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=-17.7 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,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 A67B1C433DB for ; Thu, 11 Mar 2021 23:35:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8198C64F8D for ; Thu, 11 Mar 2021 23:35:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230475AbhCKXew (ORCPT ); Thu, 11 Mar 2021 18:34:52 -0500 Received: from mail.kernel.org ([198.145.29.99]:59082 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229587AbhCKXeT (ORCPT ); Thu, 11 Mar 2021 18:34:19 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id 96BFE64F70; Thu, 11 Mar 2021 23:34:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1615505658; bh=+biSB8+9HumjYwjTHZj/++Nwwgtz3v88RjLI9E9ic8k=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=hNu33VXsHxiXTtn+zhJfbMEmZbuaGnUw4bKMwQgy+AGnsqfdC+b3EpRdv79JkhcQf rVdfegrw8i2QCFo5eyxqBhWGNzB1i24TyLKKUcv6xUBXqOf4bOvJUWg7+BeTNPycRK FqbIlUdQAOP+oxNvO0VzLZ8FPWcwnaUEQdgIbucSQ/VsOf64XC9ZnbUO/SALf4aHVS xjzgDMCHVcVX961qNEBPFlZyMixQFytwiQhl1kUbTXvTPgDv4wS4Th5vgLJa3XjnJL P+q+hE7GkQ8UUYOjm/TWoJX8IMFxQUicmin69VYDDqfjxDm95j7C3nrBPM+MkUyY5d sl5eJrcu3c9ZQ== Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 390C735239D7; Thu, 11 Mar 2021 15:34:18 -0800 (PST) Date: Thu, 11 Mar 2021 15:34:18 -0800 From: "Paul E. McKenney" To: Frederic Weisbecker 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 Subject: Re: [PATCH tip/core/rcu 07/10] rcu: Prevent dyntick-idle until ksoftirqd has been spawned Message-ID: <20210311233418.GB2696@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20210303235958.GA22373@paulmck-ThinkPad-P72> <20210304000019.22459-7-paulmck@kernel.org> <20210311232357.GA29548@lothringen> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210311232357.GA29548@lothringen> User-Agent: Mutt/1.9.4 (2018-02-28) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Mar 12, 2021 at 12:23:57AM +0100, Frederic Weisbecker wrote: > On Wed, Mar 03, 2021 at 04:00:16PM -0800, paulmck@kernel.org wrote: > > From: "Paul E. McKenney" > > > > After interrupts have enabled at boot but before some random point > > in early_initcall() processing, softirq processing is unreliable. > > If softirq sees a need to push softirq-handler invocation to ksoftirqd > > during this time, then those handlers can be delayed until the ksoftirqd > > kthreads have been spawned, which happens at some random point in the > > early_initcall() processing. In many cases, this delay is just fine. > > However, if the boot sequence blocks waiting for a wakeup from a softirq > > handler, this delay will result in a silent-hang deadlock. > > > > This commit therefore prevents these hangs by ensuring that the tick > > stays active until after the ksoftirqd kthreads have been spawned. > > This change causes the tick to eventually drain the backlog of delayed > > softirq handlers, breaking this deadlock. > > > > Signed-off-by: Paul E. McKenney > > --- > > kernel/rcu/tree_plugin.h | 11 +++++++++++ > > 1 file changed, 11 insertions(+) > > > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > > index 2d60377..36212de 100644 > > --- a/kernel/rcu/tree_plugin.h > > +++ b/kernel/rcu/tree_plugin.h > > @@ -1255,6 +1255,11 @@ static void rcu_prepare_kthreads(int cpu) > > */ > > int rcu_needs_cpu(u64 basemono, u64 *nextevt) > > { > > + /* Through early_initcall(), need tick for softirq handlers. */ > > + if (!IS_ENABLED(CONFIG_HZ_PERIODIC) && !this_cpu_ksoftirqd()) { > > + *nextevt = 1; > > + return 1; > > + } > > *nextevt = KTIME_MAX; > > return !rcu_segcblist_empty(&this_cpu_ptr(&rcu_data)->cblist) && > > !rcu_segcblist_is_offloaded(&this_cpu_ptr(&rcu_data)->cblist); > > @@ -1350,6 +1355,12 @@ int rcu_needs_cpu(u64 basemono, u64 *nextevt) > > > > lockdep_assert_irqs_disabled(); > > > > + /* Through early_initcall(), need tick for softirq handlers. */ > > + if (!IS_ENABLED(CONFIG_HZ_PERIODIC) && !this_cpu_ksoftirqd()) { > > + *nextevt = 1; > > + return 1; > > + } > > + > > /* If no non-offloaded callbacks, RCU doesn't need the CPU. */ > > if (rcu_segcblist_empty(&rdp->cblist) || > > rcu_segcblist_is_offloaded(&this_cpu_ptr(&rcu_data)->cblist)) { > > > I suspect rcutiny should be concerned as well? > > In fact this patch doesn't look necessary because can_stop_idle_tick() refuse > to stop the tick when softirqs are pending. So it does, thank you! I will drop this patch. Thanx, Paul