Messages in this thread | | | From | Vincent Guittot <> | Date | Mon, 18 Jan 2021 16:32:41 +0100 | Subject | Re: [PATCH] sched/fair: add protection for delta of wait time |
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On Mon, 18 Jan 2021 at 15:11, Jiang Biao <benbjiang@gmail.com> wrote: > > Hi, Vincent > > On Mon, 18 Jan 2021 at 15:56, Vincent Guittot > <vincent.guittot@linaro.org> wrote: > > > > On Sun, 17 Jan 2021 at 13:31, Jiang Biao <benbjiang@gmail.com> wrote: > > > > > > From: Jiang Biao <benbjiang@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.
> 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@tencent.com> > > > --- > > > 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 > > >
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