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Message-ID: <1913041e-ee67-1e65-68fa-ef08b97ed9d5@arm.com>
Date: Fri, 6 Jan 2023 12:28:26 +0100
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Chen Yu <yu.c.chen@...el.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
Vincent Guittot <vincent.guittot@...aro.org>,
Tim Chen <tim.c.chen@...el.com>,
Mel Gorman <mgorman@...hsingularity.net>,
Juri Lelli <juri.lelli@...hat.com>,
Rik van Riel <riel@...riel.com>,
Aaron Lu <aaron.lu@...el.com>,
Abel Wu <wuyun.abel@...edance.com>,
K Prateek Nayak <kprateek.nayak@....com>,
Yicong Yang <yangyicong@...ilicon.com>,
"Gautham R . Shenoy" <gautham.shenoy@....com>,
Ingo Molnar <mingo@...hat.com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Valentin Schneider <vschneid@...hat.com>,
Hillf Danton <hdanton@...a.com>,
Honglei Wang <wanghonglei@...ichuxing.com>,
Len Brown <len.brown@...el.com>,
Chen Yu <yu.chen.surf@...il.com>,
Tianchen Ding <dtcccc@...ux.alibaba.com>,
Joel Fernandes <joel@...lfernandes.org>,
Josh Don <joshdon@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v4 1/2] sched/fair: Introduce short duration task
check
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.
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