Re: [PATCH v2 1/3] process cputimer is moving faster than its corresponding clock

Olivier Langlois
2013-04-29T17:10:16Z

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

Greetings,
Olivier