Messages in this thread Patch in this message | | | Date | Wed, 20 Apr 2022 11:25:52 +0100 | Subject | Re: [PATCH v4 2/7] sched/fair: Decay task PELT values during wakeup migration | From | Vincent Donnefort <> |
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On 20/04/2022 10:34, Vincent Donnefort 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.
However, to go back to your idea of only updating when going idle: we could only do the update when the task is dequeued. We know that only after that dequeue we could have a wake-up migration.
Something like:
static void dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
...
update_load_avg(cfs_rq, se, UPDATE_TG); + if (entity_is_task(se)) + update_cfs_rq_lag(cfs_rq); se_update_runnable(se);
That would drastically reduce the number of time update_cfs_rq_lag() would be called, skipping the update during enqueue and tick.
Additionally, we could also check for nr_running in cfs_rq, to only update when cfs_rq is 'idle'?
Thoughts?
> >> >> --- >> 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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