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

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Message-ID: <D4389FC6-322C-4B0B-AF46-F3184B9E5606@tencent.com>
Date:   Wed, 19 Aug 2020 14:10:24 +0000
From:   benbjiang(蒋彪) <benbjiang@...cent.com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
CC:     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>,
        "vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
        "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 6:46 PM, 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.
> 
> So the idle task runs every ~680ms but only for 1 tick (1ms) (4 times
> less than the normal tasks). The condition 'if (delta_exec >
> ideal_runtime)' in check_preempt_tick() is only true at the 4th tick
> when a normal task runs even though the slice is 3ms.
> 
> In the 250 Hz example the sched slice diffs are hidden behind the 4ms tick.
Exactly. Tick size decides the single runtime and the interval between
two runs of idle task. 
That makes sense for most of the test results above.

Thx.
Regards,
Jiang