/*
* sched_clock for unstable cpu clocks
*
* Copyright (C) 2008 Red Hat, Inc., Peter Zijlstra
*
* Updates and enhancements:
* Copyright (C) 2008 Red Hat, Inc. Steven Rostedt <srostedt@redhat.com>
*
* Based on code by:
* Ingo Molnar <mingo@redhat.com>
* Guillaume Chazarain <guichaz@gmail.com>
*
*
* What:
*
* cpu_clock(i) provides a fast (execution time) high resolution
* clock with bounded drift between CPUs. The value of cpu_clock(i)
* is monotonic for constant i. The timestamp returned is in nanoseconds.
*
* ######################### BIG FAT WARNING ##########################
* # when comparing cpu_clock(i) to cpu_clock(j) for i != j, time can #
* # go backwards !! #
* ####################################################################
*
* There is no strict promise about the base, although it tends to start
* at 0 on boot (but people really shouldn't rely on that).
*
* cpu_clock(i) -- can be used from any context, including NMI.
* local_clock() -- is cpu_clock() on the current cpu.
*
* sched_clock_cpu(i)
*
* How:
*
* The implementation either uses sched_clock() when
* !CONFIG_HAVE_UNSTABLE_SCHED_CLOCK, which means in that case the
* sched_clock() is assumed to provide these properties (mostly it means
* the architecture provides a globally synchronized highres time source).
*
* Otherwise it tries to create a semi stable clock from a mixture of other
* clocks, including:
*
* - GTOD (clock monotomic)
* - sched_clock()
* - explicit idle events
*
* We use GTOD as base and use sched_clock() deltas to improve resolution. The
* deltas are filtered to provide monotonicity and keeping it within an
* expected window.
*
* Furthermore, explicit sleep and wakeup hooks allow us to account for time
* that is otherwise invisible (TSC gets stopped).
*
*/
#include <linux/spinlock.h>
#include <linux/hardirq.h>
#include <linux/export.h>
#include <linux/percpu.h>
#include <linux/ktime.h>
#include <linux/sched.h>
#include <linux/nmi.h>
#include <linux/sched/clock.h>
#include <linux/static_key.h>
#include <linux/workqueue.h>
#include <linux/compiler.h>
#include <linux/tick.h>
#include <linux/init.h>
/*
* Scheduler clock - returns current time in nanosec units.
* This is default implementation.
* Architectures and sub-architectures can override this.
*/
unsigned long long __weak sched_clock(void)
{
return (unsigned long long)(jiffies - INITIAL_JIFFIES)
* (NSEC_PER_SEC / HZ);
}
EXPORT_SYMBOL_GPL(sched_clock);
__read_mostly int sched_clock_running;
void sched_clock_init(void)
{
sched_clock_running = 1;
}
#ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
/*
* We must start with !__sched_clock_stable because the unstable -> stable
* transition is accurate, while the stable -> unstable transition is not.
*
* Similarly we start with __sched_clock_stable_early, thereby assuming we
* will become stable, such that there's only a single 1 -> 0 transition.
*/
static DEFINE_STATIC_KEY_FALSE(__sched_clock_stable);
static int __sched_clock_stable_early = 1;
/*
* We want: ktime_get_ns() + __gtod_offset == sched_clock() + __sched_clock_offset
*/
__read_mostly u64 __sched_clock_offset;
static __read_mostly u64 __gtod_offset;
struct sched_clock_data {
u64 tick_raw;
u64 tick_gtod;
u64 clock;
};
static DEFINE_PER_CPU_SHARED_ALIGNED(struct sched_clock_data, sched_clock_data);
static inline struct sched_clock_data *this_scd(void)
{
return this_cpu_ptr(&sched_clock_data);
}
static inline struct sched_clock_data *cpu_sdc(int cpu)
{
return &per_cpu(sched_clock_data, cpu);
}
int sched_clock_stable(void)
{
return static_branch_likely(&__sched_clock_stable);
}
static void __scd_stamp(struct sched_clock_data *scd)
{
scd->tick_gtod = ktime_get_ns();
scd->tick_raw = sched_clock();
}
static void __set_sched_clock_stable(void)
{
struct sched_clock_data *scd;
/*
* Since we're still unstable and the tick is already running, we have
* to disable IRQs in order to get a consistent scd->tick* reading.
