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=-17.8 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,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 B4421C433DB for ; Thu, 28 Jan 2021 01:29:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5DE9D64DD8 for ; Thu, 28 Jan 2021 01:29:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231243AbhA1B3U (ORCPT ); Wed, 27 Jan 2021 20:29:20 -0500 Received: from mail.kernel.org ([198.145.29.99]:38108 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229757AbhA1B3B (ORCPT ); Wed, 27 Jan 2021 20:29:01 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id A79EF60C40; Thu, 28 Jan 2021 01:28:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1611797299; bh=KPsCXP6h6RkH/EuOMyCsVxSyeQZP04vDgwTNMIY3TwQ=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=SXx0R6hkU5gI5PpUuuZpNDFFB6ERh42bmzRkHpmtdg9Bqk0+czLiFvpyCQJ6puf+3 pNFHpxGOSK49R3/yjgxMR1p0jAGdAbf5nDfZvm8HtsZ+sVsrPuYTyFV8ve7H5SBw9e 5vSH9XnNhnfBrzSlH96IoExC0uxArD1Hi6q5X65BIFHxwktnVLSUyMlhf4K055rLs+ WlAFQUPc3EcMSmosMEVdp9Z5QEKmxf2Eg/hfpFSWgQt+x9AAEuMbHCwmlkNuPdQPeV RtSQHM139MVGL8L/e4Qj0OW52Nv9K146agHET3aHXPsgNpYVyRk2LAdMKwVUOsUASv 1xE780YOkwEMw== Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 48C1B352260C; Wed, 27 Jan 2021 17:28:19 -0800 (PST) Date: Wed, 27 Jan 2021 17:28:19 -0800 From: "Paul E. McKenney" To: Boqun Feng 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, Sebastian Andrzej Siewior , Scott Wood Subject: Re: [PATCH tip/core/rcu 1/4] rcu: Expedite deboost in case of deferred quiescent state Message-ID: <20210128012819.GN2743@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20210120043144.GA7045@paulmck-ThinkPad-P72> <20210120043236.7254-1-paulmck@kernel.org> <20210127044024.GI2743@paulmck-ThinkPad-P72> <20210127191831.GM2743@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.4 (2018-02-28) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jan 28, 2021 at 09:11:20AM +0800, Boqun Feng wrote: > On Wed, Jan 27, 2021 at 11:18:31AM -0800, Paul E. McKenney wrote: > > On Wed, Jan 27, 2021 at 03:05:24PM +0800, Boqun Feng wrote: > > > On Tue, Jan 26, 2021 at 08:40:24PM -0800, Paul E. McKenney wrote: > > > > On Wed, Jan 27, 2021 at 10:42:35AM +0800, Boqun Feng wrote: > > > > > Hi Paul, > > > > > > > > > > On Tue, Jan 19, 2021 at 08:32:33PM -0800, paulmck@kernel.org wrote: > > > > > > From: "Paul E. McKenney" > > > > > > > > > > > > Historically, a task that has been subjected to RCU priority boosting is > > > > > > deboosted at rcu_read_unlock() time. However, with the advent of deferred > > > > > > quiescent states, if the outermost rcu_read_unlock() was invoked with > > > > > > either bottom halves, interrupts, or preemption disabled, the deboosting > > > > > > will be delayed for some time. During this time, a low-priority process > > > > > > might be incorrectly running at a high real-time priority level. > > > > > > > > > > > > Fortunately, rcu_read_unlock_special() already provides mechanisms for > > > > > > forcing a minimal deferral of quiescent states, at least for kernels > > > > > > built with CONFIG_IRQ_WORK=y. These mechanisms are currently used > > > > > > when expedited grace periods are pending that might be blocked by the > > > > > > current task. This commit therefore causes those mechanisms to also be > > > > > > used in cases where the current task has been or might soon be subjected > > > > > > to RCU priority boosting. Note that this applies to all kernels built > > > > > > with CONFIG_RCU_BOOST=y, regardless of whether or not they are also > > > > > > built with CONFIG_PREEMPT_RT=y. > > > > > > > > > > > > This approach assumes that kernels build for use with aggressive real-time > > > > > > applications are built with CONFIG_IRQ_WORK=y. It is likely to be far > > > > > > simpler to enable CONFIG_IRQ_WORK=y than to implement a fast-deboosting > > > > > > scheme that works correctly in its absence. > > > > > > > > > > > > While in the area, alphabetize the rcu_preempt_deferred_qs_handler() > > > > > > function's local variables. > > > > > > > > > > > > Cc: Sebastian Andrzej Siewior > > > > > > Cc: Scott Wood > > > > > > Cc: Lai Jiangshan > > > > > > Cc: Thomas Gleixner > > > > > > Signed-off-by: