 [paused]

    Totals:
        Hits: 0
        Entries: 0
        Dropped: 0

  Since we want to see the detailed list of every netif_receive_skb
  event occurring during the new run, which are in fact the same
  events being aggregated into the hash table, we add some additional
  'enable_event' events to the triggering sched_process_exec and
  sched_process_exit events as such:

    # echo 'enable_event:net:netif_receive_skb if filename==/usr/bin/wget' > \
           /sys/kernel/debug/tracing/events/sched/sched_process_exec/trigger

    # echo 'disable_event:net:netif_receive_skb if comm==wget' > \
           /sys/kernel/debug/tracing/events/sched/sched_process_exit/trigger

  If you read the trigger files for the sched_process_exec and
  sched_process_exit triggers, you should see two triggers for each:
  one enabling/disabling the hist aggregation and the other
  enabling/disabling the logging of events:

    # cat /sys/kernel/debug/tracing/events/sched/sched_process_exec/trigger
    enable_event:net:netif_receive_skb:unlimited if filename==/usr/bin/wget
    enable_hist:net:netif_receive_skb:unlimited if filename==/usr/bin/wget

    # cat /sys/kernel/debug/tracing/events/sched/sched_process_exit/trigger
    enable_event:net:netif_receive_skb:unlimited if comm==wget
    disable_hist:net:netif_receive_skb:unlimited if comm==wget

  In other words, whenever either of the sched_process_exec or
  sched_process_exit events is hit and matches 'wget', it enables or
  disables both the histogram and the event log, and what you end up
  with is a hash table and set of events just covering the specified
  duration.  Run the wget command again:

    $ wget https://www.kernel.org/pub/linux/kernel/v3.x/patch-3.19.xz

  Displaying the 'hist' file should show something similar to what you
  saw in the last run, but this time you should also see the
  individual events in the trace file:

    # cat /sys/kernel/debug/tracing/trace

    # tracer: nop
    #
    # entries-in-buffer/entries-written: 183/1426   #P:4
    #
    #                              _-----=> irqs-off
    #                             / _----=> need-resched
    #                            | / _---=> hardirq/softirq
    #                            || / _--=> preempt-depth
    #                            ||| /     delay
    #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
    #              | |       |   ||||       |         |
                wget-15108 [000] ..s1 31769.606929: netif_receive_skb: dev=lo skbaddr=ffff88009c353100 len=60
                wget-15108 [000] ..s1 31769.606999: netif_receive_skb: dev=lo skbaddr=ffff88009c353200 len=60
             dnsmasq-1382  [000] ..s1 31769.677652: netif_receive_skb: dev=lo skbaddr=ffff88009c352b00 len=130
             dnsmasq-1382  [000] ..s1 31769.685917: netif_receive_skb: dev=lo skbaddr=ffff88009c352200 len=138
    ##### CPU 2 buffer started ####
      irq/29-iwlwifi-559   [002] ..s. 31772.031529: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d433d00 len=2948
      irq/29-iwlwifi-559   [002] ..s. 31772.031572: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d432200 len=1500
      irq/29-iwlwifi-559   [002] ..s. 31772.032196: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d433100 len=2948
      irq/29-iwlwifi-559   [002] ..s. 31772.032761: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d433000 len=2948
      irq/29-iwlwifi-559   [002] ..s. 31772.033220: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d432e00 len=1500
    .
    .
    .

  The following example demonstrates how multiple hist triggers can be
  attached to a given event.  This capability can be useful for
  creating a set of different summaries derived from the same set of
  events, or for comparing the effects of different filters, among
  other things.

    # echo 'hist:keys=skbaddr.hex:vals=len if len < 0' >> \
           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
    # echo 'hist:keys=skbaddr.hex:vals=len if len > 4096' >> \
           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
    # echo 'hist:keys=skbaddr.hex:vals=len if len == 256' >> \
           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
    # echo 'hist:keys=skbaddr.hex:vals=len' >> \
           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
    # echo 'hist:keys=len:vals=common_preempt_count' >> \
           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger

  The above set of commands create four triggers differing only in
  their filters, along with a completely different though fairly
  nonsensical trigger.  Note that in order to append multiple hist
  triggers to the same file, you should use the '>>' operator to
  append them ('>' will also add the new hist trigger, but will remove
  any existing hist triggers beforehand).

