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

[Vishal Chourasia <vishalc@linux.ibm.com>]

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.

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

Thanks
-- vishal.c