linux-kernel - Re: [PATCH V6 5/5] blk-mq-sched: don't dequeue request until all in ->dispatch are flushed

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Date:   Tue, 10 Oct 2017 11:26:48 -0700
From:   Omar Sandoval <osandov@...ndov.com>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Jens Axboe <axboe@...com>, linux-block@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>,
        Mike Snitzer <snitzer@...hat.com>, dm-devel@...hat.com,
        Bart Van Assche <bart.vanassche@...disk.com>,
        Laurence Oberman <loberman@...hat.com>,
        Paolo Valente <paolo.valente@...aro.org>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        Tom Nguyen <tom81094@...il.com>, linux-kernel@...r.kernel.org,
        linux-scsi@...r.kernel.org, Omar Sandoval <osandov@...com>
Subject: Re: [PATCH V6 5/5] blk-mq-sched: don't dequeue request until all in
 ->dispatch are flushed

On Mon, Oct 09, 2017 at 07:24:24PM +0800, Ming Lei wrote:
> During dispatching, we moved all requests from hctx->dispatch to
> one temporary list, then dispatch them one by one from this list.
> Unfortunately during this period, run queue from other contexts
> may think the queue is idle, then start to dequeue from sw/scheduler
> queue and still try to dispatch because ->dispatch is empty. This way
> hurts sequential I/O performance because requests are dequeued when
> lld queue is busy.
> 
> This patch introduces the state of BLK_MQ_S_DISPATCH_BUSY to
> make sure that request isn't dequeued until ->dispatch is
> flushed.
> 
> Reviewed-by: Bart Van Assche <bart.vanassche@....com>
> Reviewed-by: Christoph Hellwig <hch@....de>

I think this will do for now.

Reviewed-by: Omar Sandoval <osandov@...com>

> Signed-off-by: Ming Lei <ming.lei@...hat.com>
> ---
>  block/blk-mq-debugfs.c |  1 +
>  block/blk-mq-sched.c   | 38 ++++++++++++++++++++++++++++++++------
>  block/blk-mq.c         |  5 +++++
>  include/linux/blk-mq.h |  1 +
>  4 files changed, 39 insertions(+), 6 deletions(-)