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=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, 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 182D1C43603 for ; Fri, 6 Dec 2019 03:10:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D7E9222525 for ; Fri, 6 Dec 2019 03:10:30 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1575601831; bh=rQhhau5BVSh7TTOxVEk8rglm0/OjQ5fR+C28jbh+rDU=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=wviB6rjVx+OcrFgPyqR2onLp0vfvMU3On0yOpFGoMzAx+9L54GZxP8NU8c9Rv0mSh z+0zNSfC+UyYDrfx8ZvhaEZgEBJ4K9mQfuEH4hoZM1ChwOLiLS3wdRAxPl2g1bBzhh rXbRxotxe8b20uYmVy9bfVEZXIbE5ZB0W8tBVhFw= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726262AbfLFDK1 (ORCPT ); Thu, 5 Dec 2019 22:10:27 -0500 Received: from mail.kernel.org ([198.145.29.99]:60822 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726097AbfLFDK0 (ORCPT ); Thu, 5 Dec 2019 22:10:26 -0500 Received: from paulmck-ThinkPad-P72.home (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78]) (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 018B021823; Fri, 6 Dec 2019 03:10:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1575601825; bh=rQhhau5BVSh7TTOxVEk8rglm0/OjQ5fR+C28jbh+rDU=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=tBJyiivRVQtC0HOi2iavJGsu9tMgJdmKypr8n84QK7/VdpefXIfxMIizXcZc9W7i/ 59EVBJDUTlwvqQJvKiOiKGjdIGVqofCmbKQnn3lxmuu4dXEivEJquqJtz9TJ+HzmYs YkP6/j8UySZ4Q4XVtHruZQT++6oFRvPrdRZ+0mUQ= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id C531E3522782; Thu, 5 Dec 2019 19:10:24 -0800 (PST) Date: Thu, 5 Dec 2019 19:10:24 -0800 From: "Paul E. McKenney" To: Joel Fernandes Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.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 Subject: Re: [PATCH tip/core/rcu] Enable tick for nohz_full CPUs not responding to expedited GP Message-ID: <20191206031024.GX2889@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20191204232544.GA17061@paulmck-ThinkPad-P72> <20191206014134.GA143492@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191206014134.GA143492@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, Dec 05, 2019 at 08:41:34PM -0500, Joel Fernandes wrote: > On Wed, Dec 04, 2019 at 03:25:44PM -0800, Paul E. McKenney wrote: > > rcu: Enable tick for nohz_full CPUs slow to provide expedited QS > > > > An expedited grace period can be stalled by a nohz_full CPU looping > > in kernel context. This possibility is currently handled by some > > carefully crafted checks in rcu_read_unlock_special() that enlist help > > from ksoftirqd when permitted by the scheduler. However, it is exactly > > these checks that require the scheduler avoid holding any of its rq or > > pi locks across rcu_read_unlock() without also having held them across > > the entire RCU read-side critical section. > > Just got a bit more free to look into the topic of RCU-related scheduler > deadlocks I was previously looking into (And I will continue working on the > WIP patch for detecting bad scenarios). > > > It would therefore be very nice if expedited grace periods could > > handle nohz_full CPUs looping in kernel context without such checks. > > Could you clarify 'without such checks'? Are you referring to these checks in > _unlock_special()? > if (irqs_were_disabled && use_softirq && > (in_interrupt() || > (exp && !t->rcu_read_unlock_special.b.deferred_qs))) { > // Using