linux-kernel - [PATCH 09/14] blk-mq: ensure that plug lists don't straddle hardware queues

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Date:   Mon, 29 Oct 2018 10:37:33 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     linux-block@...r.kernel.org, linux-scsi@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Jens Axboe <axboe@...nel.dk>
Subject: [PATCH 09/14] blk-mq: ensure that plug lists don't straddle hardware queues

Since we insert per hardware queue, we have to ensure that every
request on the plug list being inserted belongs to the same
hardware queue.

Reviewed-by: Hannes Reinecke <hare@...e.com>
Signed-off-by: Jens Axboe <axboe@...nel.dk>
---
 block/blk-mq.c | 27 +++++++++++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 60a951c4934c..52b07188b39a 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1621,6 +1621,27 @@ static int plug_ctx_cmp(void *priv, struct list_head *a, struct list_head *b)
 		  blk_rq_pos(rqa) < blk_rq_pos(rqb)));
 }
 
+/*
+ * Need to ensure that the hardware queue matches, so we don't submit
+ * a list of requests that end up on different hardware queues.
+ */
+static bool ctx_match(struct request *req, struct blk_mq_ctx *ctx,
+		      unsigned int flags)
+{
+	if (req->mq_ctx != ctx)
+		return false;
+
+	/*
+	 * If we just have one map, then we know the hctx will match
+	 * if the ctx matches
+	 */
+	if (req->q->tag_set->nr_maps == 1)
+		return true;
+
+	return blk_mq_map_queue(req->q, req->cmd_flags, ctx->cpu) ==
+		blk_mq_map_queue(req->q, flags, ctx->cpu);
+}
+
 void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
 {
 	struct blk_mq_ctx *this_ctx;
@@ -1628,7 +1649,7 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
 	struct request *rq;
 	LIST_HEAD(list);
 	LIST_HEAD(ctx_list);
-	unsigned int depth;
+	unsigned int depth, this_flags;
 
 	list_splice_init(&plug->mq_list, &list);
 
@@ -1636,13 +1657,14 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
 
 	this_q = NULL;
 	this_ctx = NULL;
+	this_flags = 0;
 	depth = 0;
 
 	while (!list_empty(&list)) {
 		rq = list_entry_rq(list.next);
 		list_del_init(&rq->queuelist);
 		BUG_ON(!rq->q);
-		if (rq->mq_ctx != this_ctx) {
+		if (!ctx_match(rq, this_ctx, this_flags)) {
 			if (this_ctx) {
 				trace_block_unplug(this_q, depth, !from_schedule);
 				blk_mq_sched_insert_requests(this_q, this_ctx,
@@ -1650,6 +1672,7 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
 								from_schedule);
 			}
 
+			this_flags = rq->cmd_flags;
 			this_ctx = rq->mq_ctx;
 			this_q = rq->q;
 			depth = 0;
-- 
2.17.1