From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934118AbeEWTNn (ORCPT ); Wed, 23 May 2018 15:13:43 -0400 Received: from mail.kernel.org ([198.145.29.99]:35358 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934122AbeEWTNk (ORCPT ); Wed, 23 May 2018 15:13:40 -0400 Date: Wed, 23 May 2018 15:13:37 -0400 From: Steven Rostedt To: "Paul E. McKenney" Cc: Joel Fernandes , linux-kernel@vger.kernel.org, "Joel Fernandes (Google)" , Peter Zilstra , Ingo Molnar , Boqun Feng , byungchul.park@lge.com, kernel-team@android.com, Josh Triplett , Lai Jiangshan , Mathieu Desnoyers Subject: Re: [PATCH 1/4] rcu: Speed up calling of RCU tasks callbacks Message-ID: <20180523151337.469bba34@gandalf.local.home> In-Reply-To: <20180523170303.GR3803@linux.vnet.ibm.com> References: <20180523063815.198302-1-joel@joelfernandes.org> <20180523063815.198302-2-joel@joelfernandes.org> <20180523155734.GK3803@linux.vnet.ibm.com> <20180523124531.7b0e972a@gandalf.local.home> <20180523170303.GR3803@linux.vnet.ibm.com> X-Mailer: Claws Mail 3.16.0 (GTK+ 2.24.32; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 23 May 2018 10:03:03 -0700 "Paul E. McKenney" wrote: > > > > diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c > > > > index 5783bdf86e5a..a28698e44b08 100644 > > > > --- a/kernel/rcu/update.c > > > > +++ b/kernel/rcu/update.c > > > > @@ -743,6 +743,12 @@ static int __noreturn rcu_tasks_kthread(void *arg) > > > > */ > > > > synchronize_srcu(&tasks_rcu_exit_srcu); > > > > > > > > + /* > > > > + * Wait a little bit incase held tasks are released > > > > > > in case > > > > > > > + * during their next timer ticks. > > > > + */ > > > > + schedule_timeout_interruptible(HZ/10); > > > > + > > > > /* > > > > * Each pass through the following loop scans the list > > > > * of holdout tasks, removing any that are no longer > > > > @@ -755,7 +761,6 @@ static int __noreturn rcu_tasks_kthread(void *arg) > > > > int rtst; > > > > struct task_struct *t1; > > > > > > > > - schedule_timeout_interruptible(HZ); > > > > rtst = READ_ONCE(rcu_task_stall_timeout); > > > > needreport = rtst > 0 && > > > > time_after(jiffies, lastreport + rtst); > > > > @@ -768,6 +773,11 @@ static int __noreturn rcu_tasks_kthread(void *arg) > > > > check_holdout_task(t, needreport, &firstreport); > > > > cond_resched(); > > > > } > > > > + > > > > + if (list_empty(&rcu_tasks_holdouts)) > > > > + break; > > > > + > > > > + schedule_timeout_interruptible(HZ); > > > > Why is this a full second wait and not the HZ/10 like the others? > > The idea is to respond quickly on small idle systems and to reduce the > number of possibly quite lengthy traversals of the task list otherwise. > I actually considered exponential backoff, but decided to keep it simple, > at least to start with. Ah, now it makes sense. Reading what you wrote, we can still do a backoff and keep it simple. What about the patch below. It appears to have the same performance improvement as Joel's -- Steve > > > > > > Is there a better way to do this? Can this be converted into a for-loop? > > > Alternatively, would it make sense to have a firsttime local variable > > > initialized to true, to keep the schedule_timeout_interruptible() at > > > the beginning of the loop, but skip it on the first pass through the loop? > > > > > > Don't get me wrong, what you have looks functionally correct, but > > > duplicating the condition might cause problems later on, for example, > > > should a bug fix be needed in the condition. > > > diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c index 68fa19a5e7bd..c6df9fa916cf 100644 --- a/kernel/rcu/update.c +++ b/kernel/rcu/update.c @@ -796,13 +796,22 @@ static int __noreturn rcu_tasks_kthread(void *arg) * holdouts. When the list is empty, we are done. */ lastreport = jiffies; - while (!list_empty(&rcu_tasks_holdouts)) { + for (;;) { bool firstreport; bool needreport; int rtst; struct task_struct *t1; + int fract = 15; + + /* Slowly back off waiting for holdouts */ + schedule_timeout_interruptible(HZ/fract); + + if (list_empty(&rcu_tasks_holdouts)) + break; + + if (fract > 1) + fract--; - schedule_timeout_interruptible(HZ); rtst = READ_ONCE(rcu_task_stall_timeout); needreport = rtst > 0 && time_after(jiffies, lastreport + rtst);