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
  */