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From: Ming Lei <tom.leiming@gmail.com>
To: "jianchao.wang" <jianchao.w.wang@oracle.com>
Cc: Jens Axboe <axboe@kernel.dk>, Ming Lei <ming.lei@redhat.com>,
	linux-block <linux-block@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH V6 3/5] blk-mq: ensure hctx to be ran on mapped cpu when issue directly
Date: Wed, 14 Nov 2018 11:02:59 +0800	[thread overview]
Message-ID: <CACVXFVNZ1p4wV6S_FRyKssbUT9CV-aXZ=iRGQ-tY05v_7vxPnw@mail.gmail.com> (raw)
In-Reply-To: <94ec3d97-f75f-645d-94f1-24d3fd476940@oracle.com>

On Wed, Nov 14, 2018 at 10:15 AM jianchao.wang
<jianchao.w.wang@oracle.com> wrote:
>
> Hi Jens
>
> Thanks for your kindly response.
>
> On 11/13/18 9:44 PM, Jens Axboe wrote:
> > On 11/13/18 2:56 AM, Jianchao Wang wrote:
> >> When issue request directly and the task is migrated out of the
> >> original cpu where it allocates request, hctx could be ran on
> >> the cpu where it is not mapped.
> >> To fix this,
> >>  - insert the request forcibly if BLK_MQ_F_BLOCKING is set.
> >>  - check whether the current is mapped to the hctx, if not, insert
> >>    forcibly.
> >>  - invoke __blk_mq_issue_directly under preemption disabled.
> >
> > I'm not too crazy about this one, adding a get/put_cpu() in the hot
> > path, and a cpumask test. The fact is that most/no drivers care
> > about strict placement. We always try to do so, if convenient,
> > since it's faster, but this seems to be doing the opposite.
> >
> > I'd be more inclined to have a driver flag if it needs guaranteed
> > placement, using one an ops BLK_MQ_F_STRICT_CPU flag or similar.
> >
> > What do you think?
> >
>
> I'd inclined blk-mq should comply with a unified rule, no matter the
> issuing directly path or inserting one. Then blk-mq would have a simpler
> model. And also this guarantee could be a little good for drivers,
> especially the case where cpu and hw queue mapping is 1:1.

I guess it is quite hard to respect this rule 100%, such as in case of
CPU hotplug.

Thanks,
Ming Lei

  reply	other threads:[~2018-11-14  3:03 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-13  9:56 [PATCH V6 0/5] blk-mq: refactor and fix on issue request directly Jianchao Wang
2018-11-13  9:56 ` [PATCH V6 1/5] blk-mq: refactor the code of " Jianchao Wang
2018-11-13  9:56 ` [PATCH V6 2/5] blk-mq: fix issue directly case when q is stopped or quiesced Jianchao Wang
2018-11-13  9:56 ` [PATCH V6 3/5] blk-mq: ensure hctx to be ran on mapped cpu when issue directly Jianchao Wang
2018-11-13 13:44   ` Jens Axboe
2018-11-14  2:15     ` jianchao.wang
2018-11-14  3:02       ` Ming Lei [this message]
2018-11-14  3:38         ` jianchao.wang
2018-11-13  9:56 ` [PATCH V6 4/5] blk-mq: issue directly with bypass 'false' in blk_mq_sched_insert_requests Jianchao Wang
2018-11-13  9:56 ` [PATCH V6 5/5] blk-mq: replace and kill blk_mq_request_issue_directly Jianchao Wang

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