Messages in this thread | | | From | Vincent Guittot <> | Date | Thu, 21 Apr 2022 09:36:08 +0200 | Subject | Re: [PATCH v4 2/7] sched/fair: Decay task PELT values during wakeup migration |
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On Wed, 20 Apr 2022 at 11:34, Vincent Donnefort <vincent.donnefort@arm.com> wrote: > > 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.
I don't get your point here. Even after 2, won't task A be correctly updated with the patch below ? As I mentioned, this doesn't take into account case where cfs can be throttled (ie CFS_BANDWIDTH)
But applying similar scheme should work
> > > > > --- > > 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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