Re: [PATCH v7 2/7] sched/fair: Decay task PELT values during wakeup migration

[Tao Zhou <tao.zhou@linux.dev>]

On Fri, Apr 29, 2022 at 10:20:00AM +0200, Vincent Guittot wrote:

> Le vendredi 29 avril 2022 à 01:22:19 (+0800), Tao Zhou a écrit :
> > Hi Vincent,
> > 
> > On Thu, Apr 28, 2022 at 03:38:39PM +0200, Vincent Guittot wrote:
> > 
> > > On Wed, 27 Apr 2022 at 19:37, Tao Zhou <tao.zhou@linux.dev> wrote:
> > > >
> 
> [..]
> 
> > > > > +                     /* sync clock_pelt_idle with last update */
> > > > > +                     if (cfs_rq->nr_running == 0)
> > > > > +                             update_idle_cfs_rq_clock_pelt(cfs_rq);
> > > >
> > > > I think that if cfs_rq->nr_running == 0 then use cfs rq pelt_idle to update
> > > > idle cfs rq.
> > > 
> > > update_blocked_averages() updates all cfs rq to be aligned with now so
> > > we don't need to calculate an estimated now. update_rq_clock(rq) is
> > > called 1st to update the rq->clock and childs
> > > 
> > > With only need to save when happened the last update which is done in
> > > update_rq_clock_pelt(rq) for rq->clock_pelt and with
> > > update_idle_cfs_rq_clock_pelt(cfs) for the cfs_rq_clock_pelt
> > 
> > I missed this.
> 
> I ended up with something a bit different:
> 
> ---
>  kernel/sched/fair.c  | 133 ++++++++++++++++++++++++++++++++++---------
>  kernel/sched/pelt.h  |  66 ++++++++++++++++++---
>  kernel/sched/sched.h |  10 ++++
>  3 files changed, 174 insertions(+), 35 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index abd1feeec0c2..63e4cf225292 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3335,27 +3335,12 @@ static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq)
>  	if (cfs_rq->load.weight)
>  		return false;
>  
> -	if (cfs_rq->avg.load_sum)
> -		return false;
> -
> -	if (cfs_rq->avg.util_sum)
> -		return false;
> -
> -	if (cfs_rq->avg.runnable_sum)
> +	if (load_avg_is_decayed(&cfs_rq->avg))
>  		return false;
>  
>  	if (child_cfs_rq_on_list(cfs_rq))
>  		return false;
>  
> -	/*
> -	 * _avg must be null when _sum are null because _avg = _sum / divider
> -	 * 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);
> -
>  	return true;
>  }
>  
> @@ -3694,6 +3679,88 @@ 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, throttled = 0;
> +
> +	/* utilization is already fully decayed */
> +	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: last update of the cfs_lock_pelt +
> +	 *   cfs_idle_lag: rq_clock_pelt delta bewteen last cfs update and last rq update +

s/bewteen/between/

> +	 *   rq_idle_lag: rq_clock delta between last rq update and now
> +	 *
> +	 * with
> +	 *
> +	 * last_update_time == cfs_clock_pelt()
> +	 *                  == rq_clock_pelt() - cfs->throttled_clock_pelt_time
> +	 *
> +	 * cfs_idle_lag: rq_clock_pelt() when rq is idle - rq_clock_pelt() when cfs is idle
> +	 *
> +	 * rq_idle_lag : sched_clock_cpu() - rq_clock() when rq is idle
> +	 *
> +	 * In fact, rq_clock_pelt() that is used for last_update_time and when
> +	 * cfs is idle are the same because their last update happens atthe

s/atthe/at the/

> +	 * same time.
> +	 *
> +	 * We can optimize "now" to be:
> +	 *   rq_clock_pelt when rq is idle - cfs->throttled_clock_pelt_time when cfs is idle +
> +	 *   sched_clock_cpu() - rq_clock() when rq is idle
> +	 *
> +	 * when rq is idle
> +	 *   rq_clock_pelt() is saved in rq->clock_pelt_idle
> +	 *   rq_clock()  is saved in rq->enter idle
> +	 *
> +	 * when cfs is idle
> +	 *    cfs->throttled_clock_pelt_time is saved in cfs_rq->throttled_pelt_idle
> +	 *
> +	 * When !CFS_BANDWIDTH, cfs->throttled_clock_pelt_time is null
> +	 */
> +
> +#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 +4496,9 @@ 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 +7016,8 @@ 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 +7025,6 @@ 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 +7036,29 @@ 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 +8224,10 @@ 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..4143c6dc64dc 100644
> --- a/kernel/sched/pelt.h
> +++ b/kernel/sched/pelt.h
> @@ -37,6 +37,29 @@ update_irq_load_avg(struct rq *rq, u64 running)
>  }
>  #endif
>  
> +static inline bool load_avg_is_decayed(struct sched_avg *sa)
> +{
> +	if (sa->load_sum)
> +		return false;
> +
> +	if (sa->util_sum)
> +		return false;
> +
> +	if (sa->runnable_sum)
> +		return false;
> +
> +	/*
> +	 * _avg must be null when _sum are null because _avg = _sum / divider
> +	 * Make sure that rounding and/or propagation of PELT values never
> +	 * break this.
> +	 */
> +	SCHED_WARN_ON(sa->load_avg ||
> +		      sa->util_avg ||
> +		      sa->runnable_avg);
> +
> +	return true;
> +}
> +
>  #define PELT_MIN_DIVIDER	(LOAD_AVG_MAX - 1024)
>  
>  static inline u32 get_pelt_divider(struct sched_avg *avg)
> @@ -61,6 +84,23 @@ 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 +116,7 @@ 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 +169,26 @@ 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.
> +	 */
> +	smp_wmb();

No smp_rmb() at read side and I feel it is in migrate_se_pelt_lag()
before this line：
    ‘now = u64_u32_load(rq->clock_pelt_idle);’
The load of cfs_rq->throttled_pelt_idle should be valid before the load
of rq->clock_pelt_idle and rq->enter_idle. This is paired with the write
side in update_idle_cfs_rq_clock_pelt().

> +	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 +198,7 @@ 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 +253,7 @@ 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;
>  
> -- 
> 2.17.1