  Displaying the contents of the 'hist' file for the event shows the
  contents of all five histograms:

    # cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist

    # event histogram
    #
    # trigger info: hist:keys=len:vals=hitcount,common_preempt_count:sort=hitcount:size=2048 [active]
    #

    { len:        176 } hitcount:          1  common_preempt_count:          0
    { len:        223 } hitcount:          1  common_preempt_count:          0
    { len:       4854 } hitcount:          1  common_preempt_count:          0
    { len:        395 } hitcount:          1  common_preempt_count:          0
    { len:        177 } hitcount:          1  common_preempt_count:          0
    { len:        446 } hitcount:          1  common_preempt_count:          0
    { len:       1601 } hitcount:          1  common_preempt_count:          0
    .
    .
    .
    { len:       1280 } hitcount:         66  common_preempt_count:          0
    { len:        116 } hitcount:         81  common_preempt_count:         40
    { len:        708 } hitcount:        112  common_preempt_count:          0
    { len:         46 } hitcount:        221  common_preempt_count:          0
    { len:       1264 } hitcount:        458  common_preempt_count:          0

    Totals:
        Hits: 1428
        Entries: 147
        Dropped: 0


    # event histogram
    #
    # trigger info: hist:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 [active]
    #

    { skbaddr: ffff8800baee5e00 } hitcount:          1  len:        130
    { skbaddr: ffff88005f3d5600 } hitcount:          1  len:       1280
    { skbaddr: ffff88005f3d4900 } hitcount:          1  len:       1280
    { skbaddr: ffff88009fed6300 } hitcount:          1  len:        115
    { skbaddr: ffff88009fe0ad00 } hitcount:          1  len:        115
    { skbaddr: ffff88008cdb1900 } hitcount:          1  len:         46
    { skbaddr: ffff880064b5ef00 } hitcount:          1  len:        118
    { skbaddr: ffff880044e3c700 } hitcount:          1  len:         60
    { skbaddr: ffff880100065900 } hitcount:          1  len:         46
    { skbaddr: ffff8800d46bd500 } hitcount:          1  len:        116
    { skbaddr: ffff88005f3d5f00 } hitcount:          1  len:       1280
    { skbaddr: ffff880100064700 } hitcount:          1  len:        365
    { skbaddr: ffff8800badb6f00 } hitcount:          1  len:         60
    .
    .
    .
    { skbaddr: ffff88009fe0be00 } hitcount:         27  len:      24677
    { skbaddr: ffff88009fe0a400 } hitcount:         27  len:      23052
    { skbaddr: ffff88009fe0b700 } hitcount:         31  len:      25589
    { skbaddr: ffff88009fe0b600 } hitcount:         32  len:      27326
    { skbaddr: ffff88006a462800 } hitcount:         68  len:      71678
    { skbaddr: ffff88006a463700 } hitcount:         70  len:      72678
    { skbaddr: ffff88006a462b00 } hitcount:         71  len:      77589
    { skbaddr: ffff88006a463600 } hitcount:         73  len:      71307
    { skbaddr: ffff88006a462200 } hitcount:         81  len:      81032

    Totals:
        Hits: 1451
        Entries: 318
        Dropped: 0


    # event histogram
    #
    # trigger info: hist:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 if len == 256 [active]
    #


    Totals:
        Hits: 0
        Entries: 0
        Dropped: 0


    # event histogram
    #
    # trigger info: hist:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 if len > 4096 [active]
    #

    { skbaddr: ffff88009fd2c300 } hitcount:          1  len:       7212
    { skbaddr: ffff8800d2bcce00 } hitcount:          1  len:       7212
    { skbaddr: ffff8800d2bcd700 } hitcount:          1  len:       7212
    { skbaddr: ffff8800d2bcda00 } hitcount:          1  len:      21492
    { skbaddr: ffff8800ae2e2d00 } hitcount:          1  len:       7212
    { skbaddr: ffff8800d2bcdb00 } hitcount:          1  len:       7212
    { skbaddr: ffff88006a4df500 } hitcount:          1  len:       4854
    { skbaddr: ffff88008ce47b00 } hitcount:          1  len:      18636
    { skbaddr: ffff8800ae2e2200 } hitcount:          1  len:      12924
    { skbaddr: ffff88005f3e1000 } hitcount:          1  len:       4356
    { skbaddr: ffff8800d2bcdc00 } hitcount:          2  len:      24420
    { skbaddr: ffff8800d2bcc200 } hitcount:          2  len:      12996

    Totals:
        Hits: 14
        Entries: 12
        Dropped: 0


    # event histogram
    #
    # trigger info: hist:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 if len < 0 [active]
    #


