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 8A239C433FE for ; Sun, 9 Oct 2022 02:29:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229463AbiJIC26 (ORCPT ); Sat, 8 Oct 2022 22:28:58 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56298 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229750AbiJIC24 (ORCPT ); Sat, 8 Oct 2022 22:28:56 -0400 Received: from szxga01-in.huawei.com (szxga01-in.huawei.com [45.249.212.187]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id AE04B3642A for ; Sat, 8 Oct 2022 19:28:51 -0700 (PDT) Received: from canpemm500006.china.huawei.com (unknown [172.30.72.56]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4MlQrP4jKZzpVbf; Sun, 9 Oct 2022 10:25:41 +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 15.1.2375.31; Sun, 9 Oct 2022 10:28:48 +0800 Message-ID: Date: Sun, 9 Oct 2022 10:28:43 +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 time backward issue about /proc/stat To: Frederic Weisbecker CC: Peter Zijlstra , , , , , , , , , , , References: <20220928033402.181530-1-zhengzucheng@huawei.com> <20220928121134.GA233658@lothringen> <5126b2dc-8624-babc-2e1e-58ac27927c31@huawei.com> <20220930121634.GA266766@lothringen> From: zhengzucheng In-Reply-To: <20220930121634.GA266766@lothringen> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-Originating-IP: [10.67.109.61] X-ClientProxiedBy: dggems702-chm.china.huawei.com (10.3.19.179) 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 在 2022/9/30 20:16, Frederic Weisbecker 写道: > On Fri, Sep 30, 2022 at 10:43:58AM +0800, zhengzucheng wrote: >> 在 2022/9/28 20:11, Frederic Weisbecker 写道: >>> @@ -1024,20 +1045,20 @@ static int kcpustat_cpu_fetch_vtime(struct kernel_cpustat *dst, >>> * add pending nohz time to the right place. >>> */ >>> if (state == VTIME_SYS) { >>> - cpustat[CPUTIME_SYSTEM] += vtime->stime + delta; >>> + cpustat[CPUTIME_SYSTEM] += delta; >>> } else if (state == VTIME_USER) { >>> if (task_nice(tsk) > 0) >>> - cpustat[CPUTIME_NICE] += vtime->utime + delta; >>> + cpustat[CPUTIME_NICE] += delta; >>> else >>> - cpustat[CPUTIME_USER] += vtime->utime + delta; >>> + cpustat[CPUTIME_USER] += delta; >> “delta” has the same problem as vtime->utime, which varies with different >> tasks. switching between different tasks may cause time statistics to be >> reversed. > I'm a bit confused, can you provide an example? Chinese National Day holiday, sorry for not replying to you in time. CONFIG_HZ=100 const struct kernel_cpustat *src = &kcpustat_cpu(cpu); struct vtime *vtime = &tsk->vtime; cpustat[CPUTIME_USER] += vtime->utime + delta; first： cat /proc/stat | grep cpu1 cpu1 319 0 496 41665 0 0 0 0 0 0 Task A is running on CPU 1，so vtime is A->vtime and delta is A's delta. kcpustat_cpu_fetch_vtime: cpu=1 src->cpustat[CPUTIME_USER]=3189000000 vtime->utime=900000 delta=100001 cpustat[CPUTIME_USER] = 3189000000 + 900000 + 100001 is 3,190,000,001, 319 ticks again： cat /proc/stat | grep cpu1 cpu1 318 0 497 41674 0 0 0 0 0 0 Task B is running on CPU 1，so vtime is B->vtime and delta is B's delta. kcpustat_cpu_fetch_vtime: cpu=1 src->cpustat[CPUTIME_USER]=3189000000 vtime->utime=900000 delta=90000 cpustat[CPUTIME_USER] = 3189000000 + 900000 + 90000 is 3,189,990,000, 318 ticks The root cause is that the value of task B's "vtime->utime + delta" may be smaller than task A. > > Thanks. > >>> } else { >>> WARN_ON_ONCE(state != VTIME_GUEST); >>> if (task_nice(tsk) > 0) { >>> - cpustat[CPUTIME_GUEST_NICE] += vtime->gtime + delta; >>> - cpustat[CPUTIME_NICE] += vtime->gtime + delta; >>> + cpustat[CPUTIME_GUEST_NICE] += delta; >>> + cpustat[CPUTIME_NICE] += delta; >>> } else { >>> - cpustat[CPUTIME_GUEST] += vtime->gtime + delta; >>> - cpustat[CPUTIME_USER] += vtime->gtime + delta; >>> + cpustat[CPUTIME_GUEST] += delta; >>> + cpustat[CPUTIME_USER] += delta; >>> } >>> } >>> } while (read_seqcount_retry(&vtime->seqcount, seq)); >>> . > .