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 D9AA6C00140 for ; Mon, 8 Aug 2022 12:23:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S242557AbiHHMXz (ORCPT ); Mon, 8 Aug 2022 08:23:55 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48190 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234685AbiHHMXu (ORCPT ); Mon, 8 Aug 2022 08:23:50 -0400 Received: from szxga01-in.huawei.com (szxga01-in.huawei.com [45.249.212.187]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 0AF6263FD for ; Mon, 8 Aug 2022 05:23:49 -0700 (PDT) Received: from canpemm500009.china.huawei.com (unknown [172.30.72.56]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4M1b0m6t4JzmVT1; Mon, 8 Aug 2022 20:21:44 +0800 (CST) Received: from [10.67.109.51] (10.67.109.51) by canpemm500009.china.huawei.com (7.192.105.203) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.24; Mon, 8 Aug 2022 20:23:47 +0800 Message-ID: <7b2586c0-c380-c856-7d63-b7bff6f3a640@huawei.com> Date: Mon, 8 Aug 2022 20:23:46 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.3.2 Subject: Re: [Question] Reading /proc/stat has a time backward issue From: "Lihua (lihua, ran)" To: Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira , "open list:SCHEDULER" , References: <85d5087b-450c-351f-270d-c61303cf3187@huawei.com> <1f1f625a-148d-0398-f840-1f9b4e964189@huawei.com> In-Reply-To: <1f1f625a-148d-0398-f840-1f9b4e964189@huawei.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-Originating-IP: [10.67.109.51] X-ClientProxiedBy: dggems703-chm.china.huawei.com (10.3.19.180) To canpemm500009.china.huawei.com (7.192.105.203) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org ping... Your suggestions are valuable, I don't have a good way to fix this. thanks all. 在 2022/8/4 15:44, Lihua (lihua, ran) 写道: > ping... > > Any good suggestions? > > thanks all. > > 在 2022/7/27 12:02, Lihua (lihua, ran) 写道: >> Hi all, >> >> I found a 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, I found that 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 >> >> Because the values ​​of utime、 stime and delta are temporarily written to cpustat. Therefore, there are two problems  read from /proc/stat: >> 1. There may be a regression phenomenon; >> 2. When there are many tasks, the statistics are not accurate enough when utime and stime do not exceed one TICK. >> The time goes back is counterintuitive, and I want to discuss whether there is a good solution without compromising performance. >> >> Thanks a lot.