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Subject: Re: [PATCH V6 3/5] blk-mq: ensure hctx to be ran on mapped cpu when
 issue directly
To:     Jianchao Wang <jianchao.w.wang@oracle.com>
Cc:     ming.lei@redhat.com, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <1542103016-21037-1-git-send-email-jianchao.w.wang@oracle.com>
 <1542103016-21037-4-git-send-email-jianchao.w.wang@oracle.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <d8d88eff-e3eb-542d-d4fa-06b6aff97ba0@kernel.dk>
Date:   Tue, 13 Nov 2018 06:44:46 -0700
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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