Messages in this thread | | | Date | Thu, 14 Jun 2018 12:32:32 +0100 | From | Patrick Bellasi <> | Subject | Re: [PATCH] sched/util_est: fix util_est_dequeue() for throttled cfs rq |
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On 14-Jun 12:33, Vincent Guittot wrote: > When a cfs_rq is throttled, parent cfs_rq->nr_running is decreased and > everything happens at cfs_rq level. Currently util_est stays unchanged > in such case and it keeps accounting the utilization of throttled tasks. > This can somewhat make sense as we don't dequeue tasks but only throttled > cfs_rq.
I think the idea here was that, if tasks are throttled, this should manifest in a reduction of their utilization... and thus the estimated utilization should still represent the amount of bandwidth required by that tasks. Although one could argue that, while a TG is throttled we would like to be able to drop the frequency if possible.
This has not been implemented that way so far because the attach/detach of TGs will require to walk them to account for all child tasks's util_est or, otherwise, to aggregate util_est across TGs.
> If a task of another group is enqueued/dequeued and root cfs_rq becomes > idle during the dequeue, util_est will be cleared whereas it was > accounting util_est of throttled tasks before.
Yep :/
> So the behavior of util_est > is not always the same regarding throttled tasks and depends of side > activity. Furthermore, util_est will not be updated when the cfs_rq is > unthrottled
right... that happens because (un)throttling does not involve (en/de)queue.
> as everything happens at cfs rq level. Main results is that > util_est will stay null whereas we now have running tasks. We have to wait > for the next dequeue/enqueue of the previously throttled tasks to get an > up to date util_est. > > Remove the assumption that cfs_rq's estimated utilization of a CPU is 0 > if there is no running task so the util_est of a task remains until the > latter is dequeued even if its cfs_rq has been throttled.
Right...
> Fixes: 7f65ea42eb00 ("sched/fair: Add util_est on top of PELT") > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> > --- > kernel/sched/fair.c | 16 ++++------------ > 1 file changed, 4 insertions(+), 12 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index e497c05..d3121fc 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -3982,18 +3982,10 @@ util_est_dequeue(struct cfs_rq *cfs_rq, struct task_struct *p, bool task_sleep) > if (!sched_feat(UTIL_EST)) > return; > > - /* > - * Update root cfs_rq's estimated utilization > - * > - * If *p is the last task then the root cfs_rq's estimated utilization > - * of a CPU is 0 by definition. > - */ > - ue.enqueued = 0;
... AFAIR, this reset what there since one of the first posts as an "optimization". But actually I was not considering the scenario you describe.
> - if (cfs_rq->nr_running) { > - ue.enqueued = cfs_rq->avg.util_est.enqueued; > - ue.enqueued -= min_t(unsigned int, ue.enqueued, > - (_task_util_est(p) | UTIL_AVG_UNCHANGED)); > - } > + /* Update root cfs_rq's estimated utilization */ > + ue.enqueued = cfs_rq->avg.util_est.enqueued; > + ue.enqueued -= min_t(unsigned int, ue.enqueued, > + (_task_util_est(p) | UTIL_AVG_UNCHANGED));
So, this should still be bound-safe thanks to the min() for the subtraction.
> WRITE_ONCE(cfs_rq->avg.util_est.enqueued, ue.enqueued); > > /* > -- > 2.7.4 >
LGTM:
Reviewed-by: Patrick Bellasi <patrick.bellasi@arm.com>
-- #include <best/regards.h>
Patrick Bellasi
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