linux-kernel - [PATCH 5.17 0011/1126] block: flush plug based on hardware and software queue order

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Date:   Tue,  5 Apr 2022 09:12:38 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Song Liu <songliubraving@...com>,
        Jens Axboe <axboe@...nel.dk>, Song Liu <song@...nel.org>
Subject: [PATCH 5.17 0011/1126] block: flush plug based on hardware and software queue order

From: Jens Axboe <axboe@...nel.dk>

commit 26fed4ac4eab09c27fbae1859696cc38f0536407 upstream.

We used to sort the plug list if we had multiple queues before dispatching
requests to the IO scheduler. This usually isn't needed, but for certain
workloads that interleave requests to disks, it's a less efficient to
process the plug list one-by-one if everything is interleaved.

Don't sort the list, but skip through it and flush out entries that have
the same target at the same time.

Fixes: df87eb0fce8f ("block: get rid of plug list sorting")
Reported-and-tested-by: Song Liu <song@...nel.org>
Reviewed-by: Song Liu <songliubraving@...com>
Signed-off-by: Jens Axboe <axboe@...nel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 block/blk-mq.c |   59 +++++++++++++++++++++++++++------------------------------
 1 file changed, 28 insertions(+), 31 deletions(-)

--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2561,13 +2561,36 @@ static void __blk_mq_flush_plug_list(str
 	q->mq_ops->queue_rqs(&plug->mq_list);
 }
 
+static void blk_mq_dispatch_plug_list(struct blk_plug *plug, bool from_sched)
+{
+	struct blk_mq_hw_ctx *this_hctx = NULL;
+	struct blk_mq_ctx *this_ctx = NULL;
+	struct request *requeue_list = NULL;
+	unsigned int depth = 0;
+	LIST_HEAD(list);
+
+	do {
+		struct request *rq = rq_list_pop(&plug->mq_list);
+
+		if (!this_hctx) {
+			this_hctx = rq->mq_hctx;
+			this_ctx = rq->mq_ctx;
+		} else if (this_hctx != rq->mq_hctx || this_ctx != rq->mq_ctx) {
+			rq_list_add(&requeue_list, rq);
+			continue;
+		}
+		list_add_tail(&rq->queuelist, &list);
+		depth++;
+	} while (!rq_list_empty(plug->mq_list));
+
+	plug->mq_list = requeue_list;
+	trace_block_unplug(this_hctx->queue, depth, !from_sched);
+	blk_mq_sched_insert_requests(this_hctx, this_ctx, &list, from_sched);
+}
+
 void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
 {
-	struct blk_mq_hw_ctx *this_hctx;
-	struct blk_mq_ctx *this_ctx;
 	struct request *rq;
-	unsigned int depth;
-	LIST_HEAD(list);
 
 	if (rq_list_empty(plug->mq_list))
 		return;
@@ -2603,35 +2626,9 @@ void blk_mq_flush_plug_list(struct blk_p
 			return;
 	}
 
-	this_hctx = NULL;
-	this_ctx = NULL;
-	depth = 0;
 	do {
-		rq = rq_list_pop(&plug->mq_list);
-
-		if (!this_hctx) {
-			this_hctx = rq->mq_hctx;
-			this_ctx = rq->mq_ctx;
-		} else if (this_hctx != rq->mq_hctx || this_ctx != rq->mq_ctx) {
-			trace_block_unplug(this_hctx->queue, depth,
-						!from_schedule);
-			blk_mq_sched_insert_requests(this_hctx, this_ctx,
-						&list, from_schedule);
-			depth = 0;
-			this_hctx = rq->mq_hctx;
-			this_ctx = rq->mq_ctx;
-
-		}
-
-		list_add(&rq->queuelist, &list);
-		depth++;
+		blk_mq_dispatch_plug_list(plug, from_schedule);
 	} while (!rq_list_empty(plug->mq_list));
-
-	if (!list_empty(&list)) {
-		trace_block_unplug(this_hctx->queue, depth, !from_schedule);
-		blk_mq_sched_insert_requests(this_hctx, this_ctx, &list,
-						from_schedule);
-	}
 }
 
 void blk_mq_try_issue_list_directly(struct blk_mq_hw_ctx *hctx,