Paul E. McKenney > > > > > > --- > > > > > > kernel/rcu/tree_plugin.h | 26 ++++++++++++++------------ > > > > > > 1 file changed, 14 insertions(+), 12 deletions(-) > > > > > > > > > > > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > > > > > > index 8b0feb2..fca31c6 100644 > > > > > > --- a/kernel/rcu/tree_plugin.h > > > > > > +++ b/kernel/rcu/tree_plugin.h > > > > > > @@ -660,9 +660,9 @@ static void rcu_preempt_deferred_qs_handler(struct irq_work *iwp) > > > > > > static void rcu_read_unlock_special(struct task_struct *t) > > > > > > { > > > > > > unsigned long flags; > > > > > > + bool irqs_were_disabled; > > > > > > bool preempt_bh_were_disabled = > > > > > > !!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)); > > > > > > - bool irqs_were_disabled; > > > > > > > > > > > > /* NMI handlers cannot block and cannot safely manipulate state. */ > > > > > > if (in_nmi()) > > > > > > @@ -671,30 +671,32 @@ static void rcu_read_unlock_special(struct task_struct *t) > > > > > > local_irq_save(flags); > > > > > > irqs_were_disabled = irqs_disabled_flags(flags); > > > > > > if (preempt_bh_were_disabled || irqs_were_disabled) { > > > > > > - bool exp; > > > > > > + bool expboost; // Expedited GP in flight or possible boosting. > > > > > > struct rcu_data *rdp = this_cpu_ptr(&rcu_data); > > > > > > struct rcu_node *rnp = rdp->mynode; > > > > > > > > > > > > - exp = (t->rcu_blocked_node && > > > > > > - READ_ONCE(t->rcu_blocked_node->exp_tasks)) || > > > > > > - (rdp->grpmask & READ_ONCE(rnp->expmask)); > > > > > > + expboost = (t->rcu_blocked_node && READ_ONCE(t->rcu_blocked_node->exp_tasks)) || > > > > > > + (rdp->grpmask & READ_ONCE(rnp->expmask)) || > > > > > > + (IS_ENABLED(CONFIG_RCU_BOOST) && irqs_were_disabled && > > > > > > + t->rcu_blocked_node); > > > > > > > > > > I take it that you check whether possible boosting is in progress via > > > > > the last expression of "||", ie: > > > > > > > > > > (IS_ENABLED(CONFIG_RCU_BOOST) && irqs_were_disabled && > > > > > t->rcu_blocked_node) > > > > > > > > > > if so, I don't see the point of using the new "expboost" in the > > > > > raise_softirq_irqoff() branch, because if in_irq() is false, we only > > > > > raise softirq if irqs_were_disabled is false (otherwise, we may take the > > > > > risk of doing a wakeup with a pi or rq lock held, IIRC), and the > > > > > boosting part of the "expboost" above is only true if irqs_were_disabled > > > > > is true, so using expboost makes no different here. > > > > > > > > I started out with two local variables, one for each side of the ||, > > > > but this looked nicer. > > > > > > > > > > // Need to defer quiescent state until everything is enabled. > > > > > > - if (use_softirq && (in_irq() || (exp && !irqs_were_disabled))) { > > > > > > + if (use_softirq && (in_irq() || (expboost && !irqs_were_disabled))) { > > > > > > // Using softirq, safe to awaken, and either the > > > > > > - // wakeup is free or there is an expedited GP. > > > > > > + // wakeup is free or there is either an expedited > > > > > > + // GP in flight or a potential need to deboost. > > > > > > > > > > and this comment will be incorrect, we won't enter here solely because > > > > > there is a potential need to deboost. > > > > > > > > You are quite right, given the !irqs_were_disabled. > > > > > > > > > That said, why the boosting condition has a "irqs_were_disabled" in it? > > > > > What if a task gets boosted because of RCU boosting, and exit the RCU > > > > > read-side c.s. with irq enabled and there is no expedited GP in flight, > > > > > will the task get deboosted quickly enough? > > > > > > > > Because if !irqs_were_disabled, there will be a local_bh_enable() or > > > > a preempt_enable(), give or take preempt_enable_no_resched(), and those > > > > will both get the scheduler involved because of the set_tsk_need_resched() > > > > and set_preempt_need_resched(). There is thus no need for the irq_work > > > > unless irqs_were_disabled. > > > > > > But if so, shouldn't we check !preemp_bh_were_disabled instead of > > > irqs_were_disabled? I.e. there is no need for the irq_work unless > > > preemption and bottom halves are all enabled (IOW, we cannot expect the > > > task to get