Re: [PATCH v6 2/7] sched/fair: Decay task PELT values during wakeup migration

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

On Tue, Apr 26, 2022 at 10:35:01AM +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 (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 biggest risk of being outdated.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

The code calulating A below is not consistent with this. The order is reversed.

>     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@arm.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);
> +#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));
>  }
>  
>  #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
>