blk-mq: improve tag waiting setup for non-shared tags

If we run out of driver tags, we currently treat shared and non-shared
tags the same - both cases hook into the tag waitqueue. This is a bit
more costly than it needs to be on unshared tags, since we have to both
grab the hctx lock, and the waitqueue lock (and disable interrupts).
For the non-shared case, we can simply mark the queue as needing a
restart.

Split blk_mq_dispatch_wait_add() to account for both cases, and
rename it to blk_mq_mark_tag_wait() to better reflect what it
does now.

Without this patch, shared and non-shared performance is about the same
with 4 fio thread hammering on a single null_blk device (~410K, at 75%
sys). With the patch, the shared case is the same, but the non-shared
tags case runs at 431K at 71% sys.
Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq.c

@@ -1006,13 +1006,26 @@ static int blk_mq_dispatch_wake(wait_queue_entry_t *wait, unsigned mode,
 	return 1;
 }
 
-static bool blk_mq_dispatch_wait_add(struct blk_mq_hw_ctx **hctx,
+/*
+ * Mark us waiting for a tag. For shared tags, this involves hooking us into
+ * the tag wakeups. For non-shared tags, we can simply mark us nedeing a
+ * restart. For both caes, take care to check the condition again after
+ * marking us as waiting.
+ */
+static bool blk_mq_mark_tag_wait(struct blk_mq_hw_ctx **hctx,
 				 struct request *rq)
 {
 	struct blk_mq_hw_ctx *this_hctx = *hctx;
-	wait_queue_entry_t *wait = &this_hctx->dispatch_wait;
+	bool shared_tags = (this_hctx->flags & BLK_MQ_F_TAG_SHARED) != 0;
 	struct sbq_wait_state *ws;
+	wait_queue_entry_t *wait;
+	bool ret;
 
+	if (!shared_tags) {
+		if (!test_bit(BLK_MQ_S_SCHED_RESTART, &this_hctx->state))
+			set_bit(BLK_MQ_S_SCHED_RESTART, &this_hctx->state);
+	} else {
+		wait = &this_hctx->dispatch_wait;
 		if (!list_empty_careful(&wait->entry))
 			return false;
 
@@ -1024,13 +1037,23 @@ static bool blk_mq_dispatch_wait_add(struct blk_mq_hw_ctx **hctx,
 
 		ws = bt_wait_ptr(&this_hctx->tags->bitmap_tags, this_hctx);
 		add_wait_queue(&ws->wait, wait);
+	}
 
 	/*
 	 * It's possible that a tag was freed in the window between the
 	 * allocation failure and adding the hardware queue to the wait
 	 * queue.
 	 */
-	if (!blk_mq_get_driver_tag(rq, hctx, false)) {
+	ret = blk_mq_get_driver_tag(rq, hctx, false);
+
+	if (!shared_tags) {
+		/*
+		 * Don't clear RESTART here, someone else could have set it.
+		 * At most this will cost an extra queue run.
+		 */
+		return ret;
+	} else {
+		if (!ret) {
 			spin_unlock(&this_hctx->lock);
 			return false;
 		}
@@ -1044,6 +1067,7 @@ static bool blk_mq_dispatch_wait_add(struct blk_mq_hw_ctx **hctx,
 		spin_unlock_irq(&ws->wait.lock);
 		spin_unlock(&this_hctx->lock);
 		return true;
+	}
 }
 
 bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
@@ -1076,9 +1100,14 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
 			 * before we add this entry back on the dispatch list,
 			 * we'll re-run it below.
 			 */
-			if (!blk_mq_dispatch_wait_add(&hctx, rq)) {
+			if (!blk_mq_mark_tag_wait(&hctx, rq)) {
 				if (got_budget)
 					blk_mq_put_dispatch_budget(hctx);
+				/*
+				 * For non-shared tags, the RESTART check
+				 * will suffice.
+				 */
+				if (hctx->flags & BLK_MQ_F_TAG_SHARED)
 					no_tag = true;
 				break;
 			}