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=-16.7 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,URIBL_BLOCKED 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 60387C432BE for ; Thu, 29 Jul 2021 01:04:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4032B60F91 for ; Thu, 29 Jul 2021 01:04:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233118AbhG2BEt (ORCPT ); Wed, 28 Jul 2021 21:04:49 -0400 Received: from mail.kernel.org ([198.145.29.99]:54528 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232837AbhG2BEs (ORCPT ); Wed, 28 Jul 2021 21:04:48 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id C6BA56054E; Thu, 29 Jul 2021 01:04:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1627520685; bh=6uCA6dq+V8ph083FJUgVYiyyZQvKKUJPCn1QR6+u9m4=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=uqy5xdH22H5q5F29Kx5oD7z26+y3Sd+n7TvVDBkRwKJWqy73l1if9EKoPtaFeJ5+D n48Ii9NKvW3cjoYp5NQv17dP06J+atMCkna8hG9qsUCodAABSafAQ2QLcXWA9Kz0rl sTNvuAozBOJnOTQzS/QsmHIHHdqQsTdkqnHOcdNTucDt5IcGclUiOn/ZO7Gm/rkSIO GYq6vFrhLyenM8s+PJWGONeaXVfZNsOn9nGb6PJlErcRxmHKt4g2sqCmEebAdTR6L9 mMnjgs+i1VRdvijzwcic/wbSl69gnO05H24+sZGMFlBzDmSpqY+0MCyI5OLOCaPmZ5 29o7pehIyE4vQ== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 93B405C048D; Wed, 28 Jul 2021 18:04:45 -0700 (PDT) Date: Wed, 28 Jul 2021 18:04:45 -0700 From: "Paul E. McKenney" To: Frederic Weisbecker Cc: Valentin Schneider , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-rt-users@vger.kernel.org, Catalin Marinas , Will Deacon , Ingo Molnar , Peter Zijlstra , Thomas Gleixner , Steven Rostedt , Daniel Bristot de Oliveira , Josh Triplett , Mathieu Desnoyers , Lai Jiangshan , Joel Fernandes , Anshuman Khandual , Vincenzo Frascino , Steven Price , Ard Biesheuvel , Sebastian Andrzej Siewior Subject: Re: [PATCH 2/3] rcu/nocb: Check for migratability rather than pure preemptability Message-ID: <20210729010445.GO4397@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20210721115118.729943-1-valentin.schneider@arm.com> <20210721115118.729943-3-valentin.schneider@arm.com> <20210727230814.GC283787@lothringen> <87pmv2kzbd.mognet@arm.com> <20210728220137.GD293265@lothringen> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210728220137.GD293265@lothringen> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jul 29, 2021 at 12:01:37AM +0200, Frederic Weisbecker wrote: > On Wed, Jul 28, 2021 at 08:34:14PM +0100, Valentin Schneider wrote: > > On 28/07/21 01:08, Frederic Weisbecker wrote: > > > On Wed, Jul 21, 2021 at 12:51:17PM +0100, Valentin Schneider wrote: > > >> Signed-off-by: Valentin Schneider > > >> --- > > >> kernel/rcu/tree_plugin.h | 3 +-- > > >> 1 file changed, 1 insertion(+), 2 deletions(-) > > >> > > >> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > > >> index ad0156b86937..6c3c4100da83 100644 > > >> --- a/kernel/rcu/tree_plugin.h > > >> +++ b/kernel/rcu/tree_plugin.h > > >> @@ -70,8 +70,7 @@ static bool rcu_rdp_is_offloaded(struct rcu_data *rdp) > > >> !(lockdep_is_held(&rcu_state.barrier_mutex) || > > >> (IS_ENABLED(CONFIG_HOTPLUG_CPU) && lockdep_is_cpus_held()) || > > >> rcu_lockdep_is_held_nocb(rdp) || > > >> - (rdp == this_cpu_ptr(&rcu_data) && > > >> - !