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, 40 insertions, 35 deletions

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index c3e596778618..25c3c29134d1 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -1139,7 +1139,7 @@ static void ops_run_check_pq(struct stripe_head *sh, struct raid5_percpu *percpu
                            &sh->ops.zero_sum_result, percpu->spare_page, &submit);
 }
 
-static void raid_run_ops(struct stripe_head *sh, unsigned long ops_request)
+static void __raid_run_ops(struct stripe_head *sh, unsigned long ops_request)
 {
         int overlap_clear = 0, i, disks = sh->disks;
         struct dma_async_tx_descriptor *tx = NULL;
@@ -1204,6 +1204,36 @@ static void raid_run_ops(struct stripe_head *sh, unsigned long ops_request)
         put_cpu();
 }
 
+#ifdef CONFIG_MULTICORE_RAID456
+static void async_run_ops(void *param, async_cookie_t cookie)
+{
+        struct stripe_head *sh = param;
+        unsigned long ops_request = sh->ops.request;
+
+        clear_bit_unlock(STRIPE_OPS_REQ_PENDING, &sh->state);
+        wake_up(&sh->ops.wait_for_ops);
+
+        __raid_run_ops(sh, ops_request);
+        release_stripe(sh);
+}
+
+static void raid_run_ops(struct stripe_head *sh, unsigned long ops_request)
+{
+        /* since handle_stripe can be called outside of raid5d context
+         * we need to ensure sh->ops.request is de-staged before another
+         * request arrives
+         */
+        wait_event(sh->ops.wait_for_ops,
+                   !test_and_set_bit_lock(STRIPE_OPS_REQ_PENDING, &sh->state));
+        sh->ops.request = ops_request;
+
+        atomic_inc(&sh->count);
+        async_schedule(async_run_ops, sh);
+}
+#else
+#define raid_run_ops __raid_run_ops
+#endif
+
 static int grow_one_stripe(raid5_conf_t *conf)
 {
         struct stripe_head *sh;
@@ -1213,6 +1243,9 @@ static int grow_one_stripe(raid5_conf_t *conf)
         memset(sh, 0, sizeof(*sh) + (conf->raid_disks-1)*sizeof(struct r5dev));
         sh->raid_conf = conf;
         spin_lock_init(&sh->lock);
+        #ifdef CONFIG_MULTICORE_RAID456
+        init_waitqueue_head(&sh->ops.wait_for_ops);
+        #endif
 
         if (grow_buffers(sh, conf->raid_disks)) {
                 shrink_buffers(sh, conf->raid_disks);
@@ -1329,6 +1362,9 @@ static int resize_stripes(raid5_conf_t *conf, int newsize)
 
                 nsh->raid_conf = conf;
                 spin_lock_init(&nsh->lock);
+                #ifdef CONFIG_MULTICORE_RAID456
+                init_waitqueue_head(&nsh->ops.wait_for_ops);
+                #endif
 
                 list_add(&nsh->lru, &newstripes);
         }
@@ -4342,37 +4378,6 @@ static int  retry_aligned_read(raid5_conf_t *conf, struct bio *raid_bio)
         return handled;
 }
 
-#ifdef CONFIG_MULTICORE_RAID456
-static void __process_stripe(void *param, async_cookie_t cookie)
-{
-        struct stripe_head *sh = param;
-
-        handle_stripe(sh);
-        release_stripe(sh);
-}
-
-static void process_stripe(struct stripe_head *sh, struct list_head *domain)
-{
-        async_schedule_domain(__process_stripe, sh, domain);
-}
-
-static void synchronize_stripe_processing(struct list_head *domain)
-{
-        async_synchronize_full_domain(domain);
-}
-#else
-static void process_stripe(struct stripe_head *sh, struct list_head *domain)
-{
-        handle_stripe(sh);
-        release_stripe(sh);
-        cond_resched();
-}
-
-static void synchronize_stripe_processing(struct list_head *domain)
-{
-}
-#endif
-
 
 /*
  * This is our raid5 kernel thread.
@@ -4386,7 +4391,6 @@ static void raid5d(mddev_t *mddev)
         struct stripe_head *sh;
         raid5_conf_t *conf = mddev->private;
         int handled;
-        LIST_HEAD(raid_domain);
 
         pr_debug("+++ raid5d active\n");
 
@@ -4423,7 +4427,9 @@ static void raid5d(mddev_t *mddev)
                 spin_unlock_irq(&conf->device_lock);
                 
                 handled++;
-                process_stripe(sh, &raid_domain);
+                handle_stripe(sh);
+                release_stripe(sh);
+                cond_resched();
 
                 spin_lock_irq(&conf->device_lock);
         }
@@ -4431,7 +4437,6 @@ static void raid5d(mddev_t *mddev)
 
         spin_unlock_irq(&conf->device_lock);
 
-        synchronize_stripe_processing(&raid_domain);
         async_tx_issue_pending_all();
         unplug_slaves(mddev);