linux-kernel - Re: [PATCH BUGFIX V3] block, bfq: add requeue-request hook

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Date:   Wed, 7 Feb 2018 15:18:16 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Paolo Valente <paolo.valente@...aro.org>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        ulf.hansson@...aro.org, broonie@...nel.org,
        linus.walleij@...aro.org, bfq-iosched@...glegroups.com,
        oleksandr@...alenko.name, alban.browaeys@...il.com,
        ming.lei@...hat.com, ivan@...ios.org, 169364@...denti.unimore.it,
        holger@...lied-asynchrony.com, efault@....de,
        Serena Ziviani <ziviani.serena@...il.com>
Subject: Re: [PATCH BUGFIX V3] block, bfq: add requeue-request hook

On 2/7/18 2:19 PM, Paolo Valente wrote:
> Commit 'a6a252e64914 ("blk-mq-sched: decide how to handle flush rq via
> RQF_FLUSH_SEQ")' makes all non-flush re-prepared requests for a device
> be re-inserted into the active I/O scheduler for that device. As a
> consequence, I/O schedulers may get the same request inserted again,
> even several times, without a finish_request invoked on that request
> before each re-insertion.
> 
> This fact is the cause of the failure reported in [1]. For an I/O
> scheduler, every re-insertion of the same re-prepared request is
> equivalent to the insertion of a new request. For schedulers like
> mq-deadline or kyber, this fact causes no harm. In contrast, it
> confuses a stateful scheduler like BFQ, which keeps state for an I/O
> request, until the finish_request hook is invoked on the request. In
> particular, BFQ may get stuck, waiting forever for the number of
> request dispatches, of the same request, to be balanced by an equal
> number of request completions (while there will be one completion for
> that request). In this state, BFQ may refuse to serve I/O requests
> from other bfq_queues. The hang reported in [1] then follows.
> 
> However, the above re-prepared requests undergo a requeue, thus the
> requeue_request hook of the active elevator is invoked for these
> requests, if set. This commit then addresses the above issue by
> properly implementing the hook requeue_request in BFQ.

Thanks, applied.

-- 
Jens Axboe