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 X-Spam-Level: X-Spam-Status: No, score=-11.7 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,MAILING_LIST_MULTI, NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4BB7AC433ED for ; Fri, 14 May 2021 06:04:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2507D613C1 for ; Fri, 14 May 2021 06:04:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231932AbhENGFt (ORCPT ); Fri, 14 May 2021 02:05:49 -0400 Received: from mail.kernel.org ([198.145.29.99]:35906 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230121AbhENGFr (ORCPT ); Fri, 14 May 2021 02:05:47 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 25E9D61442; Fri, 14 May 2021 06:04:32 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1620972276; bh=7thu+ANECwgoapk9y0rfkNoObCgHdSaeFn1wY2GgHFI=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=OBJbVCryd33z13cNpyWRwNInMMyoRKj2bV2jrZzMXTAvLt/nnwAR6Z31v+sXp+M6L 7ChtYZgbFbVn11XEWeUR75Tem8WuDh615FHBcSCX6Uvq66xVM+3wTG6YHQJnhtP3lV 4rEjhKeOIy/cVYA4beD6+n6bcxz9Grrk9FUGA3xoc0mtBMBXjhBjA/YOxRqtCcmkaa ftk5eY7tusNcY28Jz9h5TCoBVxAYuJpHpPVqMvV2YUU/8mORKDzYwh8fXQjYThT08D /AyU94aO9ItUwtYZBBnR/9Ya/iO00ATJUl6tT+wxb/vI2fiyO+et5tAI6OC/FT8/37 o3MLW5UfNVXlg== Date: Fri, 14 May 2021 15:04:31 +0900 From: Masami Hiramatsu To: paulmck@kernel.org Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org, Toke =?UTF-8?B?SMO4aWxhbmQtSsO4cmdlbnNlbg==?= Subject: Re: [PATCH tip/core/rcu 3/4] rcu-tasks: Make ksoftirqd provide RCU Tasks quiescent states Message-Id: <20210514150431.d89b0ad0a5ce1ac3971a66e5@kernel.org> In-Reply-To: <20210513191539.GF975577@paulmck-ThinkPad-P17-Gen-1> References: <20210512182747.3445812-4-paulmck@kernel.org> <20210513155417.93ab2299139ba35025ec8ef7@kernel.org> <20210513142110.GY975577@paulmck-ThinkPad-P17-Gen-1> <20210514024912.a38f755add13a0f1dc73395a@kernel.org> <20210513191539.GF975577@paulmck-ThinkPad-P17-Gen-1> X-Mailer: Sylpheed 3.7.0 (GTK+ 2.24.32; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Paul, On Thu, 13 May 2021 12:15:39 -0700 "Paul E. McKenney" wrote: > On Fri, May 14, 2021 at 02:49:12AM +0900, Masami Hiramatsu wrote: > > On Thu, 13 May 2021 07:21:10 -0700 > > "Paul E. McKenney" wrote: > > > > > On Thu, May 13, 2021 at 03:54:17PM +0900, Masami Hiramatsu wrote: > > > > Hi Paul, > > > > > > > > On Wed, 12 May 2021 11:27:46 -0700 > > > > "Paul E. McKenney" wrote: > > > > > > > > > Heavy networking load can cause a CPU to execute continuously and > > > > > indefinitely within ksoftirqd, in which case there will be no voluntary > > > > > task switches and thus no RCU-tasks quiescent states. This commit > > > > > therefore causes the exiting rcu_softirq_qs() to provide an RCU-tasks > > > > > quiescent state. > > > > > > > > > > This of course means that __do_softirq() and its callers cannot be > > > > > invoked from within a tracing trampoline. > > > > > > > > I would like to confirm that you mean "tracing trampoline" here is > > > > the code on the trampoline buffer, not the handler code which is > > > > invoked from the trampoline buffer but it is protected by preempt_disable(), > > > > am I understand correctly? > > > > > > Maybe? ;-) > > > > > > If the handler code is invoked from the trampoline buffer, but > > > returns somewhere else, then it is OK for the handler code to invoke > > > __do_softirq() or its callers. > > > > > > In addition, if the handler code is invoked from the trampoline buffer is > > > guaranteed never to be running in the context of the ksoftirqd kthread, > > > then it is also OK for the handler code to invoke __do_softirq() or > > > its callers. > > > > > > Otherwise, if the handler code might return back into the trampoline > > > buffer and if that code might be running in the context of the ksoftirqd > > > kthread, invoking __do_softirq() or one of its callers could result in > > > the trampoline buffer no longer being there when it was returned to. > > > > Hmm, the optprobe may be involved in this case. It always return to > > the trampoline and handler does not disable irqs (only disable preempt). > > BTW, what will call the __do_softirq()? Is hardirq safe? > > As long as your code does not explicitly call __do_softirq() or one of > its callers, you should be OK. > > Let's suppose that your code takes a hardirq from ksoftirqd context. > In that case, the return-from-irq path will notice the ksoftirqd > context and refrain from calling __do_softirqd(). Life is good. > (See the invoke_softirq() function for more detail.) > > On the other hand, if your code takes a hardirq from some non-ksoftirqd > context, and if this hardirq decides to handle softirqs on exit > from the hardirq, the "__this_cpu_read(ksoftirqd) == current" within > __do_softirq() will fail, so that rcu_softirq_qs() will not be called. > Life is still good. Ah, OK. This is good. > > Either way, as long as your handler does not explicitly invoke > __do_softirq(), life is good. There should be no such code, I hope. > > The bad case is when you instrument a function that is invoked in the > context of a ksoftirqd kthread, and the corresponding handler (or > some function that the handler explicitly calls) directly invokes > __do_softirq() or one of its caller. > > Is that more helpful? OK, I got it. So it would be better to be commented later. But anyway I can't imagine that there is any reason to call __do_softirq() inside kprobe handler :) Reviewed-by: Masami Hiramatsu Thank you, -- Masami Hiramatsu