    Totals:
        Hits: 0
        Entries: 0
        Dropped: 0

  Named triggers can be used to have triggers share a common set of
  histogram data.  This capability is mostly useful for combining the
  output of events generated by tracepoints contained inside inline
  functions, but names can be used in a hist trigger on any event.
  For example, these two triggers when hit will update the same 'len'
  field in the shared 'foo' histogram data:

    # echo 'hist:name=foo:keys=skbaddr.hex:vals=len' > \
           /sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
    # echo 'hist:name=foo:keys=skbaddr.hex:vals=len' > \
           /sys/kernel/debug/tracing/events/net/netif_rx/trigger

  You can see that they're updating common histogram data by reading
  each event's hist files at the same time:

    # cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist;
      cat /sys/kernel/debug/tracing/events/net/netif_rx/hist

    # event histogram
    #
    # trigger info: hist:name=foo:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 [active]
    #

    { skbaddr: ffff88000ad53500 } hitcount:          1  len:         46
    { skbaddr: ffff8800af5a1500 } hitcount:          1  len:         76
    { skbaddr: ffff8800d62a1900 } hitcount:          1  len:         46
    { skbaddr: ffff8800d2bccb00 } hitcount:          1  len:        468
    { skbaddr: ffff8800d3c69900 } hitcount:          1  len:         46
    { skbaddr: ffff88009ff09100 } hitcount:          1  len:         52
    { skbaddr: ffff88010f13ab00 } hitcount:          1  len:        168
    { skbaddr: ffff88006a54f400 } hitcount:          1  len:         46
    { skbaddr: ffff8800d2bcc500 } hitcount:          1  len:        260
    { skbaddr: ffff880064505000 } hitcount:          1  len:         46
    { skbaddr: ffff8800baf24e00 } hitcount:          1  len:         32
    { skbaddr: ffff88009fe0ad00 } hitcount:          1  len:         46
    { skbaddr: ffff8800d3edff00 } hitcount:          1  len:         44
    { skbaddr: ffff88009fe0b400 } hitcount:          1  len:        168
    { skbaddr: ffff8800a1c55a00 } hitcount:          1  len:         40
    { skbaddr: ffff8800d2bcd100 } hitcount:          1  len:         40
    { skbaddr: ffff880064505f00 } hitcount:          1  len:        174
    { skbaddr: ffff8800a8bff200 } hitcount:          1  len:        160
    { skbaddr: ffff880044e3cc00 } hitcount:          1  len:         76
    { skbaddr: ffff8800a8bfe700 } hitcount:          1  len:         46
    { skbaddr: ffff8800d2bcdc00 } hitcount:          1  len:         32
    { skbaddr: ffff8800a1f64800 } hitcount:          1  len:         46
    { skbaddr: ffff8800d2bcde00 } hitcount:          1  len:        988
    { skbaddr: ffff88006a5dea00 } hitcount:          1  len:         46
    { skbaddr: ffff88002e37a200 } hitcount:          1  len:         44
    { skbaddr: ffff8800a1f32c00 } hitcount:          2  len:        676
    { skbaddr: ffff88000ad52600 } hitcount:          2  len:        107
    { skbaddr: ffff8800a1f91e00 } hitcount:          2  len:         92
    { skbaddr: ffff8800af5a0200 } hitcount:          2  len:        142
    { skbaddr: ffff8800d2bcc600 } hitcount:          2  len:        220
    { skbaddr: ffff8800ba36f500 } hitcount:          2  len:         92
    { skbaddr: ffff8800d021f800 } hitcount:          2  len:         92
    { skbaddr: ffff8800a1f33600 } hitcount:          2  len:        675
    { skbaddr: ffff8800a8bfff00 } hitcount:          3  len:        138
    { skbaddr: ffff8800d62a1300 } hitcount:          3  len:        138
    { skbaddr: ffff88002e37a100 } hitcount:          4  len:        184
    { skbaddr: ffff880064504400 } hitcount:          4  len:        184
    { skbaddr: ffff8800a8bfec00 } hitcount:          4  len:        184
    { skbaddr: ffff88000ad53700 } hitcount:          5  len:        230
    { skbaddr: ffff8800d2bcdb00 } hitcount:          5  len:        196
    { skbaddr: ffff8800a1f90000 } hitcount:          6  len:        276
    { skbaddr: ffff88006a54f900 } hitcount:          6  len:        276

