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Subject: Re: [PATCH] sched/fair: reduce preemption with IDLE tasks
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From:   Dietmar Eggemann <dietmar.eggemann@arm.com>
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Date:   Wed, 19 Aug 2020 12:46:22 +0200
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On 17/08/2020 14:05, benbjiang(蒋彪) wrote:
> 
> 
>> On Aug 17, 2020, at 4:57 PM, Dietmar Eggemann <dietmar.eggemann@arm.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@arm.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@arm.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@arm.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@arm.com> wrote:
>>>>>>>>>>
>>>>>>>>>> On 03/08/2020 13:26, benbjiang(蒋彪) wrote:
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>> On Aug 3, 2020, at 4:16 PM, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote:
>>>>>>>>>>>>
>>>>>>>>>>>> On 01/08/2020 04:32, Jiang Biao wrote:
>>>>>>>>>>>>> From: Jiang Biao <benbjiang@tencent.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.

[...]