linux-kernel - [RFC PATCH 1/2] blk-mq: don't call callbacks for requests that bypassed the scheduler

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Date:   Fri, 27 Aug 2021 12:41:31 +0000
From:   Niklas Cassel <Niklas.Cassel@....com>
To:     Jens Axboe <axboe@...nel.dk>
CC:     Bart Van Assche <bvanassche@....org>,
        Damien Le Moal <Damien.LeMoal@....com>,
        Paolo Valente <paolo.valente@...aro.org>,
        Niklas Cassel <Niklas.Cassel@....com>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: [RFC PATCH 1/2] blk-mq: don't call callbacks for requests that
 bypassed the scheduler

From: Niklas Cassel <niklas.cassel@....com>

Currently, __blk_mq_alloc_request() calls ops.prepare_request and sets
RQF_ELVPRIV.

Therefore, (if the request is not a flush) the RQF_ELVPRIV flag will be
set for the request in blk_mq_submit_bio(), regardless if the request
was submitted to a scheduler, or bypassed the scheduler.

Later, blk_mq_free_request() checks if the RQF_ELVPRIV flag is set,
if it is, the ops.finish_request callback will be called.

The problem with this is that the finish_request scheduler callback
will be called for requests that bypassed the scheduler.

Fix this by calling the scheduler ops.prepare_request callback, and
set the RQF_ELVPRIV flag only immediately before calling the insert
callback.

This way, we can reuse the flag, and we don't need to add any
additional checks in blk_mq_sched_requeue_request() and
blk_mq_free_request(), since they already only do the callback
if RQF_ELVPRIV is set, and the existing .prepare_request callbacks
should still work without needing modifications.

Signed-off-by: Niklas Cassel <niklas.cassel@....com>
---
 block/blk-mq-sched.c   | 20 ++++++++++++++++++++
 block/blk-mq.c         | 13 -------------
 include/linux/blkdev.h |  3 ++-
 3 files changed, 22 insertions(+), 14 deletions(-)

diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index 0f006cabfd91..eacacb7088c1 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -466,6 +466,14 @@ void blk_mq_sched_insert_request(struct request *rq, bool at_head,
 	if (e) {
 		LIST_HEAD(list);
 
+		rq->elv.icq = NULL;
+		if (e && e->type->ops.prepare_request) {
+			if (e->type->icq_cache)
+				blk_mq_sched_assign_ioc(rq);
+
+			e->type->ops.prepare_request(rq);
+			rq->rq_flags |= RQF_ELVPRIV;
+		}
 		list_add(&rq->queuelist, &list);
 		e->type->ops.insert_requests(hctx, &list, at_head);
 	} else {
@@ -495,6 +503,18 @@ void blk_mq_sched_insert_requests(struct blk_mq_hw_ctx *hctx,
 
 	e = hctx->queue->elevator;
 	if (e) {
+		struct request *rq;
+
+		list_for_each_entry(rq, list, queuelist) {
+			rq->elv.icq = NULL;
+			if (e && e->type->ops.prepare_request) {
+				if (e->type->icq_cache)
+					blk_mq_sched_assign_ioc(rq);
+
+				e->type->ops.prepare_request(rq);
+				rq->rq_flags |= RQF_ELVPRIV;
+			}
+		}
 		e->type->ops.insert_requests(hctx, list, false);
 	} else {
 		/*
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 9d4fdc2be88a..3527dd9fd10e 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -328,19 +328,6 @@ static struct request *blk_mq_rq_ctx_init(struct blk_mq_alloc_data *data,
 	data->ctx->rq_dispatched[op_is_sync(data->cmd_flags)]++;
 	refcount_set(&rq->ref, 1);
 
-	if (!op_is_flush(data->cmd_flags)) {
-		struct elevator_queue *e = data->q->elevator;
-
-		rq->elv.icq = NULL;
-		if (e && e->type->ops.prepare_request) {
-			if (e->type->icq_cache)
-				blk_mq_sched_assign_ioc(rq);
-
-			e->type->ops.prepare_request(rq);
-			rq->rq_flags |= RQF_ELVPRIV;
-		}
-	}
-
 	data->hctx->queued++;
 	return rq;
 }
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 2e12320cb121..a5047c7e9448 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -81,7 +81,8 @@ typedef __u32 __bitwise req_flags_t;
 #define RQF_FAILED		((__force req_flags_t)(1 << 10))
 /* don't warn about errors */
 #define RQF_QUIET		((__force req_flags_t)(1 << 11))
-/* elevator private data attached */
+/* The request has been inserted to an elevator, and thus has private
+   data attached */
 #define RQF_ELVPRIV		((__force req_flags_t)(1 << 12))
 /* account into disk and partition IO statistics */
 #define RQF_IO_STAT		((__force req_flags_t)(1 << 13))
-- 
2.31.1