*/
local_irq_disable();
scd = this_scd();
/*
* Attempt to make the (initial) unstable->stable transition continuous.
*/
__sched_clock_offset = (scd->tick_gtod + __gtod_offset) - (scd->tick_raw);
local_irq_enable();
printk(KERN_INFO "sched_clock: Marking stable (%lld, %lld)->(%lld, %lld)\n",
scd->tick_gtod, __gtod_offset,
scd->tick_raw, __sched_clock_offset);
static_branch_enable(&__sched_clock_stable);
tick_dep_clear(TICK_DEP_BIT_CLOCK_UNSTABLE);
}
/*
* If we ever get here, we're screwed, because we found out -- typically after
* the fact -- that TSC wasn't good. This means all our clocksources (including
* ktime) could have reported wrong values.
*
* What we do here is an attempt to fix up and continue sort of where we left
* off in a coherent manner.
*
* The only way to fully avoid random clock jumps is to boot with:
* "tsc=unstable".
*/
static void __sched_clock_work(struct work_struct *work)
{
struct sched_clock_data *scd;
int cpu;
/* take a current timestamp and set 'now' */
preempt_disable();
scd = this_scd();
__scd_stamp(scd);
scd->clock = scd->tick_gtod + __gtod_offset;
preempt_enable();
/* clone to all CPUs */
for_each_possible_cpu(cpu)
per_cpu(sched_clock_data, cpu) = *scd;
printk(KERN_WARNING "TSC found unstable after boot, most likely due to broken BIOS. Use 'tsc=unstable'.\n");
printk(KERN_INFO "sched_clock: Marking unstable (%lld, %lld)<-(%lld, %lld)\n",
scd->tick_gtod, __gtod_offset,
scd->tick_raw, __sched_clock_offset);
static_branch_disable(&__sched_clock_stable);
}
static DECLARE_WORK(sched_clock_work, __sched_clock_work);
static void __clear_sched_clock_stable(void)
{
if (!sched_clock_stable())
return;
tick_dep_set(TICK_DEP_BIT_CLOCK_UNSTABLE);
schedule_work(&sched_clock_work);
}
void clear_sched_clock_stable(void)
{
__sched_clock_stable_early = 0;
smp_mb(); /* matches sched_clock_init_late() */
if (sched_clock_running == 2)
__clear_sched_clock_stable();
}
/*
* We run this as late_initcall() such that it runs after all built-in drivers,
* notably: acpi_processor and intel_idle, which can mark the TSC as unstable.
*/
static int __init sched_clock_init_late(void)
{
sched_clock_running = 2;
/*
* Ensure that it is impossible to not do a static_key update.
*
* Either {set,clear}_sched_clock_stable() must see sched_clock_running
* and do the update, or we must see their __sched_clock_stable_early
* and do the update, or both.
*/
smp_mb(); /* matches {set,clear}_sched_clock_stable() */
if (__sched_clock_stable_early)
__set_sched_clock_stable();
return 0;
}
late_initcall(sched_clock_init_late);
/*
* min, max except they take wrapping into account
*/
static inline u64 wrap_min(u64 x, u64 y)
{
return (s64)(x - y) < 0 ? x : y;
}
static inline u64 wrap_max(u64 x, u64 y)
{
return (s64)(x - y) > 0 ? x : y;
}
/*
* update the percpu scd from the raw @now value
*
* - filter out backward motion
* - use the GTOD tick value to create a window to filter crazy TSC values
*/
static u64 sched_clock_local(struct sched_clock_data *scd)
{
u64 now, clock, old_clock, min_clock, max_clock, gtod;
s64 delta;
again:
now = sched_clock();
delta = now - scd->tick_raw;
if (unlikely(delta < 0))
delta = 0;
old_clock = scd->clock;
/*
* scd->clock = clamp(scd->tick_gtod + delta,
* max(scd->tick_gtod, scd->clock),
* scd->tick_gtod + TICK_NSEC);
*/
gtod = scd->tick_gtod + __gtod_offset;
clock = gtod + delta;
min_clock = wrap_max(gtod, old_clock);
max_clock = wrap_max(old_clock, gtod + TICK_NSEC);
clock = wrap_max(clock, min_clock);
clock = wrap_min(clock, max_clock);
if (cmpxchg64(&scd->clock, old_clock, clock) != old_clock)
goto again;
return clock;
}
static u64 sched_clock_remote(struct sched_clock_data *scd)
{
struct sched_clock_data *my_scd = this_scd();
u64 this_clock, remote_clock;
u64 *ptr, old_val, val;
#if BITS_PER_LONG != 64
again:
/*
* Careful here: The local and the remote clock values need to
* be read out atomic as we need to compare the values and
* then update either the local or the remote side. So the
* cmpxchg64 below only protects one readout.
