linux-kernel - Re: [PATCH] sched/fair: reduce preemption with IDLE tasks runable(Internet mail)

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Message-ID: <A8E52241-0206-447D-9EA0-699F8F82AFAB@tencent.com>
Date:   Wed, 19 Aug 2020 14:27:35 +0000
From:   benbjiang(蒋彪) <benbjiang@...cent.com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
CC:     Vincent Guittot <vincent.guittot@...aro.org>,
        Jiang Biao <benbjiang@...il.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "juri.lelli@...hat.com" <juri.lelli@...hat.com>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "bsegall@...gle.com" <bsegall@...gle.com>,
        "mgorman@...e.de" <mgorman@...e.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched/fair: reduce preemption with IDLE tasks
 runable(Internet mail)



> On Aug 19, 2020, at 7:55 PM, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
> 
> On 19/08/2020 13:05, Vincent Guittot wrote:
>> On Wed, 19 Aug 2020 at 12:46, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>>> 
>>> On 17/08/2020 14:05, benbjiang(蒋彪) wrote:
>>>> 
>>>> 
>>>>> On Aug 17, 2020, at 4:57 PM, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>>>>> 
>>>>> On 14/08/2020 01:55, benbjiang(蒋彪) wrote:
>>>>>> Hi,
>>>>>> 
>>>>>>> On Aug 13, 2020, at 2:39 AM, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>>>>>>> 
>>>>>>> On 12/08/2020 05:19, benbjiang(蒋彪) wrote:
>>>>>>>> Hi,
>>>>>>>> 
>>>>>>>>> On Aug 11, 2020, at 11:54 PM, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>>>>>>>>> 
>>>>>>>>> On 11/08/2020 02:41, benbjiang(蒋彪) wrote:
>>>>>>>>>> Hi,
>>>>>>>>>> 
>>>>>>>>>>> On Aug 10, 2020, at 9:24 PM, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>>>>>>>>>>> 
>>>>>>>>>>> On 06/08/2020 17:52, benbjiang(蒋彪) wrote:
>>>>>>>>>>>> Hi,
>>>>>>>>>>>> 
>>>>>>>>>>>>> On Aug 6, 2020, at 9:29 PM, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>>>>>>>>>>>>> 
>>>>>>>>>>>>> On 03/08/2020 13:26, benbjiang(蒋彪) wrote:
>>>>>>>>>>>>>> 
>>>>>>>>>>>>>> 
>>>>>>>>>>>>>>> On Aug 3, 2020, at 4:16 PM, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>>>>>>>>>>>>>>> 
>>>>>>>>>>>>>>> On 01/08/2020 04:32, Jiang Biao wrote:
>>>>>>>>>>>>>>>> From: Jiang Biao <benbjiang@...cent.com>
>>> 
>>> [...]
>>> 
>>>>> Are you sure about this?
>>>> Yes. :)
>>>>> 
>>>>> The math is telling me for the:
>>>>> 
>>>>> idle task:      (3 / (1024 + 1024 + 3))^(-1) * 4ms = 2735ms
>>>>> 
>>>>> normal task: (1024 / (1024 + 1024 + 3))^(-1) * 4ms =    8ms
>>>>> 
>>>>> (4ms - 250 Hz)
>>>> My tick is 1ms - 1000HZ, which seems reasonable for 600ms? :)
>>> 
>>> OK, I see.
>>> 
>>> But here the different sched slices (check_preempt_tick()->
>>> sched_slice()) between normal tasks and the idle task play a role to.
>>> 
>>> Normal tasks get ~3ms whereas the idle task gets <0.01ms.
>> 
>> In fact that depends on the number of CPUs on the system
>> :sysctl_sched_latency = 6ms * (1 + ilog(ncpus)) . On a 8 cores system,
>> normal task will run around 12ms in one shoot and the idle task still
>> one tick period
> 
> True. This is on a single CPU.
Agree. :)

> 
>> Also, you can increase even more the period between 2 runs of idle
>> task by using cgroups and min shares value : 2
> 
> Ah yes, maybe this is what Jiang wants to do then? If his runtime does
> not have other requirements preventing this.
That could work for increasing the period between 2 runs. But could not
reduce the single runtime of idle task I guess, which means normal task
could have 1-tick schedule latency because of idle task. 
OTOH, cgroups(shares) could introduce extra complexity. :)  

I wonder if there’s any possibility to make SCHED_IDLEs’ priorities absolutely
lower than SCHED_NORMAL(OTHER), which means no weights/shares
for them, and they run only when no other task’s runnable.
I guess there may be priority inversion issue if we do that. But maybe we
could avoid it by load-balance more aggressively, or it(priority inversion)
could be ignored in some special case.

Thx. 
Regard,
Jiang
 
> 
> [...]