Messages in this thread | ![/](/images/icornerl.gif) | | Date | Wed, 20 Apr 2022 10:34:30 +0100 | Subject | Re: [PATCH v4 2/7] sched/fair: Decay task PELT values during wakeup migration | From | Vincent Donnefort <> |
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On 19/04/2022 17:27, Vincent Guittot wrote: > Le mardi 19 avril 2022 � 13:23:27 (+0100), Vincent Donnefort a �crit : >> >> >> On 19/04/2022 11:08, Vincent Guittot wrote: >>> On Tue, 12 Apr 2022 at 15:42, Vincent Donnefort >>> <vincent.donnefort@arm.com> 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> > > [...] > >>> >>> I'm worried that we will call this for each and every >>> update_cfs_rq_load_avg() whereas the content will be used only when >>> idle and not throttled. Can't we use these conditions to save values >>> only when needed and limit the number of useless updates ? >> >> I don't think we can use idle here as a condition, once it is idle, it is >> too late to get those clock values. > > As an example, the patch below should work. It doesn't handle the throttled case yet and still has to > make sure that rq->enter_idle and rq->clock_pelt_idle are coherent in regards to ILB that > update blocked load.
I had to abandon the per-rq approach from v1 to v2. This is because of the following example:
1. task A sleep 2. rq's clock updated (e.g another task runs) 3. task A migrated
With a per-rq lag, we would miss the time delta between 1 and 2. We know how old is the last clock update. But what we actually want is how old is the task's last_update_time.
> > --- > kernel/sched/fair.c | 30 ++++++++++++++++++++++++++++++ > kernel/sched/pelt.h | 21 ++++++++++++++------- > kernel/sched/sched.h | 3 ++- > 3 files changed, 46 insertions(+), 8 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index e6ecf530f19f..f00843f9dd01 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -7005,6 +7005,35 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags) > > static void detach_entity_cfs_rq(struct sched_entity *se); > > +#ifdef CONFIG_NO_HZ_COMMON > +static inline void migrate_se_pelt_lag(struct sched_entity *se) > +{ > + u64 now; > + 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(); > + > + if (!is_idle) > + return; > + > + /* TODO handle throttled cfs */ > + /* TODO handle update ilb blocked load update */ > + now = READ_ONCE(rq->clock_pelt_idle); > + now += sched_clock_cpu(cpu_of(rq)) - READ_ONCE(rq->enter_idle); > + > + __update_load_avg_blocked_se(now, se); > +} > +#else > +static void migrate_se_pelt_lag(struct sched_entity *se) {} > +#endif > + > /* > * Called immediately before a task is migrated to a new CPU; task_cpu(p) and > * cfs_rq_of(p) references at time of call are still valid and identify the > @@ -7056,6 +7085,7 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu) > * sounds not bad. > */ > remove_entity_load_avg(&p->se); > + migrate_se_pelt_lag(&p->se); > } > > /* Tell new CPU we are migrated */ > diff --git a/kernel/sched/pelt.h b/kernel/sched/pelt.h > index c336f5f481bc..ece4423026e5 100644 > --- a/kernel/sched/pelt.h > +++ b/kernel/sched/pelt.h > @@ -61,6 +61,14 @@ static inline void cfs_se_util_change(struct sched_avg *avg) > WRITE_ONCE(avg->util_est.enqueued, enqueued); > } > > +static inline u64 rq_clock_pelt(struct rq *rq) > +{ > + lockdep_assert_rq_held(rq); > + assert_clock_updated(rq); > + > + return rq->clock_pelt - rq->lost_idle_time; > +} > + > /* > * The clock_pelt scales the time to reflect the effective amount of > * computation done during the running delta time but then sync back to > @@ -78,6 +86,8 @@ static inline void update_rq_clock_pelt(struct rq *rq, s64 delta) > if (unlikely(is_idle_task(rq->curr))) { > /* The rq is idle, we can sync to clock_task */ > rq->clock_pelt = rq_clock_task(rq); > + WRITE_ONCE(rq->enter_idle, rq_clock(rq)); /* this could be factorized with idle_stamp */ > + WRITE_ONCE(rq->clock_pelt_idle, rq_clock_pelt(rq)); /* last pelt clock update when idle */ > return; > } > > @@ -130,14 +140,11 @@ static inline void update_idle_rq_clock_pelt(struct rq *rq) > */ > if (util_sum >= divider) > rq->lost_idle_time += rq_clock_task(rq) - rq->clock_pelt; > -} > > -static inline u64 rq_clock_pelt(struct rq *rq) > -{ > - lockdep_assert_rq_held(rq); > - assert_clock_updated(rq); > - > - return rq->clock_pelt - rq->lost_idle_time; > + /* The rq is idle, we can sync with clock_task */ > + rq->clock_pelt = rq_clock_task(rq); > + WRITE_ONCE(rq->enter_idle, rq_clock(rq)); /* this could be factorized with idle_stamp */ > + WRITE_ONCE(rq->clock_pelt_idle, rq_clock_pelt(rq)); /* last pelt clock update when idle */ > } > > #ifdef CONFIG_CFS_BANDWIDTH > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h > index 6ab77b171656..108698446762 100644 > --- a/kernel/sched/sched.h > +++ b/kernel/sched/sched.h > @@ -1007,7 +1007,8 @@ struct rq { > u64 clock_task ____cacheline_aligned; > u64 clock_pelt; > unsigned long lost_idle_time; > - > + u64 clock_pelt_idle; > + u64 enter_idle; > atomic_t nr_iowait; > > #ifdef CONFIG_SCHED_DEBUG
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