blk-mq: fix performance regression with shared tags

If we have shared tags enabled, then every IO completion will trigger
a full loop of every queue belonging to a tag set, and every hardware
queue for each of those queues, even if nothing needs to be done.
This causes a massive performance regression if you have a lot of
shared devices.

Instead of doing this huge full scan on every IO, add an atomic
counter to the main queue that tracks how many hardware queues have
been marked as needing a restart. With that, we can avoid looking for
restartable queues, if we don't have to.

Max reports that this restores performance. Before this patch, 4K
IOPS was limited to 22-23K IOPS. With the patch, we are running at
950-970K IOPS.

Fixes: 6d8c6c0f97ad ("blk-mq: Restart a single queue if tag sets are shared")
Reported-by: Max Gurtovoy <maxg@mellanox.com>
Tested-by: Max Gurtovoy <maxg@mellanox.com>
Reviewed-by: Bart Van Assche <bart.vanassche@sandisk.com>
Tested-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 46 insertions, 12 deletions

diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index 1f5b692526ae..0ded5e846335 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -68,6 +68,45 @@ static void blk_mq_sched_assign_ioc(struct request_queue *q,
                 __blk_mq_sched_assign_ioc(q, rq, bio, ioc);
 }
 
+/*
+ * Mark a hardware queue as needing a restart. For shared queues, maintain
+ * a count of how many hardware queues are marked for restart.
+ */
+static void blk_mq_sched_mark_restart_hctx(struct blk_mq_hw_ctx *hctx)
+{
+        if (test_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state))
+                return;
+
+        if (hctx->flags & BLK_MQ_F_TAG_SHARED) {
+                struct request_queue *q = hctx->queue;
+
+                if (!test_and_set_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state))
+                        atomic_inc(&q->shared_hctx_restart);
+        } else
+                set_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state);
+}
+
+static bool blk_mq_sched_restart_hctx(struct blk_mq_hw_ctx *hctx)
+{
+        if (!test_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state))
+                return false;
+
+        if (hctx->flags & BLK_MQ_F_TAG_SHARED) {
+                struct request_queue *q = hctx->queue;
+
+                if (test_and_clear_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state))
+                        atomic_dec(&q->shared_hctx_restart);
+        } else
+                clear_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state);
+
+        if (blk_mq_hctx_has_pending(hctx)) {
+                blk_mq_run_hw_queue(hctx, true);
+                return true;
+        }
+
+        return false;
+}
+
 struct request *blk_mq_sched_get_request(struct request_queue *q,
                                          struct bio *bio,
                                          unsigned int op,
@@ -266,18 +305,6 @@ static bool blk_mq_sched_bypass_insert(struct blk_mq_hw_ctx *hctx,
         return true;
 }
 
-static bool blk_mq_sched_restart_hctx(struct blk_mq_hw_ctx *hctx)
-{
-        if (test_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state)) {
-                clear_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state);
-                if (blk_mq_hctx_has_pending(hctx)) {
-                        blk_mq_run_hw_queue(hctx, true);
-                        return true;
-                }
-        }
-        return false;
-}
-
 /**
  * list_for_each_entry_rcu_rr - iterate in a round-robin fashion over rcu list
  * @pos:    loop cursor.
@@ -309,6 +336,13 @@ void blk_mq_sched_restart(struct blk_mq_hw_ctx *const hctx)
         unsigned int i, j;
 
         if (set->flags & BLK_MQ_F_TAG_SHARED) {
+                /*
+                 * If this is 0, then we know that no hardware queues
+                 * have RESTART marked. We're done.
+                 */
+                if (!atomic_read(&queue->shared_hctx_restart))
+                        return;
+
                 rcu_read_lock();
                 list_for_each_entry_rcu_rr(q, queue, &set->tag_list,
                                            tag_set_list) {