Date:   Wed, 27 Apr 2022 11:15:05 +0100
From:   Vincent Donnefort <vincent.donnefort@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     peterz@...radead.org, mingo@...hat.com,
        linux-kernel@...r.kernel.org, dietmar.eggemann@....com,
        morten.rasmussen@....com, chris.redpath@....com, qperret@...gle.com
Subject: Re: [PATCH v6 2/7] sched/fair: Decay task PELT values during wakeup
 migration



On 26/04/2022 17:41, Vincent Guittot wrote:
> Hi Vincent,
> 
> Le mardi 26 avril 2022 � 10:35:01 (+0100), Vincent Donnefort a �crit :
>> 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
>>
>> sched_clock_cpu() is a costly functinon. 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 biggest risk of being outdated.
>>
>> Neither clock_task, clock_pelt nor clock can be accessed without the
>> runqueue lock. We then need to store those values in a timestamp variable
>> which can be accessed during the migration. rq's enter_idle will give the
>> wall-clock time when the rq went idle. We have then:
>>
>>    B = sched_clock_cpu() - rq->enter_idle.
>>
>> Then, to catch-up the PELT clock scaling (A), two cases:
>>
>>    * !CFS_BANDWIDTH: We can simply use clock_task(). This value is stored
>>      in rq's clock_pelt_idle, before the rq enters idle. The estimated time
>>      is then:
>>
>>        rq->clock_pelt_idle + sched_clock_cpu() - rq->enter_idle.
>>
>>    * CFS_BANDWIDTH: We can't catch-up with clock_task because of the
>>      throttled_clock_task_time offset. cfs_rq's clock_pelt_idle is then
>>      giving the PELT clock when the cfs_rq becomes idle. This gives:
>>
>>        A = rq->clock_pelt_idle - cfs_rq->clock_pelt_idle
>>
>>      And gives the following estimated time:
>>
>>        cfs_rq->last_update_time +
>>        rq->clock_pelt_idle - cfs_rq->clock_pelt_idle + (A)
>>        sched_clock_cpu() - rq->enter_idle (B)
>>
>> The (B) part of the missing time is however an estimation that doesn't
>> take into account IRQ and Paravirt time.
>>
>> [1] https://lore.kernel.org/all/20190709115759.10451-1-chris.redpath@arm.com/
>>
>> Signed-off-by: Vincent Donnefort <vincent.donnefort@....com>
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index abd1feeec0c2..1256e2c0e2e2 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -3694,6 +3694,48 @@ 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 migrate_se_pelt_lag(struct sched_entity *se)
>> +{
>> +	struct cfs_rq *cfs_rq;
>> +	struct rq *rq;
>> +	bool is_idle;
>> +	u64 now;
>> +
>> +	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;
>> +
>> +#ifdef CONFIG_CFS_BANDWIDTH
>> +	now = u64_u32_load(cfs_rq->clock_pelt_idle);
>> +	/* The clock has been stopped for throttling */
>> +	if (now == U64_MAX)
>> +		return;
>> +
>> +	now += cfs_rq_last_update_time(cfs_rq);
>> +	now -= u64_u32_load(rq->clock_pelt_idle);
> 
> This is a bit difficult to read and follow.
> 
> we should have
> /* start at the last cfs pelt update time */
> now = cfs_rq_last_update_time(cfs_rq);
> /* Add delta between cfs rq becoming idle and rq becoming idle */
> now += u64_u32_load(cfs_rq->clock_pelt_idle); - u64_u32_load(rq->clock_pelt_idle);
> 
> But IIUC, you started with u64_u32_load(cfs_rq->clock_pelt_idle) to dectect throttled case

That is correct.

> 
> All this would deserve a comment explaining what we are computing here at least
And that would also helped me to identify that this part is wrong.

It should be rq->clock_pelt_idle - cfs_rq->clock_pelt_idle and not the
opposite, as rq->clock_pelt_idle is taken _after_ cfs_rq's. (especially
at idle time, rq's clock_pelt_idle is catching up with clock_task, while
cfs_rq is not).

> 
> 
>> +#else
>> +	now = u64_u32_load(rq->clock_pelt_idle);
>> +#endif
>> +	now += sched_clock_cpu(cpu_of(rq)) - u64_u32_load(rq->enter_idle);
>> +
>> +	__update_load_avg_blocked_se(now, se);
>> +}
>> +#else
>> +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()
>> @@ -4429,6 +4471,9 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
>>   	 */
>>   	if ((flags & (DEQUEUE_SAVE | DEQUEUE_MOVE)) != DEQUEUE_SAVE)
>>   		update_min_vruntime(cfs_rq);
>> +
>> +	if (cfs_rq->nr_running == 0)
>> +		update_idle_cfs_rq_clock_pelt(cfs_rq);
>>   }
>>   
>>   /*
>> @@ -6946,6 +6991,8 @@ 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 +7000,6 @@ 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 +7011,29 @@ 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 cfs_rq last_update_time
>> +		 * 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/pelt.h b/kernel/sched/pelt.h
>> index 4ff2ed4f8fa1..0380f750adbe 100644
>> --- a/kernel/sched/pelt.h
>> +++ b/kernel/sched/pelt.h
>> @@ -103,6 +103,14 @@ static inline void update_rq_clock_pelt(struct rq *rq, s64 delta)
>>   	rq->clock_pelt += delta;
>>   }
>>   
>> +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;
>> +}
>> +
>>   /*
>>    * When rq becomes idle, we have to check if it has lost idle time
>>    * because it was fully busy. A rq is fully used when the /Sum util_sum
>> @@ -130,17 +138,24 @@ 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);
>> +	 /* The rq is idle, we can sync with clock_task */
>> +	rq->clock_pelt = rq_clock_task(rq);
>>   
>> -	return rq->clock_pelt - rq->lost_idle_time;
>> +	u64_u32_store(rq->enter_idle, rq_clock(rq));
>> +	u64_u32_store(rq->clock_pelt_idle, rq_clock_pelt(rq));
> 
> rq and cfs rq clock pelt can also be updated during ILB/update_blocked_load.
> So you also need this:

