From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933805AbeEWRMu (ORCPT ); Wed, 23 May 2018 13:12:50 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:42686 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932667AbeEWRMs (ORCPT ); Wed, 23 May 2018 13:12:48 -0400 Date: Wed, 23 May 2018 10:03:03 -0700 From: "Paul E. McKenney" To: Steven Rostedt 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 Reply-To: paulmck@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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180523124531.7b0e972a@gandalf.local.home> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18052317-0028-0000-0000-000009AD1DF4 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009072; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000261; SDB=6.01036577; UDB=6.00530293; IPR=6.00815704; MB=3.00021261; MTD=3.00000008; XFM=3.00000015; UTC=2018-05-23 17:12:46 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18052317-0029-0000-0000-00003AF83607 Message-Id: <20180523170303.GR3803@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-05-23_07:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 impostorscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1709140000 definitions=main-1805230166 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 23, 2018 at 12:45:31PM -0400, Steven Rostedt wrote: > On Wed, 23 May 2018 08:57:34 -0700 > "Paul E. McKenney" wrote: > > > On Tue, May 22, 2018 at 11:38:12PM -0700, Joel Fernandes wrote: > > > From: "Joel Fernandes (Google)" > > > > > > RCU tasks callbacks can take at least 1 second before the callbacks are > > > executed. This happens even if the hold-out tasks enter their quiescent states > > > quickly. I noticed this when I was testing trampoline callback execution. > > > > > > To test the trampoline freeing, I wrote a simple script: > > > cd /sys/kernel/debug/tracing/ > > > echo '__schedule_bug:traceon' > set_ftrace_filter; > > > echo '!__schedule_bug:traceon' > set_ftrace_filter; > > > > > > In the background I had simple bash while loop: > > > while [ 1 ]; do x=1; done & > > > > > > Total time of completion of above commands in seconds: > > > > > > With this patch: > > > real 0m0.179s > > > user 0m0.000s > > > sys 0m0.054s > > > > > > Without this patch: > > > real 0m1.098s > > > user 0m0.000s > > > sys 0m0.053s > > > > > > That's a greater than 6X speed up in performance. In order to accomplish > > > this, I am waiting for HZ/10 time before entering the hold-out checking > > > loop. The loop still preserves its checking of held tasks every 1 second > > > as before, in case this first test doesn't succeed. > > > > > > Cc: Steven Rostedt > > > > Given an ack from Steven, I would be happy to take this, give or take > > some nits below. > > I'm currently testing it, and trying to understand it better. Very good, thank you! > > > Cc: Peter Zilstra > > > Cc: Ingo Molnar > > > Cc: Boqun Feng > > > Cc: Paul McKenney > > > Cc: byungchul.park@lge.com > > > Cc: kernel-team@android.com > > > Signed-off-by: Joel Fernandes (Google) > > > --- > > > kernel/rcu/update.c | 12 +++++++++++- > > > 1 file changed, 11 insertions(+), 1 deletion(-) > > > > > > 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. Thanx, Paul > -- 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. > > > > > } > > > > > > /* > > > -- > > > 2.17.0.441.gb46fe60e1d-goog > > > >