Re: [RFC PATCH v4 1/2] sched/fair: Introduce short duration task check

[Dietmar Eggemann]

On 06/01/2023 09:34, Chen Yu wrote:
> Hi Dietmar,
> thanks for reviewing the patch!
> On 2023-01-05 at 12:33:16 +0100, Dietmar Eggemann wrote:
>> On 16/12/2022 07:11, Chen Yu wrote:
>>
>> [...]
>>
>>> @@ -5995,6 +6005,18 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>>>  
>>>  static void set_next_buddy(struct sched_entity *se);
>>>  
>>> +static inline void dur_avg_update(struct task_struct *p, bool task_sleep)
>>> +{
>>> +	u64 dur;
>>> +
>>> +	if (!task_sleep)
>>> +		return;
>>> +
>>> +	dur = p->se.sum_exec_runtime - p->se.prev_sum_exec_runtime_vol;
>>> +	p->se.prev_sum_exec_runtime_vol = p->se.sum_exec_runtime;
>>
>> Shouldn't se->prev_sum_exec_runtime_vol be set in enqueue_task_fair()
>> and not in dequeue_task_fair()->dur_avg_update()? Otherwise `dur` will
>> contain sleep time.
>>
> After the task p is dequeued, p's sum_exec_runtime will not be increased.

True.

> Unless task p is switched in again, p's sum_exec_runtime will continue to
> increase. So dur should not include the sleep time, because we substract

Not sure I get this sentence? p's se->sum_exec_runtime will only
increase if p is current, so running?

> between the sum_exec_runtime rather than rq->clock_task. Not sure if I understand
> this correctly?

No, you're right. We're not dealing with time snapshots but rather with
sum_exec_runtime snapshots. So the value will not change between dequeue
and the next enqueue.

e ... enqueue_task_fair()
d ... dequeue_task_fair()
s ... set_next_entity()
p ... put_prev_entity()
u ... update_curr_fair()->update_curr()

p1:

---|---||--|--|---|--|--||---
   d   es  u  p   s  u  pd

   ^   ^
   |   |
  (A) (B)

Same se->prev_sum_exec_runtime_vol value in (A) and (B).

> My original thought was that, record the average run time of every section:
> Only consider that task voluntarily relinquishes the CPU.
> For example, suppose on CPU1, task p1 and p2 run alternatively:
> 
>  --------------------> time
> 
>  | p1 runs 1ms | p2 preempt p1 | p1 switch in, runs 0.5ms and blocks |
>                ^               ^                                     ^
>  |_____________|               |_____________________________________|
>                                                                      ^
>                                                                      |
>                                                                   p1 dequeued
> 
> p1's duration in one section is (1 + 0.5)ms. Because if p2 does not
> preempt p1, p1 can run 1.5ms. This reflects the nature of a task,
> how long it wishes to run at most.
> 
>> Like we do for se->prev_sum_exec_runtime in set_next_entity() but for
>> one `set_next_entity()-put_prev_entity()` run section.
>>
>> AFAICS, you want to measure the exec_runtime sum over all run sections
>> between enqueue and dequeue.
> Yes, we tried to record the 'decayed' average exec_runtime for each section.
> Say, task p runs for a ms , then p is dequeued and blocks for b ms, and then
> runs for c ms, its average duration is 0.875 * a + 0.125 * c , which is
> what update_avg() does.

OK.