Date:   Sat, 30 Apr 2022 01:09:25 +0800
From:   Tao Zhou <tao.zhou@...ux.dev>
To:     Vincent Donnefort <vincent.donnefort@....com>
Cc:     peterz@...radead.org, mingo@...hat.com, vincent.guittot@...aro.org,
        linux-kernel@...r.kernel.org, dietmar.eggemann@....com,
        morten.rasmussen@....com, chris.redpath@....com,
        qperret@...gle.com, Tao Zhou <tao.zhou@...ux.dev>
Subject: Re: [PATCH v8 2/7] sched/fair: Decay task PELT values during wakeup
 migration

On Fri, Apr 29, 2022 at 03:11:43PM +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 will not be accounted for in
> util_avg and a discontinuity will appear. This issue is amplified by the
> PELT clock scaling. It takes currently one tick after the CPU being idle
> to let clock_pelt catching up clock_task.
> 
> 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.
> 
> To that end, we need sched_clock_cpu() but it is a costly function. 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 the clock is having the biggest risk of being outdated. In this such case,
> let's call it cfs_idle_lag the delta time between the rq_clock_pelt value
> at rq idle and cfs_rq idle. And rq_idle_lag the delta between "now" and
> the rq_clock_pelt at rq idle.
> 
> The estimated PELT clock is then:
> 
>    last_update_time + (the cfs_rq's last_update_time)
>    cfs_idle_lag + (delta between cfs_rq's update and rq's update)
>    rq_idle_lag (delta between rq's update and now)
> 
>   last_update_time = cfs_rq_clock_pelt()
>                    = rq_clock_pelt() - cfs->throttled_clock_pelt_time
> 
>   cfs_idle_lag = rq_clock_pelt()@rq_idle -
>                  rq_clock_pelt()@cfs_rq_idle
> 
>   rq_idle_lag = sched_clock_cpu() - rq_clock()@rq_idle
> 
> The rq_clock_pelt() from last_update_time being the same as
> rq_clock_pelt()@cfs_rq_idle, we can write:
> 
>   estimation = rq_clock_pelt()@rq_idle - cfs->throttled_clock_pelt_time +
>                sched_clock_cpu() - rq_clock()@rq_idle
> 
> The clocks being not accessible without the rq lock taken, some timestamps
> are created:
> 
>       rq_clock_pelt()@rq_idle        is rq->clock_pelt_idle
>       rq_clock()@rq_idle             is rq->enter_idle
>       cfs->throttled_clock_pelt_time is cfs_rq->throttled_pelt_idle
> 
> The rq_idle_lqg part of the missing time is however an estimation that

