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From: Ming Lei <ming.lei@redhat.com>
To: Jan Kara <jack@suse.cz>
Cc: Jens Axboe <axboe@kernel.dk>,
	Paolo Valente <paolo.valente@linaro.org>,
	linux-block@vger.kernel.org
Subject: Re: [PATCH] block: Do not pull requests from the scheduler when we cannot dispatch them
Date: Fri, 21 May 2021 21:18:53 +0800	[thread overview]
Message-ID: <YKezParMURhgPiLH@T590> (raw)
In-Reply-To: <20210521112016.GH18952@quack2.suse.cz>

On Fri, May 21, 2021 at 01:20:16PM +0200, Jan Kara wrote:
> On Fri 21-05-21 09:29:33, Ming Lei wrote:
> > On Thu, May 20, 2021 at 01:25:28PM +0200, Jan Kara wrote:
> > > Provided the device driver does not implement dispatch budget accounting
> > > (which only SCSI does) the loop in __blk_mq_do_dispatch_sched() pulls
> > > requests from the IO scheduler as long as it is willing to give out any.
> > > That defeats scheduling heuristics inside the scheduler by creating
> > > false impression that the device can take more IO when it in fact
> > > cannot.
> > 
> > So hctx->dispatch_busy isn't set as true in this case?
> 
> No. blk_mq_update_dispatch_busy() has:
> 
>         if (hctx->queue->elevator)
>                 return;

ooops, the above check should have been killed in commit 6e6fcbc27e77
("blk-mq: support batching dispatch in case of io"), :-(


Thanks,
Ming


  parent reply	other threads:[~2021-05-21 13:19 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-20 11:25 [PATCH] block: Do not pull requests from the scheduler when we cannot dispatch them Jan Kara
2021-05-21  1:29 ` Ming Lei
2021-05-21 11:20   ` Jan Kara
2021-05-21 11:27     ` Jan Kara
2021-05-21 13:18     ` Ming Lei [this message]
2021-06-02  9:25 ` Ming Lei
2021-06-03 10:45   ` Jan Kara

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