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=-8.6 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SIGNED_OFF_BY, 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 F032EC433DF for ; Fri, 31 Jul 2020 03:48:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CEEE3208E4 for ; Fri, 31 Jul 2020 03:48:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1596167307; bh=r0905SOBnmpQMVEbvend0u/6eg1GHliF8EiBpEwbut4=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=AlA6CqCrSHB7NKh+Lp1IF2W+8G5DGHh1Bgwfu7y7RtJJgKN08/3uhDla5e9mn7FHA gI3+YEdNOdoUqncN9P6Dp8u6EUbYlw1ovKFh1uB9tftNab076AFITgHycvTkDsdXlx Yz1E3XKmBzoaQEo6GnrGWu9R8QxLkD0qajn1cMBA= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731308AbgGaDs0 (ORCPT ); Thu, 30 Jul 2020 23:48:26 -0400 Received: from mail.kernel.org ([198.145.29.99]:51968 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731165AbgGaDs0 (ORCPT ); Thu, 30 Jul 2020 23:48:26 -0400 Received: from paulmck-ThinkPad-P72.home (unknown [50.45.173.55]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 79EA820829; Fri, 31 Jul 2020 03:48:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1596167305; bh=r0905SOBnmpQMVEbvend0u/6eg1GHliF8EiBpEwbut4=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=g7eDudtJo2qR+/Zy7Jt6DA450rVnd41ddQqbDMvavch46nmN+kB+tQvI0u76OS43P XtIyD1U+D0hTchG9Q/o9N2ACBFqiR/lGVHP+kq1BQ27SZ4cCSzm7bLLytrIDzDA3oA CbLMjP8G+hX9aXfl8KerDCszA0T7L37P5HZ1mmqE= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 5218D3521361; Thu, 30 Jul 2020 20:48:25 -0700 (PDT) Date: Thu, 30 Jul 2020 20:48:25 -0700 From: "Paul E. McKenney" To: Joel Fernandes Cc: linux-kernel@vger.kernel.org, Neeraj Upadhyay , Josh Triplett , Lai Jiangshan , Mathieu Desnoyers , rcu@vger.kernel.org, Steven Rostedt Subject: Re: [PATCH 1/2] rcu/tree: Add a warning if CPU being onlined did not report QS already Message-ID: <20200731034825.GI9247@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200730030221.705255-1-joel@joelfernandes.org> <20200730162159.GZ9247@paulmck-ThinkPad-P72> <20200731014222.GA2349603@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200731014222.GA2349603@google.com> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jul 30, 2020 at 09:42:22PM -0400, Joel Fernandes wrote: > On Thu, Jul 30, 2020 at 09:21:59AM -0700, Paul E. McKenney wrote: > > On Wed, Jul 29, 2020 at 11:02:20PM -0400, Joel Fernandes (Google) wrote: > > > Add a warning if CPU being onlined did not report QS already. This is to > > > simplify the code in the CPU onlining path and also to make clear about > > > where QS is reported. The act of QS reporting in CPU onlining path is > > > is likely unnecessary as shown by code reading and testing with > > > rcutorture's TREE03 and hotplug parameters. > > > > How about something like this for the commit log? > > > > ------------------------------------------------------------------------ > > > > Currently, rcu_cpu_starting() checks to see if the RCU core expects a > > quiescent state from the incoming CPU. However, the current interaction > > between RCU quiescent-state reporting and CPU-hotplug operations should > > mean that the incoming CPU never needs to report a quiescent state. > > First, the outgoing CPU reports a quiescent state if needed. Second, > > the race where the CPU is leaving just as RCU is initializing a new > > grace period is handled by an explicit check for this condition. Third, > > the CPU's leaf rcu_node structure's ->lock serializes these checks. > > > > This means that if rcu_cpu_starting() ever feels the need to report > > a quiescent state, then there is a bug somewhere in the CPU hotplug > > code or the RCU grace-period handling code. This commit therefore > > adds a WARN_ON_ONCE() to bring that bug to everyone's attention. > > > > ------------------------------------------------------------------------ > > > > > Cc: Paul E. McKenney > > > Cc: Neeraj Upadhyay > > > Suggested-by: Paul E. McKenney > > > Signed-off-by: Joel Fernandes (Google) > > > > > > --- > > > kernel/rcu/tree.c | 14 +++++++++++++- > > > 1 file changed, 13 insertions(+), 1 deletion(-) > > > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > > index 65e1b5e92319..1e51962b565b 100644 > > > --- a/kernel/rcu/tree.c > > > +++ b/kernel/rcu/tree.c > > > @@ -3996,7 +3996,19 @@ void rcu_cpu_starting(unsigned int cpu) > > > rcu_gpnum_ovf(rnp, rdp); /* Offline-induced counter wrap? */ > > > rdp->rcu_onl_gp_seq = READ_ONCE(rcu_state.gp_seq); > > > rdp->rcu_onl_gp_flags = READ_ONCE(rcu_state.gp_flags); > > > - if (rnp->qsmask & mask) { /* RCU waiting on incoming CPU? */ > > > + > > > + /* > > > + * Delete QS reporting from here, by June 2021, if warning does not > > > + * fire. Let us make the rules for reporting QS for an offline CPUs > > > + * more explicit. The CPU onlining path does not need to report QS for > > > + * an offline CPU. Either the QS should have reported during CPU > > > + * offlining, or during rcu_gp_init() if it detected a race with either > > > + * CPU offlining or task unblocking on previously offlined CPUs. Note > > > + * that the FQS loop also does not report QS for an offline CPU any > > > + * longer (unless it splats due to an offline CPU blocking the GP for > > > + * too long). > > > + */ > > > > Let's leave at least the WARN_ON_ONCE() indefinitely. If you don't > > believe me, remove this code in your local tree, have someone give you > > several branches, some with bugs injected, and then try to figure out > > which have the bugs and then try to find those bugs. > > > > This is not a fastpath, so the overhead of the check is not a concern. > > Believe me, the difficulty of bug location without this check is a very > > real concern! ;-) > > > > On the other hand, I fully agree with the benefits of documenting the > > design rules. But is this really the best place to do that from the > > viewpoint of someone who is trying to figure out how RCU works? > > I can move this comment to: "Hotplug CPU" section in > Documentation/RCU/Design/Requirements/Requirements.rst That sounds like a better place for it, very good! > And I could make the comment here as: > /* > * Delete QS reporting from here, by June 2021, if the warning does not > * fire. Leave the warning indefinitely. Check RCU design requirements > * in Documentation/RCU/ about CPU hotplug requirements. > */ Rather than decide for our future selves, could we please just suggest reviewing this on June 2021? Or, given enterprise distro schedules, 2024. :-/ Thanx, Paul > I will post my v3 with changes to the requirements document. > > Let me know any other comments, thanks, > > - Joel >