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

KOSAKI Motohiro
2013-04-29T17:56:48Z

(4/29/13 1:10 PM), 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

I know. I taled thread exiting. not thread creating. And, as far as I can see, only test sig1 can fail,
not thr[12].