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

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Date:   Thu, 8 Feb 2018 08:16:51 +0100
From:   Paolo Valente <paolo.valente@...aro.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     linux-block <linux-block@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Mark Brown <broonie@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        'Paolo Valente' via bfq-iosched 
        <bfq-iosched@...glegroups.com>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        Alban Browaeys <alban.browaeys@...il.com>,
        Ming Lei <ming.lei@...hat.com>, Ivan Kozik <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



> Il giorno 07 feb 2018, alle ore 23:18, Jens Axboe <axboe@...nel.dk> ha scritto:
> 
> 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.
> 

I Jens,
I forgot to add
Tested-by: Oleksandr Natalenko <oleksandr@...alenko.name>
in the patch.

Is it still possible to add it?

Thanks,
Paolo

> -- 
> Jens Axboe