    Totals:
        Hits: 81
        Entries: 42
        Dropped: 0
    # event histogram
    #
    # trigger info: hist:name=foo:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 [active]
    #

    { skbaddr: ffff88000ad53500 } hitcount:          1  len:         46
    { skbaddr: ffff8800af5a1500 } hitcount:          1  len:         76
    { skbaddr: ffff8800d62a1900 } hitcount:          1  len:         46
    { skbaddr: ffff8800d2bccb00 } hitcount:          1  len:        468
    { skbaddr: ffff8800d3c69900 } hitcount:          1  len:         46
    { skbaddr: ffff88009ff09100 } hitcount:          1  len:         52
    { skbaddr: ffff88010f13ab00 } hitcount:          1  len:        168
    { skbaddr: ffff88006a54f400 } hitcount:          1  len:         46
    { skbaddr: ffff8800d2bcc500 } hitcount:          1  len:        260
    { skbaddr: ffff880064505000 } hitcount:          1  len:         46
    { skbaddr: ffff8800baf24e00 } hitcount:          1  len:         32
    { skbaddr: ffff88009fe0ad00 } hitcount:          1  len:         46
    { skbaddr: ffff8800d3edff00 } hitcount:          1  len:         44
    { skbaddr: ffff88009fe0b400 } hitcount:          1  len:        168
    { skbaddr: ffff8800a1c55a00 } hitcount:          1  len:         40
    { skbaddr: ffff8800d2bcd100 } hitcount:          1  len:         40
    { skbaddr: ffff880064505f00 } hitcount:          1  len:        174
    { skbaddr: ffff8800a8bff200 } hitcount:          1  len:        160
    { skbaddr: ffff880044e3cc00 } hitcount:          1  len:         76
    { skbaddr: ffff8800a8bfe700 } hitcount:          1  len:         46
    { skbaddr: ffff8800d2bcdc00 } hitcount:          1  len:         32
    { skbaddr: ffff8800a1f64800 } hitcount:          1  len:         46
    { skbaddr: ffff8800d2bcde00 } hitcount:          1  len:        988
    { skbaddr: ffff88006a5dea00 } hitcount:          1  len:         46
    { skbaddr: ffff88002e37a200 } hitcount:          1  len:         44
    { skbaddr: ffff8800a1f32c00 } hitcount:          2  len:        676
    { skbaddr: ffff88000ad52600 } hitcount:          2  len:        107
    { skbaddr: ffff8800a1f91e00 } hitcount:          2  len:         92
    { skbaddr: ffff8800af5a0200 } hitcount:          2  len:        142
    { skbaddr: ffff8800d2bcc600 } hitcount:          2  len:        220
    { skbaddr: ffff8800ba36f500 } hitcount:          2  len:         92
    { skbaddr: ffff8800d021f800 } hitcount:          2  len:         92
    { skbaddr: ffff8800a1f33600 } hitcount:          2  len:        675
    { skbaddr: ffff8800a8bfff00 } hitcount:          3  len:        138
    { skbaddr: ffff8800d62a1300 } hitcount:          3  len:        138
    { skbaddr: ffff88002e37a100 } hitcount:          4  len:        184
    { skbaddr: ffff880064504400 } hitcount:          4  len:        184
    { skbaddr: ffff8800a8bfec00 } hitcount:          4  len:        184
    { skbaddr: ffff88000ad53700 } hitcount:          5  len:        230
    { skbaddr: ffff8800d2bcdb00 } hitcount:          5  len:        196
    { skbaddr: ffff8800a1f90000 } hitcount:          6  len:        276
    { skbaddr: ffff88006a54f900 } hitcount:          6  len:        276

    Totals:
        Hits: 81
        Entries: 42
        Dropped: 0

  And here's an example that shows how to combine histogram data from
  any two events even if they don't share any 'compatible' fields
  other than 'hitcount' and 'stacktrace'.  These commands create a
  couple of triggers named 'bar' using those fields:

    # echo 'hist:name=bar:key=stacktrace:val=hitcount' > \
           /sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger
    # echo 'hist:name=bar:key=stacktrace:val=hitcount' > \
          /sys/kernel/debug/tracing/events/net/netif_rx/trigger

  And displaying the output of either shows some interesting if
  somewhat confusing output:

    # cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist
    # cat /sys/kernel/debug/tracing/events/net/netif_rx/hist

    # event histogram
    #
    # trigger info: hist:name=bar:keys=stacktrace:vals=hitcount:sort=hitcount:size=2048 [active]
    #

