linux-kernel - [PATCH 3/3] block: avoid hctx spinlock for plug with multiple queues

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Message-ID: <20250423205127.2976981-4-csander@purestorage.com>
Date: Wed, 23 Apr 2025 14:51:27 -0600
From: Caleb Sander Mateos <csander@...estorage.com>
To: Jens Axboe <axboe@...nel.dk>
Cc: linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Caleb Sander Mateos <csander@...estorage.com>
Subject: [PATCH 3/3] block: avoid hctx spinlock for plug with multiple queues

blk_mq_flush_plug_list() has a fast path if all requests in the plug
are destined for the same request_queue. It calls ->queue_rqs() with the
whole batch of requests, falling back on ->queue_rq() for any requests
not handled by ->queue_rqs(). However, if the requests are destined for
multiple queues, blk_mq_flush_plug_list() has a slow path that calls
blk_mq_dispatch_list() repeatedly to filter the requests by ctx/hctx.
Each queue's requests are inserted into the hctx's dispatch list under a
spinlock, then __blk_mq_sched_dispatch_requests() takes them out of the
dispatch list (taking the spinlock again), and finally
blk_mq_dispatch_rq_list() calls ->queue_rq() on each request.

Acquiring the hctx spinlock twice and calling ->queue_rq() instead of
->queue_rqs() makes the slow path significantly more expensive. Thus,
batching more requests into a single plug (e.g. io_uring_enter syscall)
can counterintuitively hurt performance by causing the plug to span
multiple queues. We have observed 2-3% of CPU time spent acquiring the
hctx spinlock alone on workloads issuing requests to multiple NVMe
devices in the same io_uring SQE batches.

Add a medium path in blk_mq_flush_plug_list() for plugs that don't have
elevators or come from a schedule, but do span multiple queues. Filter
the requests by queue and call ->queue_rqs()/->queue_rq() on the list of
requests destined to each request_queue.

With this change, we no longer see any CPU time spent in _raw_spin_lock
from blk_mq_flush_plug_list and throughput increases accordingly.

Signed-off-by: Caleb Sander Mateos <csander@...estorage.com>
---
 block/blk-mq.c | 45 ++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 44 insertions(+), 1 deletion(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index a777cb361ee3..f820c6c0cb1a 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2834,10 +2834,34 @@ static void __blk_mq_flush_list(struct request_queue *q, struct rq_list *rqs)
 	if (blk_queue_quiesced(q))
 		return;
 	q->mq_ops->queue_rqs(rqs);
 }
 
+static void blk_mq_extract_queue_requests(struct rq_list *rqs,
+					  struct rq_list *queue_rqs,
+					  unsigned *queue_depth)
+{
+	struct rq_list matched_rqs = {}, unmatched_rqs = {};
+	struct request *rq = rq_list_pop(rqs);
+	struct request_queue *this_q = rq->q;
+	unsigned depth = 1;
+
+	rq_list_add_tail(&matched_rqs, rq);
+	while ((rq = rq_list_pop(rqs))) {
+		if (rq->q == this_q) {
+			rq_list_add_tail(&matched_rqs, rq);
+			depth++;
+		} else {
+			rq_list_add_tail(&unmatched_rqs, rq);
+		}
+	}
+
+	*queue_rqs = matched_rqs;
+	*rqs = unmatched_rqs;
+	*queue_depth = depth;
+}
+
 static void blk_mq_dispatch_queue_requests(struct rq_list *rqs, unsigned depth)
 {
 	struct request_queue *q = rq_list_peek(rqs)->q;
 
 	trace_block_unplug(q, depth, true);
@@ -2900,10 +2924,24 @@ static void blk_mq_dispatch_list(struct rq_list *rqs, bool from_sched)
 		blk_mq_insert_requests(this_hctx, this_ctx, &list, from_sched);
 	}
 	percpu_ref_put(&this_hctx->queue->q_usage_counter);
 }
 
+static void blk_mq_dispatch_multiple_queue_requests(struct rq_list *rqs)
+{
+	do {
+		struct rq_list queue_rqs;
+		unsigned depth;
+
+		blk_mq_extract_queue_requests(rqs, &queue_rqs, &depth);
+		blk_mq_dispatch_queue_requests(&queue_rqs, depth);
+		while (!rq_list_empty(&queue_rqs)) {
+			blk_mq_dispatch_list(&queue_rqs, false);
+		}
+	} while (!rq_list_empty(rqs));
+}
+
 void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
 {
 	unsigned int depth;
 
 	/*
@@ -2916,11 +2954,16 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
 	if (plug->rq_count == 0)
 		return;
 	depth = plug->rq_count;
 	plug->rq_count = 0;
 
-	if (!plug->multiple_queues && !plug->has_elevator && !from_schedule) {
+	if (!plug->has_elevator && !from_schedule) {
+		if (plug->multiple_queues) {
+			blk_mq_dispatch_multiple_queue_requests(&plug->mq_list);
+			return;
+		}
+
 		blk_mq_dispatch_queue_requests(&plug->mq_list, depth);
 		if (rq_list_empty(&plug->mq_list))
 			return;
 	}
 
-- 
2.45.2