Messages in this thread | | | Date | Fri, 22 Apr 2022 23:14:58 +0800 | From | Tao Zhou <> | Subject | Re: [PATCH v4 2/7] sched/fair: Decay task PELT values during wakeup migration |
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On Tue, Apr 12, 2022 at 02:42:15PM +0100, Vincent Donnefort wrote: > Before being migrated to a new CPU, a task sees its PELT values > synchronized with rq last_update_time. Once done, that same task will also > have its sched_avg last_update_time reset. This means the time between > the migration and the last clock update (B) will not be accounted for in > util_avg and a discontinuity will appear. This issue is amplified by the > PELT clock scaling. If the clock hasn't been updated while the CPU is > idle, clock_pelt will not be aligned with clock_task and that time (A) > will be also lost. > > ---------|----- A -----|-----------|------- B -----|> > clock_pelt clock_task clock now > > This is especially problematic for asymmetric CPU capacity systems which > need stable util_avg signals for task placement and energy estimation. > > Ideally, this problem would be solved by updating the runqueue clocks > before the migration. But that would require taking the runqueue lock > which is quite expensive [1]. Instead estimate the missing time and update > the task util_avg with that value: > > A + B = clock_task - clock_pelt + sched_clock_cpu() - clock > > Neither clock_task, clock_pelt nor clock can be accessed without the > runqueue lock. The new cfs_rq last_update_lag is therefore created and > contains those three values when the last_update_time value for that very > same cfs_rq is updated. > > last_update_lag = clock - clock_task + clock_pelt > > And we can then write the missing time as follow: > > A + B = sched_clock_cpu() - last_update_lag > > The B. part of the missing time is however an estimation that doesn't take > into account IRQ and Paravirt time. > > Now we have an estimation for A + B, we can create an estimator for the > PELT value at the time of the migration. We need for this purpose to > inject last_update_time which is a combination of both clock_pelt and > lost_idle_time. The latter is a time value which is completely lost from a > PELT point of view and must be ignored. And finally, we can write: > > now = last_update_time + A + B > = last_update_time + sched_clock_cpu() - last_update_lag > > This estimation has a cost, mostly due to sched_clock_cpu(). Limit the > usage to the case where the source CPU is idle as we know this is when the > clock is having the biggest risk of being outdated. > > [1] https://lore.kernel.org/all/20190709115759.10451-1-chris.redpath@arm.com/ > > Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 5dd38c9df0cc..e234d015657f 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -3694,6 +3694,57 @@ static inline void add_tg_cfs_propagate(struct cfs_rq *cfs_rq, long runnable_sum > > #endif /* CONFIG_FAIR_GROUP_SCHED */ > > +#ifdef CONFIG_NO_HZ_COMMON > +static inline void update_cfs_rq_lag(struct cfs_rq *cfs_rq) > +{ > + struct rq *rq = rq_of(cfs_rq); > + > + u64_u32_store(cfs_rq->last_update_lag, > +#ifdef CONFIG_CFS_BANDWIDTH > + /* Timer stopped by throttling */ > + unlikely(cfs_rq->throttle_count) ? U64_MAX : > +#endif > + rq->clock - rq->clock_task + rq->clock_pelt); > +} > + > +static inline void migrate_se_pelt_lag(struct sched_entity *se) > +{ > + u64 now, last_update_lag; > + struct cfs_rq *cfs_rq; > + struct rq *rq; > + bool is_idle; > + > + cfs_rq = cfs_rq_of(se); > + rq = rq_of(cfs_rq); > + > + rcu_read_lock(); > + is_idle = is_idle_task(rcu_dereference(rq->curr)); > + rcu_read_unlock(); > + > + /* > + * The lag estimation comes with a cost we don't want to pay all the > + * time. Hence, limiting to the case where the source CPU is idle and > + * we know we are at the greatest risk to have an outdated clock. > + */ > + if (!is_idle) > + return; > + > + last_update_lag = u64_u32_load(cfs_rq->last_update_lag);
If CPU is idle, clock_pelt is equal to clock_task, A part is disappeared.
> + /* The clock has been stopped for throttling */ > + if (last_update_lag == U64_MAX) > + return; > + > + now = se->avg.last_update_time - last_update_lag + > + sched_clock_cpu(cpu_of(rq)); > + > + __update_load_avg_blocked_se(now, se); > +} > +#else > +static void update_cfs_rq_lag(struct cfs_rq *cfs_rq) {} > +static void migrate_se_pelt_lag(struct sched_entity *se) {} > +#endif > + > /** > * update_cfs_rq_load_avg - update the cfs_rq's load/util averages > * @now: current time, as per cfs_rq_clock_pelt() > @@ -3774,6 +3825,7 @@ update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq) > cfs_rq->last_update_time_copy, > sa->last_update_time); > #endif > + update_cfs_rq_lag(cfs_rq); > > return decayed; > } > @@ -6946,6 +6998,8 @@ static void detach_entity_cfs_rq(struct sched_entity *se); > */ > static void migrate_task_rq_fair(struct task_struct *p, int new_cpu) > { > + struct sched_entity *se = &p->se; > + > /* > * As blocked tasks retain absolute vruntime the migration needs to > * deal with this by subtracting the old and adding the new > @@ -6953,7 +7007,6 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu) > * the task on the new runqueue. > */ > if (READ_ONCE(p->__state) == TASK_WAKING) { > - struct sched_entity *se = &p->se; > struct cfs_rq *cfs_rq = cfs_rq_of(se); > > se->vruntime -= u64_u32_load(cfs_rq->min_vruntime); > @@ -6965,25 +7018,28 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu) > * rq->lock and can modify state directly. > */ > lockdep_assert_rq_held(task_rq(p)); > - detach_entity_cfs_rq(&p->se); > + 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 missing time from the rq clock and update > + * sched_avg to improve the PELT continuity after migration. > */ > - remove_entity_load_avg(&p->se); > + migrate_se_pelt_lag(se); > } > > /* Tell new CPU we are migrated */ > - p->se.avg.last_update_time = 0; > + se->avg.last_update_time = 0; > > /* We have migrated, no longer consider this task hot */ > - p->se.exec_start = 0; > + se->exec_start = 0; > > update_scan_period(p, new_cpu); > } > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h > index e2cf6e48b165..2f6446295e7d 100644 > --- a/kernel/sched/sched.h > +++ b/kernel/sched/sched.h > @@ -593,6 +593,12 @@ struct cfs_rq { > struct sched_avg avg; > #ifndef CONFIG_64BIT > u64 last_update_time_copy; > +#endif > +#ifdef CONFIG_NO_HZ_COMMON > + u64 last_update_lag; > +#ifndef CONFIG_64BIT > + u64 last_update_lag_copy; > +#endif > #endif > struct { > raw_spinlock_t lock ____cacheline_aligned; > -- > 2.25.1 >
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