From: "jianchao.wang" <email@example.com> To: Jens Axboe <firstname.lastname@example.org> Cc: email@example.com, firstname.lastname@example.org, email@example.com Subject: Re: [PATCH V6 3/5] blk-mq: ensure hctx to be ran on mapped cpu when issue directly Date: Wed, 14 Nov 2018 10:15:17 +0800 [thread overview] Message-ID: <firstname.lastname@example.org> (raw) In-Reply-To: <email@example.com> 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. Regarding with hot path, do you concern about the nvme device ? If so, how about split a standalone path for it ? Thanks Jianchao
next prev parent reply other threads:[~2018-11-14 2:15 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 [this message] 2018-11-14 3:02 ` Ming Lei 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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