Ack.


> 
> ---
>   kernel/sched/fair.c |  4 ++++
>   kernel/sched/pelt.h | 18 ++++++++++--------
>   2 files changed, 14 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index fa79d31a36bd..3c1d243079a3 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8269,6 +8269,10 @@ static bool __update_blocked_fair(struct rq *rq, bool *done)
>   		if (update_cfs_rq_load_avg(cfs_rq_clock_pelt(cfs_rq), cfs_rq)) {
>   			update_tg_load_avg(cfs_rq);
> 
> +			/* sync clock_pelt_idle with last update */
> +			if (cfs_rq->nr_running == 0)
> +				update_idle_cfs_rq_clock_pelt(cfs_rq);
> +
>   			if (cfs_rq == &rq->cfs)
>   				decayed = true;
>   		}
> diff --git a/kernel/sched/pelt.h b/kernel/sched/pelt.h
> index 0380f750adbe..6a4dbff8fdd3 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);
> +		u64_u32_store(rq->enter_idle, rq_clock(rq));
> +		u64_u32_store(rq->clock_pelt_idle, rq_clock_pelt(rq));
>   		return;
>   	}
> 
> @@ -103,14 +113,6 @@ static inline void update_rq_clock_pelt(struct rq *rq, s64 delta)
>   	rq->clock_pelt += delta;
>   }
> 
> -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;
> -}
> -
>   /*
>    * When rq becomes idle, we have to check if it has lost idle time
>    * because it was fully busy. A rq is fully used when the /Sum util_sum
> --
> 
> 
> 
>>   }
>>   
>>   #ifdef CONFIG_CFS_BANDWIDTH
>> +static inline void update_idle_cfs_rq_clock_pelt(struct cfs_rq *cfs_rq)
>> +{
>> +	if (unlikely(cfs_rq->throttle_count))
>> +		u64_u32_store(cfs_rq->clock_pelt_idle, U64_MAX);
>> +	else
>> +		u64_u32_store(cfs_rq->clock_pelt_idle,
>> +			      rq_clock_pelt(rq_of(cfs_rq)));
>> +}
>> +
>>   /* rq->task_clock normalized against any time this cfs_rq has spent throttled */
>>   static inline u64 cfs_rq_clock_pelt(struct cfs_rq *cfs_rq)
>>   {
>> @@ -150,6 +165,7 @@ static inline u64 cfs_rq_clock_pelt(struct cfs_rq *cfs_rq)
>>   	return rq_clock_pelt(rq_of(cfs_rq)) - cfs_rq->throttled_clock_pelt_time;
>>   }
>>   #else
>> +static inline void update_idle_cfs_rq_clock_pelt(struct cfs_rq *cfs_rq) { }
>>   static inline u64 cfs_rq_clock_pelt(struct cfs_rq *cfs_rq)
>>   {
>>   	return rq_clock_pelt(rq_of(cfs_rq));
>> @@ -204,6 +220,7 @@ update_rq_clock_pelt(struct rq *rq, s64 delta) { }
>>   static inline void
>>   update_idle_rq_clock_pelt(struct rq *rq) { }
>>   
>> +static inline void update_idle_cfs_rq_clock_pelt(struct cfs_rq *cfs_rq) { }
>>   #endif
>>   
>>   
>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>> index e2cf6e48b165..07014e8cbae2 100644
>> --- a/kernel/sched/sched.h
>> +++ b/kernel/sched/sched.h
>> @@ -641,6 +641,10 @@ struct cfs_rq {
>>   	int			runtime_enabled;
>>   	s64			runtime_remaining;
>>   
>> +	u64			clock_pelt_idle;
>> +#ifndef CONFIG_64BIT
>> +	u64                     clock_pelt_idle_copy;
>> +#endif
>>   	u64			throttled_clock;
>>   	u64			throttled_clock_pelt;
>>   	u64			throttled_clock_pelt_time;
>> @@ -1013,6 +1017,12 @@ struct rq {
>>   	u64			clock_task ____cacheline_aligned;
>>   	u64			clock_pelt;
>>   	unsigned long		lost_idle_time;
>> +	u64			clock_pelt_idle;
>> +	u64			enter_idle;
>> +#ifndef CONFIG_64BIT
>> +	u64			clock_pelt_idle_copy;
>> +	u64			enter_idle_copy;
>> +#endif
>>   
>>   	atomic_t		nr_iowait;
>>   
>> -- 
>> 2.25.1
>>

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