s/rq_idle_lqg/rq_idle_lag/

> 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..4e9e0d453a00 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3330,21 +3330,15 @@ static inline bool child_cfs_rq_on_list(struct cfs_rq *cfs_rq)
>  	return (prev_cfs_rq->tg->parent == cfs_rq->tg);
>  }
>  
> -static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq)
> +static inline bool load_avg_is_decayed(struct sched_avg *sa)
>  {
> -	if (cfs_rq->load.weight)
> -		return false;
> -
> -	if (cfs_rq->avg.load_sum)
> +	if (sa->load_sum)
>  		return false;
>  
> -	if (cfs_rq->avg.util_sum)
> +	if (sa->util_sum)
>  		return false;
>  
> -	if (cfs_rq->avg.runnable_sum)
> -		return false;
> -
> -	if (child_cfs_rq_on_list(cfs_rq))
> +	if (sa->runnable_sum)
>  		return false;
>  
>  	/*
> @@ -3352,9 +3346,23 @@ static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq)
>  	 * Make sure that rounding and/or propagation of PELT values never
>  	 * break this.
>  	 */
> -	SCHED_WARN_ON(cfs_rq->avg.load_avg ||
> -		      cfs_rq->avg.util_avg ||
> -		      cfs_rq->avg.runnable_avg);
> +	SCHED_WARN_ON(sa->load_avg ||
> +		      sa->util_avg ||
> +		      sa->runnable_avg);
> +
> +	return true;
> +}
> +
> +static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq)
> +{
> +	if (cfs_rq->load.weight)
> +		return false;
> +
> +	if (!load_avg_is_decayed(&cfs_rq->avg))
> +		return false;
> +
> +	if (child_cfs_rq_on_list(cfs_rq))
> +		return false;
>  
>  	return true;
>  }
> @@ -3694,6 +3702,79 @@ 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)
> +{
> +	u64 throttled = 0, now;
> +	struct cfs_rq *cfs_rq;
> +	struct rq *rq;
> +	bool is_idle;
> +
> +	if (load_avg_is_decayed(&se->avg))
> +		return;
> +
> +	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;
> +
> +	/*
> +	 * estimated "now" is:
> +	 *   last_update_time + (the cfs_rq's last_update_time)
> +	 *   cfs_idle_lag + (delta between cfs_rq's update and rq's update)
> +	 *   rq_idle_lag (delta between rq's update and now)
> +	 *
> +	 *   last_update_time = cfs_rq_clock_pelt()
> +	 *                    = rq_clock_pelt() - cfs->throttled_clock_pelt_time
> +	 *
> +	 *   cfs_idle_lag = rq_clock_pelt()@rq_idle -
> +	 *                  rq_clock_pelt()@cfs_rq_idle
> +	 *
> +	 *   rq_idle_lag = sched_clock_cpu() - rq_clock()@rq_idle
> +	 *
> +	 *   The rq_clock_pelt() from last_update_time being the same as
> +	 *   rq_clock_pelt()@cfs_rq_idle, we can write:
> +	 *
> +	 *     now = rq_clock_pelt()@rq_idle - cfs->throttled_clock_pelt_time +
> +	 *           sched_clock_cpu() - rq_clock()@rq_idle
> +	 *
> +	 *   Where:
> +	 *      rq_clock_pelt()@rq_idle        is rq->clock_pelt_idle
> +	 *      rq_clock()@rq_idle             is rq->enter_idle
> +	 *      cfs->throttled_clock_pelt_time is cfs_rq->throttled_pelt_idle
> +	 */
> +
> +#ifdef CONFIG_CFS_BANDWIDTH
> +	throttled = u64_u32_load(cfs_rq->throttled_pelt_idle);
> +	/* The clock has been stopped for throttling */
> +	if (throttled == U64_MAX)
> +		return;
> +#endif
> +	now = u64_u32_load(rq->clock_pelt_idle);
> +	now -= throttled;
> +
> +	/* An update happened while computing lag */
> +	if (now < cfs_rq_last_update_time(cfs_rq))
> +		return;
> +
> +	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 +4510,9 @@ 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 +7030,8 @@ 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 +7039,6 @@ 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 +7050,29 @@ 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);
>  }
> @@ -8149,6 +8238,10 @@ 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 4ff2ed4f8fa1..9aed92262bd9 100644
> --- a/kernel/sched/pelt.h
> +++ b/kernel/sched/pelt.h
> @@ -61,6 +61,23 @@ 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 rq is idle, we can sync to clock_task */
> +static inline void _update_idle_rq_clock_pelt(struct rq *rq)
> +{
> +	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));
> +}
> +
>  /*
>   * The clock_pelt scales the time to reflect the effective amount of
>   * computation done during the running delta time but then sync back to
> @@ -76,8 +93,7 @@ static inline void cfs_se_util_change(struct sched_avg *avg)
>  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);
> +		_update_idle_rq_clock_pelt(rq);
>  		return;
>  	}
>  
> @@ -130,17 +146,26 @@ 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;
> +
> +	_update_idle_rq_clock_pelt(rq);
>  }
>  
> -static inline u64 rq_clock_pelt(struct rq *rq)
> +#ifdef CONFIG_CFS_BANDWIDTH
> +static inline void update_idle_cfs_rq_clock_pelt(struct cfs_rq *cfs_rq)
>  {
> -	lockdep_assert_rq_held(rq);
> -	assert_clock_updated(rq);
> -
> -	return rq->clock_pelt - rq->lost_idle_time;
> +	/*
> +	 * Make sure that pending update of rq->clock_pelt_idle and
> +	 * rq->enter_idle are visible during update_blocked_average() before
> +	 * updating cfs_rq->throttled_pelt_idle.
> +	 */

Two places to call update_idle_cfs_rq_clock_pelt():

1 dequeue_entity()
    (no pending update before. and this is fast path)
    update_idle_cfs_rq_clock_pelt

2 update_blocked_averages()
    update_clock_rq() -> pending update here.
    __update_blocked_fair()
      update_idle_cfs_rq_clock_pelt

Another way will be to move the smp_wmb() to _update_idle_rq_clock_pelt()

static inline void _update_idle_rq_clock_pelt(struct rq *rq)
{
	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));
    smp_wmb();
}

But does this function called more often enough than dequeue_entity(), 

pick_next_task_fair()
  (rq will be idle)
  update_idle_rq_clock_pelt()

update_rq_clock_pelt()
  (curr is idle)
  _update_idle_rq_clock_pelt()

The condition is they are all idle.
And the migrate_se_pelt_lag() is for idle also.

If smp_wmb() is here like the patch, smp_rmb() place in 
migrate_se_pelt_lag() here:
  
#ifdef CONFIG_CFS_BANDWIDTH
	throttled = u64_u32_load(cfs_rq->throttled_pelt_idle);
    smp_rmb();
	/* The clock has been stopped for throttling */
	if (throttled == U64_MAX)
		return;
#endif

If smp_wmb() is in _update_idle_rq_clock_pelt(), smp_rmb() place in
migrate_se_pelt_lag() here:

#ifdef CONFIG_CFS_BANDWIDTH
	throttled = u64_u32_load(cfs_rq->throttled_pelt_idle);
	/* The clock has been stopped for throttling */
	if (throttled == U64_MAX)
		return;
#endif
    smp_rmb();
	now = u64_u32_load(rq->clock_pelt_idle);
	now -= throttled;

Sorry for these noise words.
> +	smp_wmb();
> +	if (unlikely(cfs_rq->throttle_count))
> +		u64_u32_store(cfs_rq->throttled_pelt_idle, U64_MAX);
> +	else
> +		u64_u32_store(cfs_rq->throttled_pelt_idle,
> +			      cfs_rq->throttled_clock_pelt_time);
>  }
>  
> -#ifdef CONFIG_CFS_BANDWIDTH
>  /* 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 +175,7 @@ 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 +230,7 @@ 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..ea9365e1a24e 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			throttled_pelt_idle;
> +#ifndef CONFIG_64BIT
> +	u64                     throttled_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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