Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758031Ab3D2RmF (ORCPT ); Mon, 29 Apr 2013 13:42:05 -0400 Received: from oproxy7-pub.bluehost.com ([67.222.55.9]:56829 "HELO oproxy7-pub.bluehost.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1757821Ab3D2RmA (ORCPT ); Mon, 29 Apr 2013 13:42:00 -0400 Message-ID: <1367257314.8833.16.camel@Wailaba2> Subject: Re: [PATCH v2 1/3] process cputimer is moving faster than its corresponding clock From: Olivier Langlois To: KOSAKI Motohiro Cc: Peter Zijlstra , Ingo Molnar , Thomas Gleixner , schwidefsky@de.ibm.com, Steven Rostedt , Frederic Weisbecker , LKML Date: Mon, 29 Apr 2013 13:41:54 -0400 In-Reply-To: <1367255407.8833.4.camel@Wailaba2> References: <1365184746.874.103.camel@Wailaba2> <1365593710.30071.52.camel@laptop> <1365608911.707.65.camel@Wailaba2> <1365763837.17140.52.camel@laptop> <1365782115.17140.68.camel@laptop> <1366951210.7911.28.camel@Wailaba2> <1366957639.7911.42.camel@Wailaba2> <517AD0AE.1030404@gmail.com> <1367037606.7911.66.camel@Wailaba2> <517DFFE9.4050808@gmail.com> <1367255407.8833.4.camel@Wailaba2> Organization: Trillion01 Inc Content-Type: text/plain; charset="ISO-8859-1" X-Mailer: Evolution 3.8.1 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit X-Identified-User: {5686:box610.bluehost.com:olivierl:trillion01.com} {sentby:smtp auth 173.178.230.31 authed with olivier@trillion01.com} Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3430 Lines: 79 On Mon, 2013-04-29 at 13:10 -0400, Olivier Langlois wrote: > On Mon, 2013-04-29 at 01:06 -0400, KOSAKI Motohiro wrote: > > (4/27/13 12:40 AM), Olivier Langlois wrote: > > > > > > > > > Forbids the cputimer to drift ahead of its process clock by > > > blocking its update when a tick occurs while a autoreaping task > > > is currently in do_exit() between the call to release_task() and > > > its final call to schedule(). > > > > > > Any task stats update after having called release_task() will > > > be lost because they are added to the global process stats located > > > in the signal struct from release_task(). > > > > > > Ideally, you should postpone the release_task() call after the > > > final context switch to get all the stats added but this is > > > more complex to achieve. > > > > > > In other words, this is slowing down the cputimer so it keep the same > > > pace than the process clock but in fact, what should be done is to > > > speed up the process clock by adding the missing stats to it. > > > > > > Signed-off-by: Olivier Langlois > > > --- > > > kernel/sched/fair.c | 10 +++++++++- > > > 1 file changed, 9 insertions(+), 1 deletion(-) > > > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > > index 7a33e59..52d7b10 100644 > > > --- a/kernel/sched/fair.c > > > +++ b/kernel/sched/fair.c > > > @@ -708,7 +708,15 @@ static void update_curr(struct cfs_rq *cfs_rq) > > > > > > trace_sched_stat_runtime(curtask, delta_exec, curr->vruntime); > > > cpuacct_charge(curtask, delta_exec); > > > - account_group_exec_runtime(curtask, delta_exec); > > > + /* > > > + * Do not update the cputimer if the task is already released by > > > + * release_task(). > > > + * > > > + * it would preferable to defer the autoreap release_task > > > + * after the last context switch but harder to do. > > > + */ > > > + if (likely(curtask->sighand)) > > > + account_group_exec_runtime(curtask, delta_exec); > > > } > > > > I'm confused. glibc's rt/tst-cputimer1 doesn't have thread exiting code. I have > > no seen any issue in this accounting. > > glibc launch a helper thread to receive timer signal and will also > create a new thread upon signal reception when a timer is created with > sigev_notify = SIGEV_THREAD; > > please see: > > glibc-2.17/nptl/sysdeps/unix/sysv/linux/timer_create.c > glibc-2.17/nptl/sysdeps/unix/sysv/linux/timer_routines.c > One very easy way to see the problem is to add a printk statement inside update_gt_cputime() if (b->sum_exec_runtime > a->sum_exec_runtime) a->sum_exec_runtime = b->sum_exec_runtime; else printk( KERN_DEBUG "cputimer %llu, process clock %llu, diff %llu\n", a->sum_exec_runtime, b->sum_exec_runtime, a->sum_exec_runtime-b->sum_exec_runtime); Check the output with and without the fair.c modif when executing tst-cputimer1. As an extra bonus, this trace will show the spurious start/stop cputimer problem that I was trying to explain to Frederic. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/