into scheduler soon via *_enable()). > > > > > > Current what we are doing is if irqs_were_disabled is true (along with > > > other checks pass), we queue a irq work, but in this situation, > > > preept_bh_were_disabled might be true as well, which means there may be > > > a preempt_enable() not far away. > > > > Except that the preempt_enable() might be executed before the > > local_irq_enable(), in which case the scheduler would not be invoked. > > > > > Consider the following simple example, if we have a in-flight gp or > > > this task has been boosted: > > > > > > preempt_disable(); > > > local_irq_disable(); > > > rcu_read_lock(); > > > ... > > > rcu_read_unlock(); > > > // current we will queue a irq work here. > > > local_irq_enable(); > > > preempt_enable(); > > > // but we will enter scheduelr here./ > > > > And consider this variation: > > > > preempt_disable(); > > local_irq_disable(); > > rcu_read_lock(); > > ... > > rcu_read_unlock(); > > // Currently, we will queue a irq work here... > > preempt_enable(); > > // ...because we cannot yet enter the scheduler. > > local_irq_enable(); > > // Now we can but we won't because no irq_work has been scheduled. > > > > Fair enough! > > But what stops the following from happening? > > preempt_disable(); > rcu_read_lock(); > ... > rcu_read_unlock(); > // irq is not disabled, so no irq_work. > > local_irq_disable(); > preempt_enable(); > // we cannot enter the scheduler > local_irq_enable(); > // Now we can but we won't because no irq_work has been scheduled > > Am I too paranoid and nobody writes code like the above? ;-) If they do write that sort of code, then a resched IPI received in the middle of the above RCU read-side critical section will be ignored until some time after the local_irq_enable(). So letting the same thing happen for RCU priority deboosting isn't making anything worse for real time. But good job finding this possibility! ;-) Thanx, Paul > Regards, > Boqun > > > However, the expboost condition is missing one thing, namely strict > > grace periods. It should read as follows: > > > > expboost = (t->rcu_blocked_node && READ_ONCE(t->rcu_blocked_node->exp_tasks)) || > > (rdp->grpmask & READ_ONCE(rnp->expmask)) || > > IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) || > > (IS_ENABLED(CONFIG_RCU_BOOST) && irqs_were_disabled && > > t->rcu_blocked_node); > > > > Not that this has anything to do with RCU priority boosting, but while > > in the area... > > > > Thanx, Paul > > > > > > I am not all that worried about preempt_enable_no_resched() because > > > > it is a problem for RT even in the absence of RCU priority boosting. > > > > And the current uses appear to be in things that one would not use while > > > > running an RT workload. > > > > > > > > > Maybe I'm missing some subtle? > > > > > > > > Or maybe I am. Thoughts? > > > > > > > > Thanx, Paul > > > > > > > > > Regards, > > > > > Boqun > > > > > > > > > > > raise_softirq_irqoff(RCU_SOFTIRQ); > > > > > > } else { > > > > > > // Enabling BH or preempt does reschedule, so... > > > > > > - // Also if no expediting, slow is OK. > > > > > > - // Plus nohz_full CPUs eventually get tick enabled. > > > > > > + // Also if no expediting and no possible deboosting, > > > > > > + // slow is OK. Plus nohz_full CPUs eventually get > > > > > > + // tick enabled. > > > > > > set_tsk_need_resched(current); > > > > > > set_preempt_need_resched(); > > > > > > if (IS_ENABLED(CONFIG_IRQ_WORK) && irqs_were_disabled && > > > > > > so the irqs_were_disabled here should be !preempt_bh_were_disabled? > > > > > > Regards, > > > Boqun > > > > > > > > > - !rdp->defer_qs_iw_pending && exp && cpu_online(rdp->cpu)) { > > > > > > + expboost && !rdp->defer_qs_iw_pending && cpu_online(rdp->cpu)) { > > > > > > // Get scheduler to re-evaluate and call hooks. > > > > > > // If !IRQ_WORK, FQS scan will eventually IPI. > > > > > > - init_irq_work(&rdp->defer_qs_iw, > > > > > > - rcu_preempt_deferred_qs_handler); > > > > > > + init_irq_work(&rdp->defer_qs_iw, rcu_preempt_deferred_qs_handler); > > > > > > rdp->defer_qs_iw_pending = true; > > > > > > irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu); > > > > > > } > > > > > > -- > > > > > > 2.9.5 > > > > > >