Messages in this thread Patch in this message | | | Date | Tue, 6 Feb 2018 16:50:56 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v4 1/3] sched/fair: add util_est on top of PELT |
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Mostly nice, I almost applied, except too many nits below.
On Tue, Feb 06, 2018 at 02:41:29PM +0000, Patrick Bellasi wrote:
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 7b6535987500..118f49c39b60 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -5193,6 +5193,20 @@ static inline void hrtick_update(struct rq *rq) > } > #endif > > +static inline unsigned long task_util(struct task_struct *p); > +static inline unsigned long _task_util_est(struct task_struct *p);
What's with the leading underscore? I don't see one without it.
> + > +static inline void util_est_enqueue(struct task_struct *p)
Also pass @rq from enqueue_task_fair() ? I see no point in computing task_rq(p) if we already have the value around.
> +{ > + struct cfs_rq *cfs_rq = &task_rq(p)->cfs; > + > + if (!sched_feat(UTIL_EST)) > + return; > + > + /* Update root cfs_rq's estimated utilization */ > + cfs_rq->avg.util_est.enqueued += _task_util_est(p); > +}
> +/* > + * Check if the specified (signed) value is within a specified margin, > + * based on the observation that: > + * abs(x) < y := (unsigned)(x + y - 1) < (2 * y - 1) * Note: this only works when x+y < INT_MAX.
> + */ > +static inline bool within_margin(long value, unsigned int margin)
This mixing of long and int is dodgy, do we want to consistently use int here?
> +{ > + return ((unsigned int)(value + margin - 1) < (2 * margin - 1)); > +} > + > +static inline void util_est_dequeue(struct task_struct *p, int flags) > +{ > + struct cfs_rq *cfs_rq = &task_rq(p)->cfs; > + unsigned long util_last; > + long last_ewma_diff; > + unsigned long ewma; > + long util_est = 0;
Why long?
> + > + 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. > + */ > + if (cfs_rq->nr_running) { > + util_est = READ_ONCE(cfs_rq->avg.util_est.enqueued);
Because util_est.enqueued is of type 'unsigned int'.
> + util_est -= min_t(long, util_est, _task_util_est(p)); > + } > + WRITE_ONCE(cfs_rq->avg.util_est.enqueued, util_est);
long to int truncate
> + > + /* > + * Skip update of task's estimated utilization when the task has not > + * yet completed an activation, e.g. being migrated. > + */ > + if (!(flags & DEQUEUE_SLEEP)) > + return; > + > + ewma = READ_ONCE(p->se.avg.util_est.ewma); > + util_last = task_util(p);
Again, all kinds of long, while the ewma type itself is of 'unsigned int'.
> + > + /* > + * Skip update of task's estimated utilization when its EWMA is > + * already ~1% close to its last activation value. > + */ > + last_ewma_diff = util_last - ewma; > + if (within_margin(last_ewma_diff, (SCHED_CAPACITY_SCALE / 100))) > + return; > + > + /* > + * Update Task's estimated utilization > + * > + * When *p completes an activation we can consolidate another sample > + * about the task size. This is done by storing the last PELT value > + * for this task and using this value to load another sample in the > + * exponential weighted moving average: > + * > + * ewma(t) = w * task_util(p) + (1-w) * ewma(t-1) > + * = w * task_util(p) + ewma(t-1) - w * ewma(t-1) > + * = w * (task_util(p) - ewma(t-1)) + ewma(t-1) > + * = w * ( last_ewma_diff ) + ewma(t-1) > + * = w * (last_ewma_diff + ewma(t-1) / w) > + * > + * Where 'w' is the weight of new samples, which is configured to be > + * 0.25, thus making w=1/4 ( >>= UTIL_EST_WEIGHT_SHIFT) > + */ > + ewma = last_ewma_diff + (ewma << UTIL_EST_WEIGHT_SHIFT); > + ewma >>= UTIL_EST_WEIGHT_SHIFT; > + > + WRITE_ONCE(p->se.avg.util_est.ewma, ewma); > + WRITE_ONCE(p->se.avg.util_est.enqueued, util_last);
Two stores to that word... can we fix that nicely?
> +}
> +static inline unsigned long _task_util_est(struct task_struct *p) > +{ > + return max(p->se.avg.util_est.ewma, p->se.avg.util_est.enqueued); > +}
Aside from the underscore thing I already noted, why is this here and not where the fwd declaration is?
> +/* > + * UtilEstimation. Use estimated CPU utilization. > + */ > +SCHED_FEAT(UTIL_EST, false)
Since you couldn't measure it, do we wants it true?
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