md/raid456: downlevel multicore operations to raid_run_ops

The percpu conversion allowed a straightforward handoff of stripe
processing to the async subsytem that initially showed some modest gains
(+4%).  However, this model is too simplistic and leads to stripes
bouncing between raid5d and the async thread pool for every invocation
of handle_stripe().  As reported by Holger this can fall into a
pathological situation severely impacting throughput (6x performance
loss).

By downleveling the parallelism to raid_run_ops the pathological
stripe_head bouncing is eliminated.  This version still exhibits an
average 11% throughput loss for:

	mdadm --create /dev/md0 /dev/sd[b-q] -n 16 -l 6
	echo 1024 > /sys/block/md0/md/stripe_cache_size
	dd if=/dev/zero of=/dev/md0 bs=1024k count=2048

...but the results are at least stable and can be used as a base for
further multicore experimentation.

Reported-by: Holger Kiehl <Holger.Kiehl@dwd.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>

1 files changed, 11 insertions, 1 deletions

diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
index 2390e0e83daf..dcefdc9629ee 100644
--- a/drivers/md/raid5.h
+++ b/drivers/md/raid5.h
@@ -214,12 +214,20 @@ struct stripe_head {
 	int			disks;		/* disks in stripe */
 	enum check_states	check_state;
 	enum reconstruct_states reconstruct_state;
-	/* stripe_operations
+	/**
+	 * struct stripe_operations
 	 * @target - STRIPE_OP_COMPUTE_BLK target
+	 * @target2 - 2nd compute target in the raid6 case
+	 * @zero_sum_result - P and Q verification flags
+	 * @request - async service request flags for raid_run_ops
 	 */
 	struct stripe_operations {
 		int 		     target, target2;
 		enum sum_check_flags zero_sum_result;
+		#ifdef CONFIG_MULTICORE_RAID456
+		unsigned long	     request;
+		wait_queue_head_t    wait_for_ops;
+		#endif
 	} ops;
 	struct r5dev {
 		struct bio	req;
@@ -294,6 +302,8 @@ struct r6_state {
 #define	STRIPE_FULL_WRITE	13 /* all blocks are set to be overwritten */
 #define	STRIPE_BIOFILL_RUN	14
 #define	STRIPE_COMPUTE_RUN	15
+#define	STRIPE_OPS_REQ_PENDING	16
+
 /*
  * Operation request flags
  */