Messages in this thread | | | Subject | Re: [PATCH v2 1/3] process cputimer is moving faster than its corresponding clock | From | Olivier Langlois <> | Date | Mon, 29 Apr 2013 13:41:54 -0400 |
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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 <olivier@trillion01.com> > > > --- > > > 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.
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