Re: [PATCH v4 2/7] sched/fair: Decay task PELT values during wakeup migration

[Vincent Donnefort <vincent.donnefort@arm.com>]

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.

> 
> ---
>   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