linux-kernel - Re: [PATCH 1/2] blk-mq: bypass IO scheduler's limit

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Message-ID: <c8c572f3-ac49-5a9b-0ca0-6ec150bdb27c@kernel.dk>
Date:   Fri, 16 Apr 2021 06:09:18 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Lin Feng <linf@...gsu.com>
Cc:     linux-block@...r.kernel.org, ming.lei@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] blk-mq: bypass IO scheduler's limit_depth for
 passthrough request

On 4/14/21 9:39 PM, Lin Feng wrote:
> Commit 01e99aeca39796003 ("blk-mq: insert passthrough request into
> hctx->dispatch directly") gives high priority to passthrough requests and
> bypass underlying IO scheduler. But as we allocate tag for such request it
> still runs io-scheduler's callback limit_depth, while we really want is to
> give full sbitmap-depth capabity to such request for acquiring available
> tag.
> blktrace shows PC requests(dmraid -s -c -i) hit bfq's limit_depth:
>   8,0    2        0     0.000000000 39952 1,0  m   N bfq [bfq_limit_depth] wr_busy 0 sync 0 depth 8
>   8,0    2        1     0.000008134 39952  D   R 4 [dmraid]
>   8,0    2        2     0.000021538    24  C   R [0]
>   8,0    2        0     0.000035442 39952 1,0  m   N bfq [bfq_limit_depth] wr_busy 0 sync 0 depth 8
>   8,0    2        3     0.000038813 39952  D   R 24 [dmraid]
>   8,0    2        4     0.000044356    24  C   R [0]
> 
> This patch introduce a new wrapper to make code not that ugly.

Applied, thanks.

-- 
Jens Axboe