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

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Date:   Wed, 19 Aug 2020 13:55:44 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     benbjiang(蒋彪) <benbjiang@...cent.com>,
        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 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.

> 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.

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