From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Subject: Re: [PATCH BUGFIX V3] block, bfq: add requeue-request hook To: Oleksandr Natalenko , Paolo Valente Cc: linux-block , Linux Kernel Mailing List , Ulf Hansson , Mark Brown , Linus Walleij , 'Paolo Valente' via bfq-iosched , Alban Browaeys , Ming Lei , Ivan Kozik , 169364@studenti.unimore.it, holger@applied-asynchrony.com, efault@gmx.de, Serena Ziviani References: <20180207211920.6343-1-paolo.valente@linaro.org> <17c57205-7cc0-5577-0322-dc35914e50e5@kernel.dk> <34041F0A-F460-4736-9A6C-76D861EA0070@linaro.org> From: Jens Axboe Message-ID: <760ae85e-be4c-cf13-53f2-579ab4646b09@kernel.dk> Date: Fri, 9 Feb 2018 10:17:34 -0700 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 List-ID: On 2/9/18 6:21 AM, Oleksandr Natalenko wrote: > Hi. > > 08.02.2018 08:16, Paolo Valente wrote: >>> Il giorno 07 feb 2018, alle ore 23:18, Jens Axboe 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 >> 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. I can't add the tested-by anymore, but it's easy enough to target for stable after-the-fact. -- Jens Axboe