Re: [PATCH] sched/fair: add protection for delta of wait time

[Vincent Guittot]

On Mon, 18 Jan 2021 at 15:11, Jiang Biao <benbjiang@xxxxxxxxx> wrote:
>
> Hi, Vincent
>
> On Mon, 18 Jan 2021 at 15:56, Vincent Guittot
> <vincent.guittot@xxxxxxxxxx> wrote:
> >
> > On Sun, 17 Jan 2021 at 13:31, Jiang Biao <benbjiang@xxxxxxxxx> wrote:
> > >
> > > From: Jiang Biao <benbjiang@xxxxxxxxxxx>
> > >
> > > delta in update_stats_wait_end() might be negative, which would
> > > make following statistics go wrong.
> >
> > Could you describe the use case that generates a negative delta ?
> >
> > rq_clock is always increasing so this should not lead to a negative
> > value even if update_stats_wait_end/start are not called in the right
> > order,
> Yes, indeed.
>
> > This situation could happen after a migration if we forgot to call
> > update_stats_wait_start
> The migration case was what I worried about, but no regular use case
> comes into my mind. :)

IIUC, you haven't faced the problem and it's only based on studying the code.

> As an extreme case, would it be a problem if we disable/re-enable
> sched_schedstats during migration?
>
> static inline void
> update_stats_wait_start(struct cfs_rq *cfs_rq, struct sched_entity *se)
> {
>         u64 wait_start, prev_wait_start;
>
>         if (!schedstat_enabled()) // disable during migration
>                 return; // return here, and skip updating wait_start
> ...
> }
>
> static inline void
> update_stats_wait_end(struct cfs_rq *cfs_rq, struct sched_entity *se)
> {
>         struct task_struct *p;
>         u64 delta;
>
>         if (!schedstat_enabled())  // re-enable again
>                 return;
>
>         /*
>          * When the sched_schedstat changes from 0 to 1, some sched se
>          * maybe already in the runqueue, the se->statistics.wait_start
>          * will be 0.So it will let the delta wrong. We need to avoid this
>          * scenario.
>          */
>         if (unlikely(!schedstat_val(se->statistics.wait_start)))
>                 return;
>          //stale wait_start which might be bigger than rq_clock would
> be used. -)
>         delta = rq_clock(rq_of(cfs_rq)) -
> schedstat_val(se->statistics.wait_start);
> ...
>
> Thanks a lot.
> Regards,
> Jiang
>
> }
> >
> > >
> > > Add protection for delta of wait time, like what have been done in
> > > update_stats_enqueue_sleeper() for deltas of sleep/block time.
> > >
> > > Signed-off-by: Jiang Biao <benbjiang@xxxxxxxxxxx>
> > > ---
> > >  kernel/sched/fair.c | 3 +++
> > >  1 file changed, 3 insertions(+)
> > >
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index c0374c1152e0..ac950ac950bc 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -917,6 +917,9 @@ update_stats_wait_end(struct cfs_rq *cfs_rq, struct sched_entity *se)
> > >
> > >         delta = rq_clock(rq_of(cfs_rq)) - schedstat_val(se->statistics.wait_start);
> > >
> > > +       if ((s64)delta < 0)
> > > +               delta = 0;
> > > +
> > >         if (entity_is_task(se)) {
> > >                 p = task_of(se);
> > >                 if (task_on_rq_migrating(p)) {
> > > --
> > > 2.21.0
> > >