From 9712c57ec1117d6b5fe1cc8ad420049171140b26 Mon Sep 17 00:00:00 2001 From: Caleb Sander Mateos Date: Fri, 25 Apr 2025 19:17:28 -0600 Subject: [PATCH] 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 Reviewed-by: Christoph Hellwig Link: https://lore.kernel.org/r/20250426011728.4189119-4-csander@purestorage.com [axboe: fix whitespace damage] Signed-off-by: Jens Axboe --- block/blk-mq.c | 49 ++++++++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 48 insertions(+), 1 deletion(-) diff --git a/block/blk-mq.c b/block/blk-mq.c index a777cb361ee3..796baeccd37b 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -2836,6 +2836,35 @@ static void __blk_mq_flush_list(struct request_queue *q, struct rq_list *rqs) q->mq_ops->queue_rqs(rqs); } +static unsigned blk_mq_extract_queue_requests(struct rq_list *rqs, + struct rq_list *queue_rqs) +{ + struct request *rq = rq_list_pop(rqs); + struct request_queue *this_q = rq->q; + struct request **prev = &rqs->head; + struct rq_list matched_rqs = {}; + struct request *last = NULL; + unsigned depth = 1; + + rq_list_add_tail(&matched_rqs, rq); + while ((rq = *prev)) { + if (rq->q == this_q) { + /* move rq from rqs to matched_rqs */ + *prev = rq->rq_next; + rq_list_add_tail(&matched_rqs, rq); + depth++; + } else { + /* leave rq in rqs */ + prev = &rq->rq_next; + last = rq; + } + } + + rqs->tail = last; + *queue_rqs = matched_rqs; + return depth; +} + static void blk_mq_dispatch_queue_requests(struct rq_list *rqs, unsigned depth) { struct request_queue *q = rq_list_peek(rqs)->q; @@ -2902,6 +2931,19 @@ static void blk_mq_dispatch_list(struct rq_list *rqs, bool 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; + + depth = blk_mq_extract_queue_requests(rqs, &queue_rqs); + 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; @@ -2918,7 +2960,12 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule) 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; -- 2.25.1