Re: [PATCH v3] sched/fair: Sync se's load_avg with cfs_rq in reweight_task

[Chengming Zhou <chengming.zhou@linux.dev>]

On 2024/7/24 07:00, Vishal Chourasia wrote:
> On 24/07/24 2:40 am, Dietmar Eggemann wrote:
>> On 23/07/2024 17:48, Vishal Chourasia wrote:
>>> On Tue, Jul 23, 2024 at 07:42:47PM +0800, Chuyi Zhou wrote:
>>>> In reweight_task(), there are two situations:
>>>>
>>>> 1. The task was on_rq, then the task's load_avg is accurate because we
>>>> synchronized it with cfs_rq through update_load_avg() in dequeue_task().
>>>>
>>>> 2. The task is sleeping, its load_avg might not have been updated for some
>>>> time, which can result in inaccurate dequeue_load_avg() in
>>>> reweight_entity().
>>>>
>>>> This patch solves this by using sync_entity_load_avg() to synchronize the
>>>> load_avg of se with cfs_rq before dequeue_load_avg() in reweight_entity().
>>>> For tasks were on_rq, since we already update load_avg to accurate values
>>>> in dequeue_task(), this change will not have other effects due to the short
>>>> time interval between the two updates.
>>>>
>>>> Suggested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
>>>> Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
>>>> ---
>>>> Changes in v3:
>>>> - use sync_entity_load_avg() rather than update_load_avg() to sync the
>>>> sleeping task with its cfs_rq suggested by Dietmar.
>>>> - Link t0 v2: https://lore.kernel.org/lkml/20240720051248.59608-1-zhouchuyi@bytedance.com/
>>>> Changes in v2:
>>>> - change the description in commit log.
>>>> - use update_load_avg() in reweight_task() rather than in reweight_entity
>>>> suggested by chengming.
>>>> - Link to v1: https://lore.kernel.org/lkml/20240716150840.23061-1-zhouchuyi@bytedance.com/
>>>> ---
>>>>   kernel/sched/fair.c | 43 ++++++++++++++++++++++++-------------------
>>>>   1 file changed, 24 insertions(+), 19 deletions(-)
>>>>
>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>>> index 9057584ec06d..da3cdd86ab2e 100644
>>>> --- a/kernel/sched/fair.c
>>>> +++ b/kernel/sched/fair.c
>>>> @@ -3669,11 +3669,32 @@ dequeue_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se)
>>>>   	cfs_rq->avg.load_sum = max_t(u32, cfs_rq->avg.load_sum,
>>>>   					  cfs_rq->avg.load_avg * PELT_MIN_DIVIDER);
>>>>   }
>>>> +
>>>> +static inline u64 cfs_rq_last_update_time(struct cfs_rq *cfs_rq)
>>>> +{
>>>> +	return u64_u32_load_copy(cfs_rq->avg.last_update_time,
>>>> +				 cfs_rq->last_update_time_copy);
>>>> +}
>>>> +
>>>> +/*
>>>> + * Synchronize entity load avg of dequeued entity without locking
>>>> + * the previous rq.
>>>> + */
>>>> +static void sync_entity_load_avg(struct sched_entity *se)
>>>> +{
>>>> +	struct cfs_rq *cfs_rq = cfs_rq_of(se);
>>>> +	u64 last_update_time;
>>>> +
>>>> +	last_update_time = cfs_rq_last_update_time(cfs_rq);
>>>> +	__update_load_avg_blocked_se(last_update_time, se);
>>>> +}
>>>> +
>>> The difference between using update_load_avg() and
>>> sync_entity_load_avg() is:
>>> 1. update_load_avg() uses the updated PELT clock value from the rq
>>>     structure.
>>> 2. sync_entity_load_avg() uses the last updated time of
>>>     the cfs_rq where the scheduling entity (se) is attached.
>>>
>>> Won't this affect the entity load sync?
>>
>> Not sure what you mean exactly by entity load sync here.
> load avg sync for the wakee
>>
>> The task has been sleeping for a long time, i.e. its PELT values haven't
>> been updated or a long time (its last_update_time (lut) value is pretty
>> old).
>>
>> In this meantime the task's cfs_rq has potentially seen other PELT
>> updates due to PELT updates of other non-sleeping tasks related to this
>> cfs_rq. I.e. the cfs_rq lut is much more recent.
>>
>> What we want to do here is to sync the sleeping task with its cfs_rq. If
>> the task was sleeping for more than 1us (1024ns) and we cross a 1ms PELT
>> period (1024us) when we use cfs_rq's lut as the 'now' value for
>> __update_load_avg_blocked_se() then we will see the task PELT values decay.
>>
>> We rely on sync_entity_load_avg() for instance in EAS wakeup where the
>> task's util_avg influences on which CPU type the task will run next. So
>> we sync the wakee with its cfs_rq to be able to work with a current task
>> util_avg.
> I was talking about the case where all the tasks on a cfs_rq are sleeping.
> In this case, lut of the cfs_rq will be same as, at the time of last dequeue.

In this case, cfs_rq is not on_rq, its load_sum/avg will be updated when
enqueue next time. (Or periodically updated from load balance)

> 
> And, wakee is been woken up (suppose) after 1us window
> 
> 
> I guess, in this case pelt metric would not have changed for the cfs_rq

IMHO, so long as task_se load is synced with cfs_rq there should be ok.

Thanks.

> 
> Thanks
> -- vishal.c
>