From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752411AbeBIRRl (ORCPT ); Fri, 9 Feb 2018 12:17:41 -0500 Received: from mail-io0-f169.google.com ([209.85.223.169]:46098 "EHLO mail-io0-f169.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751277AbeBIRRi (ORCPT ); Fri, 9 Feb 2018 12:17:38 -0500 X-Google-Smtp-Source: AH8x22434ctwbWW7v0IpWPRoeZsjhQvD12MhoH/qWM6a7Wi3VJASRlWdESAQMpLeH1cfmFZJv7u/vw== 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 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:58.0) Gecko/20100101 Thunderbird/58.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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