softirq, safe to awaken, and we get > // no help from enabling irqs, unlike bh/preempt. > raise_softirq_irqoff(RCU_SOFTIRQ); > > I am sorry but I did not fully understand this patch motivation but -- you > are just saying you want EXP GPs to be handled quickly on nohz_full CPUs, > right? Are we not turning the tick on already in rcu_nmi_enter_common() for > handling slow EXP GPs as well? And if not, can we do it from there instead? Currently, if the scheduler holds one of its rq or pi locks across rcu_read_unlock(), it must do so across the entire RCU read-side critical section. Although this odd requirement isn't a flaming disaster, it would be good to lift if feasible. According to Lai's analysis (which seems correct to me), the only remaining need for this requirement is the raise_softirq_irqoff() you call out above. If expedited grace periods can turn on the tick for holdout nohz_full CPUs, it seems to me that the above check can be changed so as to avoid invoking raise_softirq_irqoff() except in cases where it doesn't go into the scheduler. This could be done by removing the portion of the "if" condition following the "||". > > This commit therefore adds code to the expedited grace period's wait > > and cleanup code that forces the scheduler-clock interrupt on for CPUs > > that fail to quickly supply a quiescent state. "Quickly" is currently > > a hard-coded single-jiffy delay. > > > > Signed-off-by: Paul E. McKenney > > > > --- > > > > This is needed to avoid rcu_read_unlock_special() needing to enter > > the scheduler for the benefit of expedited grace periods on nohz_full > > CPUs, thus enabling more of Lai Jiangshan's patchset. > > Still need to go through Lai's patches :-(. Will be looking to make more time > for the same. It would indeed be good to get another set of eyes on this, as it is a bit on the subtle side! Thanx, Paul > thanks! > > - Joel > > > > diff --git a/include/linux/tick.h b/include/linux/tick.h > > index 4ed788c..72a2a21 100644 > > --- a/include/linux/tick.h > > +++ b/include/linux/tick.h > > @@ -109,9 +109,10 @@ enum tick_dep_bits { > > TICK_DEP_BIT_PERF_EVENTS = 1, > > TICK_DEP_BIT_SCHED = 2, > > TICK_DEP_BIT_CLOCK_UNSTABLE = 3, > > - TICK_DEP_BIT_RCU = 4 > > + TICK_DEP_BIT_RCU = 4, > > + TICK_DEP_BIT_RCU_EXP = 5 > > }; > > -#define TICK_DEP_BIT_MAX TICK_DEP_BIT_RCU > > +#define TICK_DEP_BIT_MAX TICK_DEP_BIT_RCU_EXP > > > > #define TICK_DEP_MASK_NONE 0 > > #define TICK_DEP_MASK_POSIX_TIMER (1 << TICK_DEP_BIT_POSIX_TIMER) > > @@ -119,6 +120,7 @@ enum tick_dep_bits { > > #define TICK_DEP_MASK_SCHED (1 << TICK_DEP_BIT_SCHED) > > #define TICK_DEP_MASK_CLOCK_UNSTABLE (1 << TICK_DEP_BIT_CLOCK_UNSTABLE) > > #define TICK_DEP_MASK_RCU (1 << TICK_DEP_BIT_RCU) > > +#define TICK_DEP_MASK_RCU_EXP (1 << TICK_DEP_BIT_RCU_EXP) > > > > #ifdef CONFIG_NO_HZ_COMMON > > extern bool tick_nohz_enabled; > > diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h > > index 634c1db..0c87e4c 100644 > > --- a/kernel/rcu/tree.h > > +++ b/kernel/rcu/tree.h > > @@ -181,6 +181,7 @@ struct rcu_data { > > bool rcu_need_heavy_qs; /* GP old, so heavy quiescent state! */ > > bool rcu_urgent_qs; /* GP old need light quiescent state. */ > > bool rcu_forced_tick; /* Forced tick to provide QS. */ > > + bool rcu_forced_tick_exp; /* ... provide QS to expedited GP. */ > > #ifdef CONFIG_RCU_FAST_NO_HZ > > unsigned long last_accelerate; /* Last jiffy CBs were accelerated. */ > > unsigned long last_advance_all; /* Last jiffy CBs were all advanced. */ > > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h > > index 726ba20..6935a9e 100644 > > --- a/kernel/rcu/tree_exp.h > > +++ b/kernel/rcu/tree_exp.h > > @@ -230,7 +230,9 @@ static void __maybe_unused rcu_report_exp_rnp(struct rcu_node *rnp, bool wake) > > static void rcu_report_exp_cpu_mult(struct rcu_node *rnp, > > unsigned long mask, bool wake) > > { > > + int cpu; > > unsigned long flags; > > + struct rcu_data *rdp; > > > > raw_spin_lock_irqsave_rcu_node(rnp, flags); > > if (!(rnp->expmask & mask)) { > > @@ -238,6 +240,13 @@ static void rcu_report_exp_cpu_mult(struct rcu_node *rnp, > > return; > > } > > WRITE_ONCE(rnp->expmask, rnp->expmask & ~mask); > > + for_each_leaf_node_cpu_mask(rnp, cpu, mask) { > > + rdp = per_cpu_ptr(&rcu_data, cpu); > > + if (!IS_ENABLED(CONFIG_NO_HZ_FULL) || !rdp->rcu_forced_tick_exp) > > + continue; > > + rdp->rcu_forced_tick_exp = false; > > + tick_dep_clear_cpu(cpu, TICK_DEP_BIT_RCU_EXP); > > + } > > __rcu_report_exp_rnp(rnp, wake, flags); /* Releases rnp->lock. */ > > } > > > > @@ -450,6 +459,26 @@ static void sync_rcu_exp_select_cpus(void) > > } > > > > /* > > + * Wait for the expedited grace period to elapse, within time limit. > > + * If the time limit is exceeded without the grace period elapsing, > > + * return false, otherwise return true. > > + */ > > +static bool synchronize_rcu_expedited_wait_once(long tlimit) > > +{ > > + int t; > > + struct rcu_node *rnp_root = rcu_get_root(); > > + > > + t = swait_event_timeout_exclusive(rcu_state.expedited_wq, > > + sync_rcu_exp_done_unlocked(rnp_root), > > + tlimit); > > + // Workqueues should not be signaled. > > + if (t > 0 || sync_rcu_exp_done_unlocked(rnp_root)) > > + return true; > > + WARN_ON(t < 0); /* workqueues should not be signaled. */ > > + return false; > > +} > > + > > +/* > > * Wait for the expedited grace period to elapse, issuing any needed > > * RCU CPU stall warnings along the way. > > */ > > @@ -460,22 +489,31 @@ static void synchronize_rcu_expedited_wait(void) > > unsigned long jiffies_start; > > unsigned long mask; > > int ndetected; > > + struct rcu_data *rdp; > > struct rcu_node *rnp; > > struct rcu_node *rnp_root = rcu_get_root(); > > - int ret; > > > > trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("startwait")); > > jiffies_stall = rcu_jiffies_till_stall_check(); > > jiffies_start = jiffies; > > + if (IS_ENABLED(CONFIG_NO_HZ_FULL)) { > > + if (synchronize_rcu_expedited_wait_once(1)) > > + return; > > + rcu_for_each_leaf_node(rnp) { > > + for_each_leaf_node_cpu_mask(rnp, cpu, rnp->expmask) { > > + rdp = per_cpu_ptr(&rcu_data, cpu); > > + if (rdp->rcu_forced_tick_exp) > > + continue; > > + rdp->rcu_forced_tick_exp = true; > > + tick_dep_set_cpu(cpu, TICK_DEP_BIT_RCU_EXP); > > + } > > + } > > + WARN_ON_ONCE(1); > > + } > > > > for (;;) { > > - ret = swait_event_timeout_exclusive( > > - rcu_state.expedited_wq, > > - sync_rcu_exp_done_unlocked(rnp_root), > > - jiffies_stall); > > - if (ret > 0 || sync_rcu_exp_done_unlocked(rnp_root)) > > + if (synchronize_rcu_expedited_wait_once(jiffies_stall)) > > return; > > - WARN_ON(ret < 0); /* workqueues should not be signaled. */ > > if (rcu_cpu_stall_suppress) > > continue; > > panic_on_rcu_stall();