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 6A0F3C19F28 for ; Wed, 27 Jul 2022 04:02:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240202AbiG0EC4 (ORCPT ); Wed, 27 Jul 2022 00:02:56 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:45690 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231448AbiG0ECy (ORCPT ); Wed, 27 Jul 2022 00:02:54 -0400 Received: from szxga01-in.huawei.com (szxga01-in.huawei.com [45.249.212.187]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 9D1C315820 for ; Tue, 26 Jul 2022 21:02:53 -0700 (PDT) Received: from canpemm500009.china.huawei.com (unknown [172.30.72.53]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4Lt0RL2rhFzjXMK; Wed, 27 Jul 2022 11:59:58 +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; Wed, 27 Jul 2022 12:02:51 +0800 Message-ID: <85d5087b-450c-351f-270d-c61303cf3187@huawei.com> Date: Wed, 27 Jul 2022 12:02:51 +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 To: Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira , "open list:SCHEDULER" From: "Lihua (lihua, ran)" Subject: [Question] Reading /proc/stat has a time backward issue Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-Originating-IP: [10.67.109.51] X-ClientProxiedBy: dggems702-chm.china.huawei.com (10.3.19.179) 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 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.