From: Mike Snitzer <snitzer@redhat.com>
To: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: axboe@kernel.dk, device-mapper development <dm-devel@redhat.com>,
hch@lst.de, linux-block@vger.kernel.org
Subject: should blk-mq halt requeue processing while queue is frozen? [was: Re: [PATCH 8/9] dm: Fix two race conditions related to stopping and starting queues]
Date: Fri, 2 Sep 2016 12:10:59 -0400 [thread overview]
Message-ID: <20160902161059.GB17508@redhat.com> (raw)
In-Reply-To: <20160902151213.GA17508@redhat.com>
On Fri, Sep 02 2016 at 11:12am -0400,
Mike Snitzer <snitzer@redhat.com> wrote:
> So in the case of blk-mq request-based DM: we cannot expect
> blk_mq_freeze_queue(), during suspend, to complete if requests are
> getting requeued to the blk-mq queue via BLK_MQ_RQ_QUEUE_BUSY.
Looking closer at blk-mq. Currently __blk_mq_run_hw_queue() will move
any requeued requests to the hctx->dispatch list and then performs async
blk_mq_run_hw_queue().
To do what you hoped (have blk_mq_freeze_queue() discontinue all use of
blk-mq hw queues during DM suspend) I think we'd need blk-mq to:
1) avoid processing requeued IO if blk_mq_freeze_queue() was used to
freeze the queue. Meaning it'd have to hold requeued work longer
than it currently does.
2) Then once blk_mq_unfreeze_queue() is called it'd allow requeues to
proceed.
This would be catering to a very specific requirement of DM (given it
re-queues IO back to the request_queue during suspend).
BUT all said, relative to request-based DM multipath, what we have is
perfectly fine on a correctness level: the requests are re-queued
because the blk-mq DM device is suspended.
Unfortunately on an efficiency level DM suspend creates a lot of busy
looping in blk-mq, with 100% cpu usage in a threads with names
"kworker/3:1H", ideally we'd avoid that!
next prev parent reply other threads:[~2016-09-02 16:10 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-08-31 22:14 [PATCH 0/9] dm patches for kernel v4.9 Bart Van Assche
2016-08-31 22:15 ` [PATCH 1/9] blk-mq: Introduce blk_mq_queue_stopped() Bart Van Assche
2016-08-31 22:16 ` [PATCH 2/9] dm: Rename a function argument Bart Van Assche
2016-09-01 3:29 ` Mike Snitzer
2016-09-01 14:17 ` Bart Van Assche
2016-08-31 22:16 ` [PATCH 3/9] dm: Introduce signal_pending_state() Bart Van Assche
2016-08-31 22:16 ` [PATCH 4/9] dm: Convert wait loops Bart Van Assche
2016-08-31 22:17 ` [PATCH 5/9] dm: Add two lockdep_assert_held() statements Bart Van Assche
2016-08-31 22:17 ` [PATCH 6/9] dm: Simplify dm_old_stop_queue() Bart Van Assche
2016-08-31 22:17 ` [PATCH 7/9] dm: Mark block layer queue dead before destroying the dm device Bart Van Assche
2016-08-31 22:18 ` [PATCH 8/9] dm: Fix two race conditions related to stopping and starting queues Bart Van Assche
2016-09-01 3:13 ` Mike Snitzer
2016-09-01 14:23 ` Bart Van Assche
2016-09-01 15:05 ` Mike Snitzer
2016-09-01 15:31 ` Bart Van Assche
2016-09-01 15:50 ` Mike Snitzer
2016-09-01 16:12 ` Mike Snitzer
2016-09-01 17:59 ` Bart Van Assche
2016-09-01 19:05 ` Mike Snitzer
2016-09-01 19:35 ` Mike Snitzer
2016-09-01 20:15 ` Bart Van Assche
2016-09-01 20:33 ` Mike Snitzer
2016-09-01 20:39 ` Bart Van Assche
2016-09-01 20:48 ` Mike Snitzer
2016-09-01 20:52 ` Bart Van Assche
2016-09-01 21:17 ` Mike Snitzer
2016-09-01 22:18 ` Mike Snitzer
2016-09-01 22:22 ` Bart Van Assche
2016-09-01 22:26 ` Mike Snitzer
2016-09-01 23:17 ` Bart Van Assche
2016-09-01 23:47 ` Mike Snitzer
2016-09-02 0:03 ` Bart Van Assche
2016-09-02 15:12 ` Mike Snitzer
2016-09-02 16:10 ` Mike Snitzer [this message]
2016-09-02 22:42 ` [dm-devel] should blk-mq halt requeue processing while queue is frozen? Bart Van Assche
2016-09-03 0:34 ` Mike Snitzer
2016-09-07 16:41 ` Mike Snitzer
2016-09-13 8:01 ` [dm-devel] " Bart Van Assche
2016-09-13 14:36 ` Mike Snitzer
2016-08-31 22:18 ` [PATCH 9/9] dm path selector: Avoid that device removal triggers an infinite loop Bart Van Assche
2016-09-01 2:49 ` Mike Snitzer
2016-09-01 14:14 ` Bart Van Assche
2016-09-01 15:06 ` Mike Snitzer
2016-09-01 15:22 ` Bart Van Assche
2016-09-01 15:26 ` Mike Snitzer
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