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 6BEEFC00140 for ; Mon, 15 Aug 2022 08:17:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S241751AbiHOIQB (ORCPT ); Mon, 15 Aug 2022 04:16:01 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:43038 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231163AbiHOIPh (ORCPT ); Mon, 15 Aug 2022 04:15:37 -0400 Received: from desiato.infradead.org (desiato.infradead.org [IPv6:2001:8b0:10b:1:d65d:64ff:fe57:4e05]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 10DBF639E; Mon, 15 Aug 2022 01:15:34 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=In-Reply-To:Content-Transfer-Encoding: Content-Type:MIME-Version:References:Message-ID:Subject:Cc:To:From:Date: Sender:Reply-To:Content-ID:Content-Description; bh=8Ewz/NjlMw51NrevL6mP6tY09Paxwi5mAgndnXxIDBg=; b=l0fFEmCLMSXx9W2xsG+a7cIUXK Wa/U1dxWGmmtRQ5aP1D0E55ZzWtF6se5QPpXFADHuyFZtEG3qBMDdpkI+44S5+lVHDWZcKopE57Se zrPO8UKMLMRtUEbhd9j7+c7i23fyo6IFAtuVZbStiVxy6d23p2e5UKD/JzJYxta7Pp82UN1ql/a7W TIzHyUpthpkFuErAFlCh6t/LcHzj95hJgoxf+JHTb5I/REVOqnFVxVzE8xzp1FqjnKw9WEdIJCrvI +gH1SqrnnhhXpNMYwRAXxJU2pwyEgnZ+CaKBZR53RQsf6NvwKPp/+wbSG3EttTq+F6dPmJRHPH8Mr RGFVfGfA==; Received: from j130084.upc-j.chello.nl ([24.132.130.84] helo=worktop.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1oNVFk-002cVb-B6; Mon, 15 Aug 2022 08:15:16 +0000 Received: by worktop.programming.kicks-ass.net (Postfix, from userid 1000) id 3D37B980153; Mon, 15 Aug 2022 10:15:15 +0200 (CEST) Date: Mon, 15 Aug 2022 10:15:15 +0200 From: Peter Zijlstra To: Li Hua Cc: mingo@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, bristot@redhat.com, vschneid@redhat.com, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH -next] sched/cputime: Fix the bug of reading time backward from /proc/stat Message-ID: References: <20220813000102.42051-1-hucool.lihua@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20220813000102.42051-1-hucool.lihua@huawei.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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?!?