    { stacktrace:
             _do_fork+0x18e/0x330
             kernel_thread+0x29/0x30
             kthreadd+0x154/0x1b0
             ret_from_fork+0x3f/0x70
    } hitcount:          1
    { stacktrace:
             netif_rx_internal+0xb2/0xd0
             netif_rx_ni+0x20/0x70
             dev_loopback_xmit+0xaa/0xd0
             ip_mc_output+0x126/0x240
             ip_local_out_sk+0x31/0x40
             igmp_send_report+0x1e9/0x230
             igmp_timer_expire+0xe9/0x120
             call_timer_fn+0x39/0xf0
             run_timer_softirq+0x1e1/0x290
             __do_softirq+0xfd/0x290
             irq_exit+0x98/0xb0
             smp_apic_timer_interrupt+0x4a/0x60
             apic_timer_interrupt+0x6d/0x80
             cpuidle_enter+0x17/0x20
             call_cpuidle+0x3b/0x60
             cpu_startup_entry+0x22d/0x310
    } hitcount:          1
    { stacktrace:
             netif_rx_internal+0xb2/0xd0
             netif_rx_ni+0x20/0x70
             dev_loopback_xmit+0xaa/0xd0
             ip_mc_output+0x17f/0x240
             ip_local_out_sk+0x31/0x40
             ip_send_skb+0x1a/0x50
             udp_send_skb+0x13e/0x270
             udp_sendmsg+0x2bf/0x980
             inet_sendmsg+0x67/0xa0
             sock_sendmsg+0x38/0x50
             SYSC_sendto+0xef/0x170
             SyS_sendto+0xe/0x10
             entry_SYSCALL_64_fastpath+0x12/0x6a
    } hitcount:          2
    { stacktrace:
             netif_rx_internal+0xb2/0xd0
             netif_rx+0x1c/0x60
             loopback_xmit+0x6c/0xb0
             dev_hard_start_xmit+0x219/0x3a0
             __dev_queue_xmit+0x415/0x4f0
             dev_queue_xmit_sk+0x13/0x20
             ip_finish_output2+0x237/0x340
             ip_finish_output+0x113/0x1d0
             ip_output+0x66/0xc0
             ip_local_out_sk+0x31/0x40
             ip_send_skb+0x1a/0x50
             udp_send_skb+0x16d/0x270
             udp_sendmsg+0x2bf/0x980
             inet_sendmsg+0x67/0xa0
             sock_sendmsg+0x38/0x50
             ___sys_sendmsg+0x14e/0x270
    } hitcount:         76
    { stacktrace:
             netif_rx_internal+0xb2/0xd0
             netif_rx+0x1c/0x60
             loopback_xmit+0x6c/0xb0
             dev_hard_start_xmit+0x219/0x3a0
             __dev_queue_xmit+0x415/0x4f0
             dev_queue_xmit_sk+0x13/0x20
             ip_finish_output2+0x237/0x340
             ip_finish_output+0x113/0x1d0
             ip_output+0x66/0xc0
             ip_local_out_sk+0x31/0x40
             ip_send_skb+0x1a/0x50
             udp_send_skb+0x16d/0x270
             udp_sendmsg+0x2bf/0x980
             inet_sendmsg+0x67/0xa0
             sock_sendmsg+0x38/0x50
             ___sys_sendmsg+0x269/0x270
    } hitcount:         77
    { stacktrace:
             netif_rx_internal+0xb2/0xd0
             netif_rx+0x1c/0x60
             loopback_xmit+0x6c/0xb0
             dev_hard_start_xmit+0x219/0x3a0
             __dev_queue_xmit+0x415/0x4f0
             dev_queue_xmit_sk+0x13/0x20
             ip_finish_output2+0x237/0x340
             ip_finish_output+0x113/0x1d0
             ip_output+0x66/0xc0
             ip_local_out_sk+0x31/0x40
             ip_send_skb+0x1a/0x50
             udp_send_skb+0x16d/0x270
             udp_sendmsg+0x2bf/0x980
             inet_sendmsg+0x67/0xa0
             sock_sendmsg+0x38/0x50
             SYSC_sendto+0xef/0x170
    } hitcount:         88
    { stacktrace:
             _do_fork+0x18e/0x330
             SyS_clone+0x19/0x20
             entry_SYSCALL_64_fastpath+0x12/0x6a
    } hitcount:        244

    Totals:
        Hits: 489
        Entries: 7
        Dropped: 0