linux-kernel - Re: [PATCH v2] blk-wbt: fix performance regression in wbt scale_up/scale

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Message-ID: <946f4a47-c4ad-0b2e-4a5b-d0873cbcb510@kernel.dk>
Date:   Sun, 6 Oct 2019 09:25:10 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Harshad Shirwadkar <harshadshirwadkar@...il.com>,
        linux-kernel@...r.kernel.org
Cc:     linux-block@...r.kernel.org, vaibhavrustagi@...gle.com,
        Josef Bacik <jbacik@...com>
Subject: Re: [PATCH v2] blk-wbt: fix performance regression in wbt
 scale_up/scale_down

On 10/5/19 12:59 PM, Harshad Shirwadkar wrote:
> scale_up wakes up waiters after scaling up. But after scaling max, it
> should not wake up more waiters as waiters will not have anything to
> do. This patch fixes this by making scale_up (and also scale_down)
> return when threshold is reached.
> 
> This bug causes increased fdatasync latency when fdatasync and dd
> conv=sync are performed in parallel on 4.19 compared to 4.14. This
> bug was introduced during refactoring of blk-wbt code.

Nice catch, thanks, applied.

-- 
Jens Axboe