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From: Jens Axboe <>
To: Jianchao Wang <>
Subject: Re: [PATCH V6 3/5] blk-mq: ensure hctx to be ran on mapped cpu when issue directly
Date: Tue, 13 Nov 2018 06:44:46 -0700	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

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?

Jens Axboe

  reply	other threads:[~2018-11-13 13:44 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 [this message]
2018-11-14  2:15     `
2018-11-14  3:02       ` Ming Lei
2018-11-14  3:38         `
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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