Messages in this thread | | | Date | Mon, 20 Dec 2021 17:09:42 +0100 | From | Vincent Donnefort <> | Subject | Re: [PATCH 2/4] sched/fair: Decay task PELT values during migration |
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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 - * throw away the out-of-date time.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.
> > [...] >
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