From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-13.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4F0BCC04FF3 for ; Mon, 24 May 2021 17:14:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 25F4F613EC for ; Mon, 24 May 2021 17:14:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233242AbhEXRPd (ORCPT ); Mon, 24 May 2021 13:15:33 -0400 Received: from mail.itouring.de ([85.10.202.141]:58410 "EHLO mail.itouring.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232543AbhEXRPc (ORCPT ); Mon, 24 May 2021 13:15:32 -0400 Received: from tux.applied-asynchrony.com (p5ddd760d.dip0.t-ipconnect.de [93.221.118.13]) by mail.itouring.de (Postfix) with ESMTPSA id D1C9EE0; Mon, 24 May 2021 19:13:59 +0200 (CEST) Received: from [192.168.100.221] (hho.applied-asynchrony.com [192.168.100.221]) by tux.applied-asynchrony.com (Postfix) with ESMTP id 7D39DF01600; Mon, 24 May 2021 19:13:59 +0200 (CEST) Subject: Re: [PATCH BUGFIX] block, bfq: fix delayed stable merge check To: Paolo Valente Cc: Luca Mariotti , Jens Axboe , linux-block , linux-kernel@vger.kernel.org, Pietro Pedroni References: From: =?UTF-8?Q?Holger_Hoffst=c3=a4tte?= Organization: Applied Asynchrony, Inc. Message-ID: Date: Mon, 24 May 2021 19:13:59 +0200 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 2021-05-24 18:57, Paolo Valente wrote: > > >> Il giorno 20 mag 2021, alle ore 09:15, Holger Hoffstätte ha scritto: >> >> On 2021-05-18 12:43, Luca Mariotti wrote: >>> When attempting to schedule a merge of a given bfq_queue with the currently >>> in-service bfq_queue or with a cooperating bfq_queue among the scheduled >>> bfq_queues, delayed stable merge is checked for rotational or non-queueing >>> devs. For this stable merge to be performed, some conditions must be met. >>> If the current bfq_queue underwent some split from some merged bfq_queue, >>> one of these conditions is that two hundred milliseconds must elapse from >>> split, otherwise this condition is always met. >>> Unfortunately, by mistake, time_is_after_jiffies() was written instead of >>> time_is_before_jiffies() for this check, verifying that less than two >>> hundred milliseconds have elapsed instead of verifying that at least two >>> hundred milliseconds have elapsed. >>> Fix this issue by replacing time_is_after_jiffies() with >>> time_is_before_jiffies(). >>> Signed-off-by: Luca Mariotti >>> Signed-off-by: Paolo Valente >>> Signed-off-by: Pietro Pedroni >>> --- >>> block/bfq-iosched.c | 2 +- >>> 1 file changed, 1 insertion(+), 1 deletion(-) >>> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c >>> index acd1f881273e..2adb1e69c9d2 100644 >>> --- a/block/bfq-iosched.c >>> +++ b/block/bfq-iosched.c >>> @@ -2697,7 +2697,7 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq, >>> if (unlikely(!bfqd->nonrot_with_queueing)) { >>> if (bic->stable_merge_bfqq && >>> !bfq_bfqq_just_created(bfqq) && >>> - time_is_after_jiffies(bfqq->split_time + >>> + time_is_before_jiffies(bfqq->split_time + >>> msecs_to_jiffies(200))) { >>> struct bfq_queue *stable_merge_bfqq = >>> bic->stable_merge_bfqq; >> >> Not sure why but with this patch I quickly got a division-by-zero in BFQ and >> complete system halt. Unfortunately I couldn't capture the exact stack trace, >> but it read something like bfq_calc_weight() or something ike that. >> I looked through the code and found bfq_delta(), so maybe weight got >> reduced to 0? >> > > Hi Holger, > is this (easily) reproducible for you? If so, I'd like to propose you > a candidate fix. Yes, it's easily reproducible (should be reproducible on 5.13-rc as well). Simple read/write I/O on a cold FS (rotational disk obviously) will crash pretty much immediately; without it everything works fine, likely because the bug (in the recent queue merging patches?) is never triggered due to the accidentally-wrong time calculation. Will gladly test your patch! :) cheers Holger