Re: [BUG nohz]: wrong user and system time accounting

[Wanpeng Li <kernellwp@gmail.com>]

2017-03-30 21:38 GMT+08:00 Frederic Weisbecker <fweisbec@gmail.com>:
> On Thu, Mar 30, 2017 at 02:47:11PM +0800, Wanpeng Li wrote:
>> Cc Peterz, Thomas,
>> 2017-03-30 12:27 GMT+08:00 Mike Galbraith <efault@gmx.de>:
>> > On Wed, 2017-03-29 at 16:08 -0400, Rik van Riel wrote:
>> >
>> >> In other words, the tick on cpu0 is aligned
>> >> with the tick on the nohz_full cpus, and
>> >> jiffies is advanced while the nohz_full cpus
>> >> with an active tick happen to be in kernel
>> >> mode?
>> >
>> > You really want skew_tick=1, especially on big boxen.
>> >
>> >> Frederic, can you think of any reason why
>> >> the tick on nohz_full CPUs would end up aligned
>> >> with the tick on cpu0, instead of running at some
>> >> random offset?
>> >
>> > (I or low rq->clock bits as crude NOHZ collision avoidance)
>> >
>> >> A random offset, or better yet a somewhat randomized
>> >> tick length to make sure that simultaneous ticks are
>> >> fairly rare and the vtime sampling does not end up
>> >> "in phase" with the jiffies incrementing, could make
>> >> the accounting work right again.
>> >
>> > That improves jitter, especially on big boxen.  I have an 8 socket box
>> > that thinks it's an extra large PC, there, collision avoidance matters
>> > hugely.  I couldn't reproduce bean counting woes, no idea if collision
>> > avoidance will help that.
>>
>> So I implement two methods, one is from Rik's random offset proposal
>> through skew tick, the other one is from Frederic's proposal and it is
>> the same as my original idea through use nanosecond granularity to
>> check deltas but only perform an actual cputime update when that delta
>> >= TICK_NSEC. Both methods can solve the bug which Luiz reported.
>> Peterz, Thomas, any ideas?
>>
>> --------------------------->8-------------------------------------------------------------
>>
>> skew tick:
>>
>> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
>> index 7fe53be..9981437 100644
>> --- a/kernel/time/tick-sched.c
>> +++ b/kernel/time/tick-sched.c
>> @@ -1198,7 +1198,11 @@ void tick_setup_sched_timer(void)
>>      hrtimer_set_expires(&ts->sched_timer, tick_init_jiffy_update());
>>
>>      /* Offset the tick to avert jiffies_lock contention. */
>> +#ifdef CONFIG_NO_HZ_FULL
>> +    if (sched_skew_tick || tick_nohz_full_running) {
>> +#else
>>      if (sched_skew_tick) {
>> +#endif
>
> Please rather use tick_nohz_full_enabled() to avoid ifdeffery.
>
>>          u64 offset = ktime_to_ns(tick_period) >> 1;
>>          do_div(offset, num_possible_cpus());
>>          offset *= smp_processor_id();
>
> If it works, we may want to take that solution, likely less performance sensitive
> than using sched_clock(). In fact sched_clock() is fast, especially as we require it to
> be stable for nohz_full, but using it involves costly conversion back and forth to jiffies.

So both Rik and you agree with the skew tick solution, I will try it
tomorrow. Btw, if we should just add random offset to the cpu in the
nohz_full mode or add random offset to all cpus like the codes above?

Regards,
Wanpeng Li

>
>>
>> -------------------------------------->8-----------------------------------------------------
>>
>> use nanosecond granularity to check deltas but only perform an actual
>> cputime update when that delta >= TICK_NSEC.
>>
>> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
>> index f3778e2b..f1ee393 100644
>> --- a/kernel/sched/cputime.c
>> +++ b/kernel/sched/cputime.c
>> @@ -676,18 +676,21 @@ void thread_group_cputime_adjusted(struct
>> task_struct *p, u64 *ut, u64 *st)
>>  #ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
>>  static u64 vtime_delta(struct task_struct *tsk)
>>  {
>> -    unsigned long now = READ_ONCE(jiffies);
>> +    u64 now = local_clock();
>
> I fear we need a global clock, because the reader (task_cputime()) needs
> to compute the delta and therefore use the same clock from any CPU.
>
> Or we can use the local_clock() but the reader must access the same.
>
> So there would be vtime_delta_writer() which uses local_clock and stores
> the current CPU to tsk->vtime_cpu (under the vtime_seqcount). And then
> vtime_delta_reader() which calls sched_clock_cpu(tsk->vtime_cpu) which
> is protected by vtime_seqcount as well.
>
> Although those sched_clock_cpu() things seem to only matter when the
> sched_clock() is unstable. And that stability is a condition for nohz_full
> to work anyway. So probably sched_clock() alone would be enough.
>
> Thanks.