*
* We must reread via sched_clock_local() in the retry case on
* 32bit as an NMI could use sched_clock_local() via the
* tracer and hit between the readout of
* the low32bit and the high 32bit portion.
*/
this_clock = sched_clock_local(my_scd);
/*
* We must enforce atomic readout on 32bit, otherwise the
* update on the remote cpu can hit inbetween the readout of
* the low32bit and the high 32bit portion.
*/
remote_clock = cmpxchg64(&scd->clock, 0, 0);
#else
/*
* On 64bit the read of [my]scd->clock is atomic versus the
* update, so we can avoid the above 32bit dance.
*/
sched_clock_local(my_scd);
again:
this_clock = my_scd->clock;
remote_clock = scd->clock;
#endif
/*
* Use the opportunity that we have both locks
* taken to couple the two clocks: we take the
* larger time as the latest time for both
* runqueues. (this creates monotonic movement)
*/
if (likely((s64)(remote_clock - this_clock) < 0)) {
ptr = &scd->clock;
old_val = remote_clock;
val = this_clock;
} else {
/*
* Should be rare, but possible:
*/
ptr = &my_scd->clock;
old_val = this_clock;
val = remote_clock;
}
if (cmpxchg64(ptr, old_val, val) != old_val)
goto again;
return val;
}
/*
* Similar to cpu_clock(), but requires local IRQs to be disabled.
*
* See cpu_clock().
*/
u64 sched_clock_cpu(int cpu)
{
struct sched_clock_data *scd;
u64 clock;
if (sched_clock_stable())
return sched_clock() + __sched_clock_offset;
if (unlikely(!sched_clock_running))
return 0ull;
preempt_disable_notrace();
scd = cpu_sdc(cpu);
if (cpu != smp_processor_id())
clock = sched_clock_remote(scd);
else
clock = sched_clock_local(scd);
preempt_enable_notrace();
return clock;
}
EXPORT_SYMBOL_GPL(sched_clock_cpu);
void sched_clock_tick(void)
{
struct sched_clock_data *scd;
if (sched_clock_stable())
return;
if (unlikely(!sched_clock_running))
return;
lockdep_assert_irqs_disabled();
scd = this_scd();
__scd_stamp(scd);
sched_clock_local(scd);
}
void sched_clock_tick_stable(void)
{
u64 gtod, clock;
if (!sched_clock_stable())
return;
/*
* Called under watchdog_lock.
*
* The watchdog just found this TSC to (still) be stable, so now is a
* good moment to update our __gtod_offset. Because once we find the
* TSC to be unstable, any computation will be computing crap.
*/
local_irq_disable();
gtod = ktime_get_ns();
clock = sched_clock();
__gtod_offset = (clock + __sched_clock_offset) - gtod;
local_irq_enable();
}
/*
* We are going deep-idle (irqs are disabled):
*/
void sched_clock_idle_sleep_event(void)
{
sched_clock_cpu(smp_processor_id());
}
EXPORT_SYMBOL_GPL(sched_clock_idle_sleep_event);
/*
* We just idled; resync with ktime.
*/
void sched_clock_idle_wakeup_event(void)
{
unsigned long flags;
if (sched_clock_stable())
return;
if (unlikely(timekeeping_suspended))
return;
local_irq_save(flags);
sched_clock_tick();
local_irq_restore(flags);
}
EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event);
#else /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
u64 sched_clock_cpu(int cpu)
{
if (unlikely(!sched_clock_running))
return 0;
return sched_clock();
}
#endif /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
/*
* Running clock - returns the time that has elapsed while a guest has been
* running.
* On a guest this value should be local_clock minus the time the guest was
* suspended by the hypervisor (for any reason).
* On bare metal this function should return the same as local_clock.
* Architectures and sub-architectures can override this.
*/
u64 __weak running_clock(void)
{
return local_clock();
}
