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 E3BDEC433FE for ; Wed, 9 Nov 2022 09:38:37 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230014AbiKIJig (ORCPT ); Wed, 9 Nov 2022 04:38:36 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42134 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230212AbiKIJia (ORCPT ); Wed, 9 Nov 2022 04:38:30 -0500 Received: from szxga01-in.huawei.com (szxga01-in.huawei.com [45.249.212.187]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7E56E5F8C; Wed, 9 Nov 2022 01:38:28 -0800 (PST) Received: from dggpemm500024.china.huawei.com (unknown [172.30.72.54]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4N6fz93D0WzmVjL; Wed, 9 Nov 2022 17:38:13 +0800 (CST) Received: from dggpemm500006.china.huawei.com (7.185.36.236) by dggpemm500024.china.huawei.com (7.185.36.203) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.31; Wed, 9 Nov 2022 17:38:26 +0800 Received: from thunder-town.china.huawei.com (10.174.178.55) by dggpemm500006.china.huawei.com (7.185.36.236) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.31; Wed, 9 Nov 2022 17:38:25 +0800 From: Zhen Lei To: "Paul E . McKenney" , Frederic Weisbecker , Neeraj Upadhyay , "Josh Triplett" , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Joel Fernandes , , CC: Zhen Lei , Robert Elliott Subject: [PATCH v6 2/2] doc: Document CONFIG_RCU_CPU_STALL_CPUTIME=y stall information Date: Wed, 9 Nov 2022 17:37:38 +0800 Message-ID: <20221109093739.187-3-thunder.leizhen@huawei.com> X-Mailer: git-send-email 2.37.3.windows.1 In-Reply-To: <20221109093739.187-1-thunder.leizhen@huawei.com> References: <20221109093739.187-1-thunder.leizhen@huawei.com> MIME-Version: 1.0 Content-Transfer-Encoding: 7BIT Content-Type: text/plain; charset=US-ASCII X-Originating-IP: [10.174.178.55] X-ClientProxiedBy: dggems704-chm.china.huawei.com (10.3.19.181) To dggpemm500006.china.huawei.com (7.185.36.236) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This commit doucments how to quickly determine the bug causing a given RCU CPU stall fault warning based on the output information provided by CONFIG_RCU_CPU_STALL_CPUTIME=y. [ paulmck: Apply wordsmithing. ] Signed-off-by: Zhen Lei Signed-off-by: Paul E. McKenney --- Documentation/RCU/stallwarn.rst | 88 +++++++++++++++++++++++++++++++++ 1 file changed, 88 insertions(+) diff --git a/Documentation/RCU/stallwarn.rst b/Documentation/RCU/stallwarn.rst index dfa4db8c0931eaf..5e24e849290a286 100644 --- a/Documentation/RCU/stallwarn.rst +++ b/Documentation/RCU/stallwarn.rst @@ -390,3 +390,91 @@ for example, "P3421". It is entirely possible to see stall warnings from normal and from expedited grace periods at about the same time during the same run. + +RCU_CPU_STALL_CPUTIME +===================== + +In kernels built with CONFIG_RCU_CPU_STALL_CPUTIME=y or booted with +rcupdate.rcu_cpu_stall_cputime=1, the following additional information +is supplied with each RCU CPU stall warning:: + +rcu: hardirqs softirqs csw/system +rcu: number: 624 45 0 +rcu: cputime: 69 1 2425 ==> 2500(ms) + +These statistics are collected during the sampling period. The values +in row "number:" are the number of hard interrupts, number of soft +interrupts, and number of context switches on the stalled CPU. The +first three values in row "cputime:" indicate the CPU time in +milliseconds consumed by hard interrupts, soft interrupts, and tasks +on the stalled CPU. The last number is the measurement interval, again +in milliseconds. Because user-mode tasks normally do not cause RCU CPU +stalls, these tasks are typically kernel tasks, which is why only the +system CPU time are considered. + +The sampling period is shown as follows: +|<------------first timeout---------->|<-----second timeout----->| +|<--half timeout-->|<--half timeout-->| | +| |<--first period-->| | +| |<-----------second sampling period---------->| +| | | | +| sampling time point 1st-stall 2nd-stall + + +The following describes four typical scenarios: + +1. A CPU looping with interrupts disabled.:: + + rcu: hardirqs softirqs csw/system + rcu: number: 0 0 0 + rcu: cputime: 0 0 0 ==> 2500(ms) + + Because interrupts have been disabled throughout the measurement + interval, there are no interrupts and no context switches. + Furthermore, because CPU time consumption was measured using interrupt + handlers, the system CPU consumption is misleadingly measured as zero. + This scenario will normally also have "(0 ticks this GP)" printed on + this CPU's summary line. + +2. A CPU looping with bottom halves disabled. + + This is similar to the previous example, but with non-zero number of + and CPU time consumed by hard interrupts, along with non-zero CPU + time consumed by in-kernel execution.:: + + rcu: hardirqs softirqs csw/system + rcu: number: 624 0 0 + rcu: cputime: 49 0 2446 ==> 2500(ms) + + The fact that there are zero softirqs gives a hint that these were + disabled, perhaps via local_bh_disable(). It is of course possible + that there were no softirqs, perhaps because all events that would + result in softirq execution are confined to other CPUs. In this case, + the diagnosis should continue as shown in the next example. + +3. A CPU looping with preemption disabled. + + Here, only the number of context switches is zero.:: + + rcu: hardirqs softirqs csw/system + rcu: number: 624 45 0 + rcu: cputime: 69 1 2425 ==> 2500(ms) + + This situation hints that the stalled CPU was looping with preemption + disabled. + +4. No looping, but massive hard and soft interrupts.:: + + rcu: hardirqs softirqs csw/system + rcu: number: xx xx 0 + rcu: cputime: xx xx 0 ==> 2500(ms) + + Here, the number and CPU time of hard interrupts are all non-zero, + but the number of context switches and the in-kernel CPU time consumed + are zero. The number and cputime of soft interrupts will usually be + non-zero, but could be zero, for example, if the CPU was spinning + within a single hard interrupt handler. + + If this type of RCU CPU stall warning can be reproduced, you can + narrow it down by looking at /proc/interrupts or by writing code to + trace each interrupt, for example, by referring to show_interrupts(). -- 2.25.1