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=-11.4 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,MAILING_LIST_MULTI, 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 138CEC433ED for ; Mon, 17 May 2021 18:24:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id EBCC961285 for ; Mon, 17 May 2021 18:24:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236966AbhEQS0J (ORCPT ); Mon, 17 May 2021 14:26:09 -0400 Received: from mail.kernel.org ([198.145.29.99]:38854 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233592AbhEQS0H (ORCPT ); Mon, 17 May 2021 14:26:07 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id E3EDF61241; Mon, 17 May 2021 18:24:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1621275890; bh=agtqME+MKmOmL3aHSZ0CiwFpj5LnjrmqdL388mt/8PI=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=cNIzKIMnVD7el6yhUFHTVuSo/cF8AFYjDx/7JWn9swaI2SV07JxBLMHPY1pMY1a6E N2Y0AagWNaWu8W90Fw6R45oFm5hiCyx/46EzqHrpLlQUlBSg3R2EseybSk6rLmfUlh LSx/HYqxJvwLDIWOxuzF9NhVGRktueq+1i95AyH5tto4RDmaEbRPwwAwr2F+TgIDbB Gkfl3y4z/+UGKrlsf+D4QrRVogKM14kpytdWPlqsmgOvfcHW5QdH1jX/ZemrPdA0vv tWwnUdVTvQbWVHVeI7HO8tEyTFGig8fVww3Du+ahYAc5hWKd3+1Xp10IzIqsIgl8TQ EnhI8X2oxO5wA== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id C019B5C00C6; Mon, 17 May 2021 11:24:50 -0700 (PDT) Date: Mon, 17 May 2021 11:24:50 -0700 From: "Paul E. McKenney" To: Masami Hiramatsu 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 =?iso-8859-1?Q?H=F8iland-J=F8rgensen?= Subject: Re: [PATCH tip/core/rcu 3/4] rcu-tasks: Make ksoftirqd provide RCU Tasks quiescent states Message-ID: <20210517182450.GL4441@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org 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> <20210514150431.d89b0ad0a5ce1ac3971a66e5@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210514150431.d89b0ad0a5ce1ac3971a66e5@kernel.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, May 14, 2021 at 03:04:31PM +0900, Masami Hiramatsu wrote: > 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 I will apply on the next rebase, thank you! Thanx, Paul