Messages in this thread Patch in this message | | | From | Jiang Biao <> | Date | Mon, 18 Jan 2021 22:11:01 +0800 | Subject | Re: [PATCH] sched/fair: add protection for delta of wait time |
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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. :) 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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