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 E5386C38A02 for ; Sat, 29 Oct 2022 09:51:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229634AbiJ2Jv6 (ORCPT ); Sat, 29 Oct 2022 05:51:58 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60260 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229482AbiJ2Jv5 (ORCPT ); Sat, 29 Oct 2022 05:51:57 -0400 Received: from szxga08-in.huawei.com (szxga08-in.huawei.com [45.249.212.255]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 31421107A92; Sat, 29 Oct 2022 02:51:54 -0700 (PDT) Received: from dggpemm500023.china.huawei.com (unknown [172.30.72.53]) by szxga08-in.huawei.com (SkyGuard) with ESMTP id 4MzvhH4TpVz15Lx9; Sat, 29 Oct 2022 17:46:55 +0800 (CST) Received: from dggpemm500006.china.huawei.com (7.185.36.236) by dggpemm500023.china.huawei.com (7.185.36.83) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.31; Sat, 29 Oct 2022 17:51:52 +0800 Received: from [10.174.178.55] (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; Sat, 29 Oct 2022 17:51:51 +0800 Subject: Re: [PATCH v2 1/3] sched: Add helper kstat_cpu_softirqs_sum() To: CC: "Elliott, Robert (Servers)" , Frederic Weisbecker , Neeraj Upadhyay , "Josh Triplett" , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Joel Fernandes , "rcu@vger.kernel.org" , "linux-kernel@vger.kernel.org" References: <20221022124525.2080-1-thunder.leizhen@huawei.com> <20221022124525.2080-2-thunder.leizhen@huawei.com> <20221028223514.GV5600@paulmck-ThinkPad-P17-Gen-1> From: "Leizhen (ThunderTown)" Message-ID: <40cf1a2c-821d-51da-14b1-cdf6d5b8aeef@huawei.com> Date: Sat, 29 Oct 2022 17:51:51 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0 MIME-Version: 1.0 In-Reply-To: <20221028223514.GV5600@paulmck-ThinkPad-P17-Gen-1> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit 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 On 2022/10/29 6:35, Paul E. McKenney wrote: > On Fri, Oct 28, 2022 at 10:38:15AM +0800, Leizhen (ThunderTown) wrote: >> >> >> On 2022/10/28 3:04, Elliott, Robert (Servers) wrote: >>> >>>> Similar to kstat_cpu_irqs_sum(), it counts the sum of all software >>>> interrupts on a specified CPU. >>>> >>>> diff --git a/include/linux/kernel_stat.h b/include/linux/kernel_stat.h >>>> @@ -67,6 +67,17 @@ static inline unsigned int kstat_softirqs_cpu(unsigned int irq, int cpu) >>>> return kstat_cpu(cpu).softirqs[irq]; >>>> } >>>> >>>> +static inline unsigned int kstat_cpu_softirqs_sum(int cpu) >>>> +{ >>>> + int i; >>>> + unsigned int sum = 0; >>>> + >>>> + for (i = 0; i < NR_SOFTIRQS; i++) >>>> + sum += kstat_softirqs_cpu(i, cpu); >>>> + >>>> + return sum; >>>> +} >>> >>> In the function upon which this is based: >>> >>> struct kernel_stat { >>> unsigned long irqs_sum; >>> unsigned int softirqs[NR_SOFTIRQS]; >>> }; >>> >>> static inline unsigned int kstat_cpu_irqs_sum(unsigned int cpu) >>> { >>> return kstat_cpu(cpu).irqs_sum; >>> } >>> >>> kstat_cpu_irqs_sum returns an unsigned long as an unsigned int, which >>> could cause large values to be truncated. Should that return >>> unsigned long? The only existing caller is fs/proc/stat.c which >> >> This should be a mistake on: >> commit f2c66cd8eeddedb4 ("/proc/stat: scalability of irq num per cpu") >> >> I'll correct it to "unsigned long" in the next version. Thanks. >> >>> puts it into a u64: >>> u64 sum = 0; >>> ... >>> sum += kstat_cpu_irqs_sum(i); >>> >>> The softirqs field is an unsigned int, so the new function doesn't have >>> this inconsistency. >> >> OK. >> >> To be honest, I did the math. CONFIG_HZ=250 >> 2^32 / 250 / 3600 / 24 / 365 = 0.545 < 1 year > > For this to be a problem, our RCU CPU stall warning would have to be > for a months-long grace period, even on systems with HZ=1000. In almost > all cases, the system would have OOMed long before then. Yes. > >> So, in theory, for those 32-bit processors, we should use "unsigned long long". >> Of course, from a programming point of view, 64-bit consists of two 32-bits, >> and there is an atomicity problem. I think that's probably why members of >> struct kernel_stat don't use u64. >> >> However, it seems that the type of member softirqs can currently be changed to >> unsigned long. So, at least on a 64-bit processor, it won't have a count >> overflow problem. > > An unsigned long should suffice. ;-) include/linux/irqdesc.h:58: unsigned int __percpu *kstat_irqs; I found another place where the hard interrupt count was stored with type "unsigned int", it's used by "/proc/interrupts". Maybe the user-mode program gets it periodically and accumulates it to a 64-bit value. Of course, maybe half a year later, no one cares about the specific interrupts count anymore. So, apart from what Elliott mentioned, I won't change the rest. > > Thanx, Paul > . > -- Regards, Zhen Lei