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SubjectRe: [PATCH v5 4/4] sched/fair: update util_est only on util_avg updates
On Thu, Feb 22, 2018 at 05:01:53PM +0000, Patrick Bellasi wrote:
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 8364771f7301..1bf9a86ebc39 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3047,6 +3047,29 @@ static inline void cfs_rq_util_change(struct cfs_rq *cfs_rq)
> }
> }
>
> +/*
> + * When a task is dequeued, its estimated utilization should not be update if
> + * its util_avg has not been updated at least once.
> + * This flag is used to synchronize util_avg updates with util_est updates.
> + * We map this information into the LSB bit of the utilization saved at
> + * dequeue time (i.e. util_est.dequeued).
> + */
> +#define UTIL_EST_NEED_UPDATE_FLAG 0x1
> +
> +static inline void cfs_se_util_change(struct sched_avg *avg)
> +{
if (!sched_feat(UTIL_EST))
return;

> + if (sched_feat(UTIL_EST)) {
> + struct util_est ue = READ_ONCE(avg->util_est);
> +
> + if (!(ue.enqueued & UTIL_EST_NEED_UPDATE_FLAG))
> + return;
> +
> + /* Reset flag to report util_avg has been updated */
> + ue.enqueued &= ~UTIL_EST_NEED_UPDATE_FLAG;
> + WRITE_ONCE(avg->util_est, ue);
> + }

and loose the indent. Also, since we only update the enqueued value, we
don't need to load/store the entire util_est thing here.

> +}
> +
> #ifdef CONFIG_SMP
> /*
> * Approximate:
> @@ -3308,6 +3331,7 @@ __update_load_avg_se(u64 now, int cpu, struct cfs_rq *cfs_rq, struct sched_entit
> cfs_rq->curr == se)) {
>
> ___update_load_avg(&se->avg, se_weight(se), se_runnable(se));
> + cfs_se_util_change(&se->avg);
> return 1;
> }
>

So we only clear the bit for @se updates.

> @@ -5218,7 +5242,7 @@ static inline void util_est_enqueue(struct cfs_rq *cfs_rq,
>
> /* Update root cfs_rq's estimated utilization */
> enqueued = READ_ONCE(cfs_rq->avg.util_est.enqueued);
> - enqueued += _task_util_est(p);
> + enqueued += (_task_util_est(p) | 0x1);

UTIL_EST_NEED_UPDATE_FLAG, although I do agree that 0x1 is much easier
to type ;-)

But you set it for the cfs_rq value ?! That doesn't seem right.

> WRITE_ONCE(cfs_rq->avg.util_est.enqueued, enqueued);
> }
>
> @@ -5310,7 +5334,7 @@ static inline void util_est_dequeue(struct cfs_rq *cfs_rq,
> if (cfs_rq->nr_running) {
> ue.enqueued = READ_ONCE(cfs_rq->avg.util_est.enqueued);
> ue.enqueued -= min_t(unsigned int, ue.enqueued,
> - _task_util_est(p));
> + (_task_util_est(p) | UTIL_EST_NEED_UPDATE_FLAG));
> }
> WRITE_ONCE(cfs_rq->avg.util_est.enqueued, ue.enqueued);
>

Would it be really horrible if you separate the value and flag using a
bitfield/shifts?

> @@ -5321,12 +5345,19 @@ static inline void util_est_dequeue(struct cfs_rq *cfs_rq,
> if (!task_sleep)
> return;
>
> + /*
> + * Skip update of task's estimated utilization if the PELT signal has
> + * never been updated (at least once) since last enqueue time.
> + */
> + ue = READ_ONCE(p->se.avg.util_est);
> + if (ue.enqueued & UTIL_EST_NEED_UPDATE_FLAG)
> + return;
> +
> /*
> * Skip update of task's estimated utilization when its EWMA is
> * already ~1% close to its last activation value.
> */
> + ue.enqueued = (task_util(p) | UTIL_EST_NEED_UPDATE_FLAG);
> last_ewma_diff = ue.enqueued - ue.ewma;
> if (within_margin(last_ewma_diff, (SCHED_CAPACITY_SCALE / 100)))
> return;


I see what you do, but Yuck! that's really nasty. Then again, I've not
actually got a better suggestion.


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