Re: [PATCH 3/3] sched,time: atomically increment stime & utime

[Oleg Nesterov]

Aaah, Rik, I am sorry!

You documented this in 0/3 which I didn't bother to read.

Sorry for noise.

On 08/16, Oleg Nesterov wrote:
>
> On 08/15, riel@xxxxxxxxxx wrote:
> >
> > --- a/kernel/sched/cputime.c
> > +++ b/kernel/sched/cputime.c
> > @@ -605,9 +605,12 @@ static void cputime_adjust(struct task_cputime *curr,
> >  	 * If the tick based count grows faster than the scheduler one,
> >  	 * the result of the scaling may go backward.
> >  	 * Let's enforce monotonicity.
> > +	 * Atomic exchange protects against concurrent cputime_adjust.
> >  	 */
> > -	prev->stime = max(prev->stime, stime);
> > -	prev->utime = max(prev->utime, utime);
> > +	while (stime > (rtime = ACCESS_ONCE(prev->stime)))
> > +		cmpxchg(&prev->stime, rtime, stime);
> > +	while (utime > (rtime = ACCESS_ONCE(prev->utime)))
> > +		cmpxchg(&prev->utime, rtime, utime);
> >
> >  out:
> >  	*ut = prev->utime;
> 
> I am still not sure about this change. At least I think it needs some
> discussion.
> 
> Let me repeat, afaics this can lead to inconsistent results. Just
> suppose that the caller of thread_group_cputime_adjusted() gets a long
> preemption between thread_group_cputime() and cputime_adjust(), and
> the numbers in signal->prev_cputime grow significantly when this task
> resumes. If cputime_adjust() sees both prev->stime and prev->utime
> updated everything is fine. But we can race with cputime_adjust() on
> another CPU and miss, say, the change in ->utime.
> 
> IOW. To simplify, suppose that thread_group_cputime(T) fills task_cputime
> with zeros. Then the caller X is preempted.
> 
> Another task does thread_group_cputime(T) and this time task_cputime is
> { .utime = A_LOT_U, .stime = A_LOT_S }. This task calls cputime_adjust()
> and sets prev->stime = A_LOT_S.
> 
> X resumes, calls cputime_adjust(), and returns { 0, A_LOT_S }.
> 
> If you think that we do not care, probably I won't argue. But at least
> this should be documented/discussed. And if we can tolerate this, then we
> can probably simply remove the scale_stime recalculation and change it to
> just do
> 
> 	static void cputime_adjust(struct task_cputime *curr,
> 				   struct cputime *prev,
> 				   cputime_t *ut, cputime_t *st)
> 	{
> 		cputime_t rtime, stime, utime;
> 		/*
> 		 * Let's enforce monotonicity.
> 		 * Atomic exchange protects against concurrent cputime_adjust.
> 		 */
> 		while (stime > (rtime = ACCESS_ONCE(prev->stime)))
> 			cmpxchg(&prev->stime, rtime, stime);
> 		while (utime > (rtime = ACCESS_ONCE(prev->utime)))
> 			cmpxchg(&prev->utime, rtime, utime);
> 
> 		*ut = prev->utime;
> 		*st = prev->stime;
> 	}
> 
> Oleg.

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