linux-kernel - [PATCH 5.15 177/179] block: avoid to quiesce queue in elevator_init

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Message-Id: <20211129181724.761570426@linuxfoundation.org>
Date:   Mon, 29 Nov 2021 19:19:31 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, yangerkun <yangerkun@...wei.com>,
        Damien Le Moal <damien.lemoal@....com>,
        Ming Lei <ming.lei@...hat.com>, Jens Axboe <axboe@...nel.dk>
Subject: [PATCH 5.15 177/179] block: avoid to quiesce queue in elevator_init_mq

From: Ming Lei <ming.lei@...hat.com>

commit 245a489e81e13dd55ae46d27becf6d5901eb7828 upstream.

elevator_init_mq() is only called before adding disk, when there isn't
any FS I/O, only passthrough requests can be queued, so freezing queue
plus canceling dispatch work is enough to drain any dispatch activities,
then we can avoid synchronize_srcu() in blk_mq_quiesce_queue().

Long boot latency issue can be fixed in case of lots of disks added
during booting.

Fixes: 737eb78e82d5 ("block: Delay default elevator initialization")
Reported-by: yangerkun <yangerkun@...wei.com>
Cc: Damien Le Moal <damien.lemoal@....com>
Signed-off-by: Ming Lei <ming.lei@...hat.com>
Link: https://lore.kernel.org/r/20211117115502.1600950-1-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@...nel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 block/elevator.c |   10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

--- a/block/elevator.c
+++ b/block/elevator.c
@@ -694,12 +694,18 @@ void elevator_init_mq(struct request_que
 	if (!e)
 		return;
 
+	/*
+	 * We are called before adding disk, when there isn't any FS I/O,
+	 * so freezing queue plus canceling dispatch work is enough to
+	 * drain any dispatch activities originated from passthrough
+	 * requests, then no need to quiesce queue which may add long boot
+	 * latency, especially when lots of disks are involved.
+	 */
 	blk_mq_freeze_queue(q);
-	blk_mq_quiesce_queue(q);
+	blk_mq_cancel_work_sync(q);
 
 	err = blk_mq_init_sched(q, e);
 
-	blk_mq_unquiesce_queue(q);
 	blk_mq_unfreeze_queue(q);
 
 	if (err) {