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=-12.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 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 1BAD2C433E0 for ; Mon, 22 Feb 2021 18:44:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D322864E20 for ; Mon, 22 Feb 2021 18:44:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231664AbhBVSno (ORCPT ); Mon, 22 Feb 2021 13:43:44 -0500 Received: from mail.kernel.org ([198.145.29.99]:44888 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231569AbhBVSmg (ORCPT ); Mon, 22 Feb 2021 13:42:36 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id 5BEF264F44; Mon, 22 Feb 2021 18:16:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1614017768; bh=Hjf816WA/+lRyr0PoVVMaBNdOkCdcL7XT7zVlSLoU8g=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=mlsB6asw56Og4rf/ku+ySjRWZHw4LMBFe8EFSwyIeMLQRhsXNlczGgiFrYWeYqZGZ jyEcDOHCFLTcS4FSo0uaHHZVF5ckDfNCRAMQDdfjsZzLCDklpc0OIGsriI2vS7hEbJ T5wikt+epe5AOHP/q1k8rNFbeUevAmNiJ2zScnfEgXdoIqCWc3pzvttmQWjUESD5XD 8i1Av0B1Juc+OKew5JQ+nZQfgd4Dv92QW1ffiHGeAPAax9IWigUCMvwydL3TpHwU6b aBRcpK5e/DAgVrTS+4lQH/e9YbxVLR6zRK3aQ+MRhAfmFVmd+4o0+WE3x0l1Ns83kL cePg3G3T9EINg== Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 16ADB35227D5; Mon, 22 Feb 2021 10:16:08 -0800 (PST) Date: Mon, 22 Feb 2021 10:16:08 -0800 From: "Paul E. McKenney" To: Uladzislau Rezki Cc: Sebastian Andrzej Siewior , Masami Hiramatsu , Ingo Molnar , Steven Rostedt , Peter Zijlstra , Thomas Gleixner , LKML , RCU , Michael Ellerman , Andrew Morton , Daniel Axtens , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Michal Hocko , "Theodore Y . Ts'o" , Oleksiy Avramchenko Subject: Re: [PATCH] kprobes: Fix to delay the kprobes jump optimization Message-ID: <20210222181608.GK2743@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20210219111301.GA34441@pc638.lan> <20210219111738.go6i2fdzvavpotxd@linutronix.de> <20210219112357.GA34462@pc638.lan> <20210219112751.GA34528@pc638.lan> <20210219181811.GY2743@paulmck-ThinkPad-P72> <20210219183336.GA23049@paulmck-ThinkPad-P72> <20210222102104.v3pr7t57hmpwijpi@linutronix.de> <20210222125431.GA41939@pc638.lan> <20210222150903.GH2743@paulmck-ThinkPad-P72> <20210222171605.GA42169@pc638.lan> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20210222171605.GA42169@pc638.lan> User-Agent: Mutt/1.9.4 (2018-02-28) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Feb 22, 2021 at 06:16:05PM +0100, Uladzislau Rezki wrote: > On Mon, Feb 22, 2021 at 07:09:03AM -0800, Paul E. McKenney wrote: > > On Mon, Feb 22, 2021 at 01:54:31PM +0100, Uladzislau Rezki wrote: > > > On Mon, Feb 22, 2021 at 11:21:04AM +0100, Sebastian Andrzej Siewior wrote: > > > > On 2021-02-19 10:33:36 [-0800], Paul E. McKenney wrote: > > > > > For definiteness, here is the first part of the change, posted earlier. > > > > > The commit log needs to be updated. I will post the change that keeps > > > > > the tick going as a reply to this email. > > > > … > > > > > diff --git a/kernel/softirq.c b/kernel/softirq.c > > > > > index 9d71046..ba78e63 100644 > > > > > --- a/kernel/softirq.c > > > > > +++ b/kernel/softirq.c > > > > > @@ -209,7 +209,7 @@ static inline void invoke_softirq(void) > > > > > if (ksoftirqd_running(local_softirq_pending())) > > > > > return; > > > > > > > > > > - if (!force_irqthreads) { > > > > > + if (!force_irqthreads || !__this_cpu_read(ksoftirqd)) { > > > > > #ifdef CONFIG_HAVE_IRQ_EXIT_ON_IRQ_STACK > > > > > /* > > > > > * We can safely execute softirq on the current stack if > > > > > @@ -358,8 +358,8 @@ asmlinkage __visible void __softirq_entry __do_softirq(void) > > > > > > > > > > pending = local_softirq_pending(); > > > > > if (pending) { > > > > > - if (time_before(jiffies, end) && !need_resched() && > > > > > - --max_restart) > > > > > + if (!__this_cpu_read(ksoftirqd) || > > > > > + (time_before(jiffies, end) && !need_resched() && --max_restart)) > > > > > goto restart; > > > > > > > > This is hunk shouldn't be needed. The reason for it is probably that the > > > > following wakeup_softirqd() would avoid further invoke_softirq() > > > > performing the actual softirq work. It would leave early due to > > > > ksoftirqd_running(). Unless I'm wrong, any raise_softirq() invocation > > > > outside of an interrupt would do the same. > > > > And it does pass the rcutorture test without that hunk: > > > > tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --duration 2 --configs "TREE03" --kconfig "CONFIG_DEBUG_LOCK_ALLOC=y CONFIG_PROVE_LOCKING=y" --bootargs "threadirqs=1" --trust-make > > > Yep. I have tested that patch also. It works for me as well. So > technically i do not see any issues from the first glance but of > course it should be reviewed by the softirq people to hear their > opinion. > > IRQs are enabled, so it can be handled from an IRQ tail until > ksoftirqd threads are spawned. And if I add "CONFIG_NO_HZ_IDLE=y CONFIG_HZ_PERIODIC=n" it still works, even if I revert my changes to rcu_needs_cpu(). Should I rely on this working globally? ;-) Thanx, Paul > > > > I would like PeterZ / tglx to comment on this one. Basically I'm not > > > > sure if it is okay to expect softirqs beeing served and waited on that > > > > early in the boot. > > > > It would be good to get other eyes on this. > > > > I do agree that "don't wait on softirq handlers until after completion > > of all early_initcall() handlers" is a nice simple rule, but debugging > > violations of it is not so simple. Adding warnings to ease debugging > > of violations of this rule is quite a bit more complex than is either of > > the methods of making the rule unnecessary, at least from what I can see > > at this point. The complexity of the warnings is exactly what Sebastian > > pointed out earlier, that it is currently legal to raise_softirq() but > > not to wait on the resulting handlers. But even waiting is OK if that > > waiting does not delay the boot sequence. But if the boot kthread waits > > on the kthread that does the waiting, it is once again not OK. > > > > So am I missing something subtle here? > > > I agree here. Seems like we are on the same page in understanding :) > > > > The ksoftirqd threads get spawned during early_initcall() phase. Why not > > > just spawn them one step earlier what is totally safe? I mean before > > > do_pre_smp_initcalls() that calls early callbacks. > > > > > > + spawn_ksoftirqd(); > > > rcu_init_tasks_generic(); > > > do_pre_smp_initcalls(); > > > > > > With such change the spawning will not be depended on linker/compiler > > > i.e. when and in which order an early_initcall(spawn_ksoftirqd) callback > > > is executed. > > > > We both posted patches similar to this, so I am not opposed. One caveat, > > though, namely that this narrows the window quite a bit but does not > > entirely close it. But it does allow the early_initcall()s to wait on > > softirq handlers. > > > Yep, that was an intention. At least to provide such functionality for early > callbacks. What happens before it(init/main.c) is pretty controllable. > > -- > Vlad Rezki