From: "jianchao.wang" <email@example.com> To: Ming Lei <firstname.lastname@example.org> Cc: Jens Axboe <email@example.com>, Ming Lei <firstname.lastname@example.org>, linux-block <email@example.com>, Linux Kernel Mailing List <firstname.lastname@example.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 11:38:16 +0800 [thread overview] Message-ID: <email@example.com> (raw) In-Reply-To: <CACVXFVNZ1p4wV6S_FRyKssbUT9CV-aXZ=iRGQ-tY05v_7vxPnw@mail.gmail.com> On 11/14/18 11:02 AM, Ming Lei wrote: > On Wed, Nov 14, 2018 at 10:15 AM jianchao.wang > <firstname.lastname@example.org> 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. > Yes, it is indeed the case. Looks like this patch is contentious. I will drop this one and post later as a standalone one if necessary. Thanks Jianchao
next prev parent reply other threads:[~2018-11-14 3:38 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 2018-11-14 3:38 ` jianchao.wang [this message] 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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