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 9FA84C433EF for ; Tue, 15 Feb 2022 17:39:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240021AbiBORkF (ORCPT ); Tue, 15 Feb 2022 12:40:05 -0500 Received: from mxb-00190b01.gslb.pphosted.com ([23.128.96.19]:51518 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232294AbiBORkD (ORCPT ); Tue, 15 Feb 2022 12:40:03 -0500 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id DFEC675E5E; Tue, 15 Feb 2022 09:39:52 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 75C0C615F4; Tue, 15 Feb 2022 17:39:52 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id D7D5AC340EB; Tue, 15 Feb 2022 17:39:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1644946791; bh=nUNc9NCmuYO7Z96Z2aJ5YseB2bw6sCO4T+CqaRXfLy4=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=ZHqzvRuWz09Sq3BLyc32W2AQzBDS73OTft6+Uift/wx0NxR5LdMjzdYMUVKx0sWne XqUOfNdHVnP8aBq9L5xIEtYb76fq3/AF8sdOVJ/jJTVGQkL4tSAy+dseNUn8A5nGR1 PtxZtxOha7N2Pvdyayh7RAdOKnCNxpAS/60YkCx3FLfjuepKk+aSHaMpUwYearAbBh IGnWxcPvMvWibambSQXXCFGND5ri42Q7j35Fu0gxuCXXw5hNdDyQ+LNf1eL3nti/3s NKaFnR7vYFX5ry8jjp4eXuq4nemZ0yJ/YQYKtz846BjUlW0Xea4xOHAtGhE9CuPnLS AfUZ7a2l6i4Yw== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 9A0415C0641; Tue, 15 Feb 2022 09:39:51 -0800 (PST) Date: Tue, 15 Feb 2022 09:39:51 -0800 From: "Paul E. McKenney" To: Mukesh Ojha Cc: kernel-team@fb.com, linux-kernel@vger.kernel.org, rcu@vger.kernel.org, rostedt@goodmis.org, tj@kernel.org, Peter Zijlstra Subject: Re: [PATCH rcu 3/3] rcu: Allow expedited RCU grace periods on incoming CPUs Message-ID: <20220215173951.GH4285@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20220209233811.GC557593@lothringen> <20220214164435.GA2805255@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Feb 15, 2022 at 07:53:10PM +0530, Mukesh Ojha wrote: > > On 2/14/2022 10:14 PM, Paul E. McKenney wrote: > > On Thu, Feb 10, 2022 at 12:38:11AM +0100, Frederic Weisbecker wrote: > > > On Fri, Feb 04, 2022 at 02:55:07PM -0800, Paul E. McKenney wrote: > > > > Although it is usually safe to invoke synchronize_rcu_expedited() from a > > > > preemption-enabled CPU-hotplug notifier, if it is invoked from a notifier > > > > between CPUHP_AP_RCUTREE_ONLINE and CPUHP_AP_ACTIVE, its attempts to > > > > invoke a workqueue handler will hang due to RCU waiting on a CPU that > > > > the scheduler is not paying attention to. This commit therefore expands > > > > use of the existing workqueue-independent synchronize_rcu_expedited() > > > > from early boot to also include CPUs that are being hotplugged. > > > > > > > > Link:https://lore.kernel.org/lkml/7359f994-8aaf-3cea-f5cf-c0d3929689d6@quicinc.com/ > > > > Reported-by: Mukesh Ojha > > > > Cc: Tejun Heo > > > > Signed-off-by: Paul E. McKenney > > > I'm surprised by this scheduler behaviour. > > > > > > Since sched_cpu_activate() hasn't been called yet, > > > rq->balance_callback = balance_push_callback. As a result, balance_push() should > > > be called at the end of schedule() when the workqueue is picked as the next task. > > > Then eventually the workqueue should be immediately preempted by the stop task to > > > be migrated elsewhere. > > > > > > So I must be missing something. For the fun, I booted the following and it > > > didn't produce any issue: > > > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > > index 80faf2273ce9..b1e74a508881 100644 > > > --- a/kernel/rcu/tree.c > > > +++ b/kernel/rcu/tree.c > > > @@ -4234,6 +4234,8 @@ int rcutree_online_cpu(unsigned int cpu) > > > // Stop-machine done, so allow nohz_full to disable tick. > > > tick_dep_clear(TICK_DEP_BIT_RCU); > > > + if (cpu != 0) > > > + synchronize_rcu_expedited(); > > > return 0; > > > } > > That does seem compelling. And others have argued that the workqueue > > system's handling of offline CPUs should deal with this. > > > > Mukesh, was this a theoretical bug, or did you actually make it happen? > > If you made it happen, as seems to have been the case given your original > > email [1], could you please post your reproducer? > > No, it was not theoretical one. We saw this issue only once in our testing > and i don't think it is easy to reproduce otherwise > it would been fixed by now. > > When one of thread calling synchronize_expedite_rcu with timer of 20s but it > did not get the exp funnel > lock for 20s and there we crash it with panic() on timeout. > > The other thread cpuhp which was having the lock got stuck at the point > mentioned at the below link. > > https://lore.kernel.org/lkml/7359f994-8aaf-3cea-f5cf-c0d3929689d6@quicinc.com/ OK. Are you able to create an in-kernel reproducer, perhaps similar to Frederic's change above? I am worried that the patch that I am carrying might be fixing some other bug by accident... Thanx, Paul > e.g Below sample test in combination of many other test in parallel > > :loop > > adb shell "echo 0 > /sys/devices/system/cpu/cpu0/online" > > adb shell "echo 1 > /sys/devices/system/cpu/cpu0/online" > > adb shell "echo 0 > /sys/devices/system/cpu/cpu1/online" > > adb shell "echo 1 > /sys/devices/system/cpu/cpu1/online" > > adb shell "echo 0 > /sys/devices/system/cpu/cpu2/online" > > adb shell "echo 1 > /sys/devices/system/cpu/cpu2/online" > > adb shell "echo 0 > /sys/devices/system/cpu/cpu3/online" > > adb shell "echo 1 > /sys/devices/system/cpu/cpu3/online" > > adb shell "echo 0 > /sys/devices/system/cpu/cpu4/online" > > adb shell "echo 1 > /sys/devices/system/cpu/cpu4/online" > > adb shell "echo 0 > /sys/devices/system/cpu/cpu5/online" > > adb shell "echo 1 > /sys/devices/system/cpu/cpu5/online" > > adb shell "echo 0 > /sys/devices/system/cpu/cpu6/online" > > adb shell "echo 1 > /sys/devices/system/cpu/cpu6/online" > > adb shell "echo 0 > /sys/devices/system/cpu/cpu7/online" > > adb shell "echo 1 > /sys/devices/system/cpu/cpu7/online" > > goto loop > > > > Thanks, Mukesh > > > > > Thanx, Paul > > > > [1]https://lore.kernel.org/lkml/7359f994-8aaf-3cea-f5cf-c0d3929689d6@quicinc.com/