Re: [bug-report] possible s64 overflow in max_vruntime()

[Vincent Guittot]

On Fri, 27 Jan 2023 at 23:10, Benjamin Segall <bsegall@xxxxxxxxxx> wrote:
>
> Vincent Guittot <vincent.guittot@xxxxxxxxxx> writes:
>
> > On Fri, 27 Jan 2023 at 12:44, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >>
> >> On Thu, Jan 26, 2023 at 07:31:02PM +0100, Roman Kagan wrote:
> >>
> >> > > All that only matters for small sleeps anyway.
> >> > >
> >> > > Something like:
> >> > >
> >> > >         sleep_time = U64_MAX;
> >> > >         if (se->avg.last_update_time)
> >> > >           sleep_time = cfs_rq_clock_pelt(cfs_rq) - se->avg.last_update_time;
> >> >
> >> > Interesting, why not rq_clock_task(rq_of(cfs_rq)) - se->exec_start, as
> >> > others were suggesting?  It appears to better match the notion of sleep
> >> > wall-time, no?
> >>
> >> Should also work I suppose. cfs_rq_clock takes throttling into account,
> >> but that should hopefully also not be *that* long, so either should
> >> work.
> >
> > yes rq_clock_task(rq_of(cfs_rq)) should be fine too
>
> No, last_update_time is based on cfs_rq_clock_pelt(cfs_rq), and it will
> get more and more out of sync as time goes on, every time the cfs_rq
> throttles. It won't reset when the throttle is done.

I was referring to the rq_clock_task(rq_of(cfs_rq)) - se->exec_start
that Roman was asking for

>
> >
> > Another thing to take into account is the sleeper credit that the
> > waking task deserves so the detection should be done once it has been
> > subtracted from vruntime.
> >
> > Last point, when a nice -20 task runs on a rq, it will take a bit more
> > than 2 seconds for the vruntime to be increased by more than 24ms (the
> > maximum credit that a waking task can get) so threshold must be
> > significantly higher than 2 sec. On the opposite side, the lowest
> > possible weight of a cfs rq is 2 which means that the problem appears
> > for a sleep longer or equal to 2^54 = 2^63*2/1024. We should use this
> > value instead of an arbitrary 200 days