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=-6.7 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, UNPARSEABLE_RELAY 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 03370C432C0 for ; Fri, 29 Nov 2019 06:37:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D68282086A for ; Fri, 29 Nov 2019 06:37:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726793AbfK2Ghk convert rfc822-to-8bit (ORCPT ); Fri, 29 Nov 2019 01:37:40 -0500 Received: from out30-45.freemail.mail.aliyun.com ([115.124.30.45]:42853 "EHLO out30-45.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726167AbfK2Ghj (ORCPT ); Fri, 29 Nov 2019 01:37:39 -0500 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R111e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04394;MF=xiejingfeng@linux.alibaba.com;NM=1;PH=DS;RN=7;SR=0;TI=SMTPD_---0TjMhZfY_1575009453; Received: from 30.18.122.9(mailfrom:xiejingfeng@linux.alibaba.com fp:SMTPD_---0TjMhZfY_1575009453) by smtp.aliyun-inc.com(127.0.0.1); Fri, 29 Nov 2019 14:37:34 +0800 User-Agent: Microsoft-MacOutlook/10.1f.0.191110 Date: Fri, 29 Nov 2019 14:37:57 +0800 Subject: Re: [PATCH] psi:fix divide by zero in psi_update_stats From: Jingfeng Xie To: Johannes Weiner CC: Ingo Molnar , Peter Zijlstra , , Suren Baghdasaryan , Xunlei Pang , "=?UTF-8?B?6b2Q5rGf?=(=?UTF-8?B?56qF6buY?=)" Message-ID: Thread-Topic: [PATCH] psi:fix divide by zero in psi_update_stats References: <20191112154144.GC168812@cmpxchg.org> <20191112154844.GD168812@cmpxchg.org> <20191112160821.GE168812@cmpxchg.org> Mime-version: 1.0 Content-type: text/plain; charset="UTF-8" Content-transfer-encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Weiner, The crash does not happen right after boot, in my case, it happens in 58914 ~ 815463 seconds range since boot With my coredump，some values are extracted as below： period = 001df2dc00000000 now = 001df2dc00000000， same as period expires = group->next_update = rdi = 00003594f700648e group->avg_last_update could not be known missed_periods = 0 在 2019/11/13 上午12:08，“Johannes Weiner” 写入: On Tue, Nov 12, 2019 at 10:48:46AM -0500, Johannes Weiner wrote: > On Tue, Nov 12, 2019 at 10:41:46AM -0500, Johannes Weiner wrote: > > On Fri, Nov 08, 2019 at 03:33:24PM +0800, tim wrote: > > > In psi_update_stats, it is possible that period has value like > > > 0xXXXXXXXX00000000 where the lower 32 bit is 0, then it calls div_u64 which > > > truncates u64 period to u32, results in zero divisor. > > > Use div64_u64() instead of div_u64() if the divisor is u64 to avoid > > > truncation to 32-bit on 64-bit platforms. > > > > > > Signed-off-by: xiejingfeng > > > > This is legit. When we stop the periodic averaging worker due to an > > idle CPU, the period after restart can be much longer than the ~4 sec > > in the lower 32 bits. See the missed_periods logic in update_averages. > > Argh, that's not right. Of course I notice right after hitting send. > > missed_periods are subtracted out of the difference between now and > the last update, so period should be not much bigger than 2s. > > Something else is going on here. Tim, does this happen right after boot? I wonder if it's because we're not initializing avg_last_update, and the initial delta between the last update (0) and the first scheduled update (sched_clock() + 2s) ends up bigger than 4 seconds somehow. Later on, the delta between the last and the scheduled update should always be ~2s. But for that to happen, it would require a pretty slow boot, or a sched_clock() that does not start at 0. Tim, if you have a coredump, can you extract the value of the other variables printed in the following patch? diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c index 84af7aa158bf..1b6836d23091 100644 --- a/kernel/sched/psi.c +++ b/kernel/sched/psi.c @@ -374,6 +374,10 @@ static u64 update_averages(struct psi_group *group, u64 now) */ avg_next_update = expires + ((1 + missed_periods) * psi_period); period = now - (group->avg_last_update + (missed_periods * psi_period)); + + WARN(period >> 32, "period=%ld now=%ld expires=%ld last=%ld missed=%ld\n", + period, now, expires, group->avg_last_update, missed_periods); + group->avg_last_update = now; for (s = 0; s < NR_PSI_STATES - 1; s++) { And we may need something like this to make the tick initialization more robust regardless of the reported bug here: diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c index 84af7aa158bf..ce8f6748678a 100644 --- a/kernel/sched/psi.c +++ b/kernel/sched/psi.c @@ -185,7 +185,8 @@ static void group_init(struct psi_group *group) for_each_possible_cpu(cpu) seqcount_init(&per_cpu_ptr(group->pcpu, cpu)->seq); - group->avg_next_update = sched_clock() + psi_period; + group->avg_last_update = sched_clock(); + group->avg_next_update = group->avg_last_update + psi_period; INIT_DELAYED_WORK(&group->avgs_work, psi_avgs_work); mutex_init(&group->avgs_lock); /* Init trigger-related members */