lkml.org 
[lkml]   [2021]   [Dec]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 2/4] sched/fair: Decay task PELT values during migration
On Mon, Dec 20, 2021 at 12:26:23PM +0100, Dietmar Eggemann wrote:
> On 09.12.21 17:11, Vincent Donnefort wrote:
>
> [...]
>
> > @@ -6899,6 +6899,14 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
> >
> > static void detach_entity_cfs_rq(struct sched_entity *se);
> >
> > +static u64 rq_clock_pelt_estimator(struct rq *rq)
> > +{
> > + u64 pelt_lag = sched_clock_cpu(cpu_of(rq)) -
> > + u64_u32_load(rq->clock_pelt_lag);
> > +
> > + return cfs_rq_last_update_time(&rq->cfs) + pelt_lag;
>
> Why do you use `avg.last_update_time` (lut) of the root cfs_rq here?
>
> p's lut was just synced to cfs_rq_of(se)'s lut in
>
> migrate_task_rq_fair() (1) -> remove_entity_load_avg() ->
> sync_entity_load_avg(se) (2)

Huum, indeed, the estimation is an offset on top of the se's last_update_time,
which I suppose could be different from the rq's cfs_rq.

I'll add a sched_entity argument for this function, to use either cfs_rq_of(se)
or se last_update_time


> [...]
>
> > @@ -6924,26 +6934,29 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
> > * In case of TASK_ON_RQ_MIGRATING we in fact hold the 'old'
> > * rq->lock and can modify state directly.
> > */
> > - lockdep_assert_rq_held(task_rq(p));
> > - detach_entity_cfs_rq(&p->se);
> > + lockdep_assert_rq_held(rq);
> > + detach_entity_cfs_rq(se);
> >
> > } else {
> > + remove_entity_load_avg(se);
> > +
> > /*
> > - * We are supposed to update the task to "current" time, then
> > - * its up to date and ready to go to new CPU/cfs_rq. But we
> > - * have difficulty in getting what current time is, so simply
> > - * throw away the out-of-date time. This will result in the
> > - * wakee task is less decayed, but giving the wakee more load
> > - * sounds not bad.
> > + * Here, the task's PELT values have been updated according to
> > + * the current rq's clock. But if that clock hasn't been
> > + * updated in a while, a substantial idle time will be missed,
> > + * leading to an inflation after wake-up on the new rq.
> > + *
> > + * Estimate the PELT clock lag, and update sched_avg to ensure
> > + * PELT continuity after migration.
> > */
> > - remove_entity_load_avg(&p->se);
> > + __update_load_avg_blocked_se(rq_clock_pelt_estimator(rq), se);
>
> We do __update_load_avg_blocked_se() now twice for p, 1. in (2) and then
> in (1) again.

the first __update_load_avg_blocked_se() ensures the se is aligned with the
cfs_rq's clock and then, update the "removed" struct accordingly. We couldn't
use the estimator there, it would break that structure.

>
> [...]
>

\
 
 \ /
  Last update: 2021-12-20 17:10    [W:0.050 / U:0.328 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site