[PATCH] mq-deadline: Fix request accounting

[Bart Van Assche <bvanassche@acm.org>]

The block layer may call the I/O scheduler .finish_request() callback
without having called the .insert_requests() callback. Make sure that the
mq-deadline I/O statistics are correct if the block layer inserts an I/O
request that bypasses the I/O scheduler. This patch prevents that lower
priority I/O is delayed longer than necessary for mixed I/O priority
workloads.

Cc: Niklas Cassel <Niklas.Cassel@wdc.com>
Cc: Damien Le Moal <damien.lemoal@wdc.com>
Cc: Hannes Reinecke <hare@suse.de>
Reported-by: Niklas Cassel <Niklas.Cassel@wdc.com>
Fixes: 08a9ad8bf607 ("block/mq-deadline: Add cgroup support")
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 block/mq-deadline.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/block/mq-deadline.c b/block/mq-deadline.c
index dbd74bae9f11..28f2e7655a5f 100644
--- a/block/mq-deadline.c
+++ b/block/mq-deadline.c
@@ -713,6 +713,7 @@ static void dd_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
 
 	prio = ioprio_class_to_prio[ioprio_class];
 	dd_count(dd, inserted, prio);
+	rq->elv.priv[0] = (void *)(uintptr_t)1;
 
 	if (blk_mq_sched_try_insert_merge(q, rq, &free)) {
 		blk_mq_free_requests(&free);
@@ -761,12 +762,10 @@ static void dd_insert_requests(struct blk_mq_hw_ctx *hctx,
 	spin_unlock(&dd->lock);
 }
 
-/*
- * Nothing to do here. This is defined only to ensure that .finish_request
- * method is called upon request completion.
- */
+/* Callback from inside blk_mq_rq_ctx_init(). */
 static void dd_prepare_request(struct request *rq)
 {
+	rq->elv.priv[0] = NULL;
 }
 
 /*
@@ -793,7 +792,14 @@ static void dd_finish_request(struct request *rq)
 	const enum dd_prio prio = ioprio_class_to_prio[ioprio_class];
 	struct dd_per_prio *per_prio = &dd->per_prio[prio];
 
-	dd_count(dd, completed, prio);
+	/*
+	 * The block layer core may call dd_finish_request() without having
+	 * called dd_insert_requests(). Hence only update statistics for
+	 * requests for which dd_insert_requests() has been called. See also
+	 * blk_mq_request_bypass_insert().
+	 */
+	if (rq->elv.priv[0])
+		dd_count(dd, completed, prio);
 
 	if (blk_queue_is_zoned(q)) {
 		unsigned long flags;