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Date: Fri, 09 Feb 2018 14:21:57 +0100
From: Oleksandr Natalenko <oleksandr@natalenko.name>
To: Paolo Valente <paolo.valente@linaro.org>
Cc: Jens Axboe <axboe@kernel.dk>, linux-block <linux-block@vger.kernel.org>,
 Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Ulf Hansson
 <ulf.hansson@linaro.org>, Mark Brown <broonie@kernel.org>, Linus Walleij
 <linus.walleij@linaro.org>, 'Paolo Valente' via bfq-iosched
 <bfq-iosched@googlegroups.com>, Alban Browaeys <alban.browaeys@gmail.com>,
 Ming Lei <ming.lei@redhat.com>, Ivan Kozik <ivan@ludios.org>,
 169364@studenti.unimore.it, holger@applied-asynchrony.com, efault@gmx.de,
 Serena Ziviani <ziviani.serena@gmail.com>
Subject: Re: [PATCH BUGFIX V3] block, bfq: add requeue-request hook
In-Reply-To: <34041F0A-F460-4736-9A6C-76D861EA0070@linaro.org>
References: <20180207211920.6343-1-paolo.valente@linaro.org>
 <17c57205-7cc0-5577-0322-dc35914e50e5@kernel.dk>
 <34041F0A-F460-4736-9A6C-76D861EA0070@linaro.org>
Message-ID: <b5d4a3bc8946db0a696e8398b68adba0@natalenko.name>
List-ID: <linux-block@vger.kernel.org>

Hi.

08.02.2018 08:16, Paolo Valente wrote:
>> Il giorno 07 feb 2018, alle ore 23:18, Jens Axboe <axboe@kernel.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@natalenko.name>
> in the patch.
> 
> Is it still possible to add it?
> 

In addition to this I think it should be worth considering CC'ing Greg 
to pull this fix into 4.15 stable tree.

Oleksandr