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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id D5683ECAAA1 for ; Mon, 5 Sep 2022 03:47:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236065AbiIEDrw (ORCPT ); Sun, 4 Sep 2022 23:47:52 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42768 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229480AbiIEDrt (ORCPT ); Sun, 4 Sep 2022 23:47:49 -0400 Received: from szxga01-in.huawei.com (szxga01-in.huawei.com [45.249.212.187]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 494442BB00; Sun, 4 Sep 2022 20:47:48 -0700 (PDT) Received: from canpemm500006.china.huawei.com (unknown [172.30.72.55]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4MLZBS53lqzkWvg; Mon, 5 Sep 2022 11:44:00 +0800 (CST) Received: from [10.67.109.61] (10.67.109.61) by canpemm500006.china.huawei.com (7.192.105.130) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.24; Mon, 5 Sep 2022 11:47:46 +0800 Message-ID: <0b2f6919-7ed4-b30d-e92b-355c09bbfd25@huawei.com> Date: Mon, 5 Sep 2022 11:47:41 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.7.0 Subject: Re: [PATCH -next] sched/cputime: Fix the bug of reading time backward from /proc/stat To: Peter Zijlstra , Li Hua CC: , , , , , , , , , , References: <20220813000102.42051-1-hucool.lihua@huawei.com> From: zhengzucheng In-Reply-To: Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-Originating-IP: [10.67.109.61] X-ClientProxiedBy: dggems705-chm.china.huawei.com (10.3.19.182) To canpemm500006.china.huawei.com (7.192.105.130) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Assume that a CPU time“ A” is read from /proc/stat， and after a while,  a CPU time “B” is read. If T = B – A < 0, T is identified as a large number as an unsigned integer. As a result, the CPU usage calculated by this way will be abnormally high. It seems to be a problem to be fixed. original link: https://lore.kernel.org/lkml/20220813000102.42051-1-hucool.lihua@huawei.com/ 在 2022/8/15 16:15, Peter Zijlstra 写道: > On Sat, Aug 13, 2022 at 08:01:02AM +0800, Li Hua wrote: >> The problem that the statistical time goes backward, the value read first is 319, and the value read again is 318. As follows： >> first： >> cat /proc/stat | grep cpu1 >> cpu1 319 0 496 41665 0 0 0 0 0 0 >> then： >> cat /proc/stat | grep cpu1 >> cpu1 318 0 497 41674 0 0 0 0 0 0 >> >> Time goes back, which is counterintuitive. >> >> After debug this, The problem is caused by the implementation of kcpustat_cpu_fetch_vtime. As follows： >> >> CPU0 CPU1 >> First: >> show_stat(): >> ->kcpustat_cpu_fetch() >> ->kcpustat_cpu_fetch_vtime() >> ->cpustat[CPUTIME_USER] = kcpustat_cpu(cpu) + vtime->utime + delta; rq->curr is in user mod >> ---> When CPU1 rq->curr running on userspace, need add utime and delta >> ---> rq->curr->vtime->utime is less than 1 tick >> Then: >> show_stat(): >> ->kcpustat_cpu_fetch() >> ->kcpustat_cpu_fetch_vtime() >> ->cpustat[CPUTIME_USER] = kcpustat_cpu(cpu); rq->curr is in kernel mod >> ---> When CPU1 rq->curr running on kernel space, just got kcpustat > This is unreadable, what?!? > .