(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible())) || > > >> + (rdp == this_cpu_ptr(&rcu_data) && is_pcpu_safe()) || > > > > > > I fear that won't work. We really need any caller of rcu_rdp_is_offloaded() > > > on the local rdp to have preemption disabled and not just migration disabled, > > > because we must protect against concurrent offloaded state changes. > > > > > > The offloaded state is changed by a workqueue that executes on the target rdp. > > > > > > Here is a practical example where it matters: > > > > > > CPU 0 > > > ----- > > > // =======> task rcuc running > > > rcu_core { > > > rcu_nocb_lock_irqsave(rdp, flags) { > > > if (!rcu_segcblist_is_offloaded(rdp->cblist)) { > > > // is not offloaded right now, so it's going > > > // to just disable IRQs. Oh no wait: > > > // preemption > > > // ========> workqueue running > > > rcu_nocb_rdp_offload(); > > > // ========> task rcuc resume > > > local_irq_disable(); > > > } > > > } > > > .... > > > rcu_nocb_unlock_irqrestore(rdp, flags) { > > > if (rcu_segcblist_is_offloaded(rdp->cblist)) { > > > // is offloaded right now so: > > > raw_spin_unlock_irqrestore(rdp, flags); > > > > > > And that will explode because that's an impaired unlock on nocb_lock. > > > > Harumph, that doesn't look good, thanks for pointing this out. > > > > AFAICT PREEMPT_RT doesn't actually require to disable softirqs here (since > > it forces RCU callbacks on the RCU kthreads), but disabled softirqs seem to > > be a requirement for much of the underlying functions and even some of the > > callbacks (delayed_put_task_struct() ~> vfree() pays close attention to > > in_interrupt() for instance). > > > > Now, if the offloaded state was (properly) protected by a local_lock, do > > you reckon we could then keep preemption enabled? > > I guess we could take such a local lock on the update side > (rcu_nocb_rdp_offload) and then take it on rcuc kthread/softirqs > and maybe other places. > > But we must make sure that rcu_core() is preempt-safe from a general perspective > in the first place. From a quick glance I can't find obvious issues...yet. > > Paul maybe you can see something? Let's see... o Extra context switches in rcu_core() mean extra quiescent states. It therefore might be necessary to wrap rcu_core() in an rcu_read_lock() / rcu_read_unlock() pair, because otherwise an RCU grace period won't wait for rcu_core(). Actually, better have local_bh_disable() imply rcu_read_lock() and local_bh_enable() imply rcu_read_unlock(). But I would hope that this already happened. o The rcu_preempt_deferred_qs() check should still be fine, unless there is a raw_bh_disable() in -rt. o The set_tsk_need_resched() and set_preempt_need_resched() might preempt immediately. I cannot think of a problem with that, but careful testing is clearly in order. o The values checked by rcu_check_quiescent_state() could now change while this function is running. I don't immediately see a problematic sequence of events, but here be dragons. I therefore suggest disabling preemption across this function. Or if that is impossible, taking a very careful look at the proposed expansion of the state space of this function. o I don't see any new races in the grace-period/callback check. New callbacks can appear in interrupt handlers, after all. o The rcu_check_gp_start_stall() function looks similarly unproblematic. o Callback invocation can now be preempted, but then again it recently started being concurrent, so this should be no added risk over offloading/de-offloading. o I don't see any problem with do_nocb_deferred_wakeup(). o The CONFIG_RCU_STRICT_GRACE_PERIOD check should not be impacted. So some adjustments might be needed, but I don't see a need for major surgery. This of course might be a failure of imagination on my part, so it wouldn't hurt to double-check my observations. > > From a naive outsider PoV, rdp->nocb_lock looks like a decent candidate, > > but it's a *raw* spinlock (I can't tell right now whether changing this is > > a horrible idea or not), and then there's > > Yeah that's not possible, nocb_lock is too low level and has to be called with > IRQs disabled. So if we take that local_lock solution, we need a new lock. No argument here! Thanx, Paul