Hi, Vincent
On Mon, 18 Jan 2021 at 23:32, Vincent Guittot
<vincent.guit...@linaro.org> wrote:
>
> On Mon, 18 Jan 2021 at 15:11, Jiang Biao <benbji...@gmail.com> wrote:
> >
> > Hi, Vincent
> >
> > On Mon, 18 Jan 2021 at 15:56, Vincent Guittot
> > <vincent.guit...@linaro.org> wrote:
> > >
> > > On Sun, 17 Jan 2021 at 13:31, Jiang Biao <benbji...@gmail.com> wrote:
> > > >
> > > > From: Jiang Biao <benbji...@tencent.com>
> > > >
> > > > 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.
Not yet. :). Just found there are protections for
sleep_time/block_time, but no protection
for wait_time.
Think more later, the sleep_time/block_time do need to be protected
for the migration case,
because update_stats_enqueue_sleeper could be called right after
migration with src cpu's
sleep_start/block_start. But wait_time is not the case.
The following case might be too extreme to happen. :)
Thanks a lot for your patience.
Regards,
Jiang
>
> > 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
