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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:   Mon, 17 Aug 2020 10:57:24 +0200
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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>

[...]

>>> ** 2normal+1idle: idle preempt normal every 600+ms **
>>
>> During the 3.2s the 2 SCHED_OTHER tasks run, the SCHED_IDLE task is
>> switched in only once, after ~2.5s.
> Use your config with loop increased from 200 to 2000, to observe longer,
> 
>            <...>-37620 [002] d... 47950.446191: sched_switch: prev_comm=task_other-1 prev_pid=37620 prev_prio=120 prev_state=S ==> next_comm=task_idle-2 next_pid=37621 next_prio=120
>            <...>-37619 [002] d... 47955.687709: sched_switch: prev_comm=task_other-0 prev_pid=37619 prev_prio=120 prev_state=R ==> next_comm=task_idle-2 next_pid=37621 next_prio=120
> // The first preemption interval is 5.2s.
>            <...>-37620 [002] d... 47956.375716: sched_switch: prev_comm=task_other-1 prev_pid=37620 prev_prio=120 prev_state=R ==> next_comm=task_idle-2 next_pid=37621 next_prio=120
>            <...>-37619 [002] d... 47957.060722: sched_switch: prev_comm=task_other-0 prev_pid=37619 prev_prio=120 prev_state=R ==> next_comm=task_idle-2 next_pid=37621 next_prio=120
>            <...>-37620 [002] d... 47957.747728: sched_switch: prev_comm=task_other-1 prev_pid=37620 prev_prio=120 prev_state=R ==> next_comm=task_idle-2 next_pid=37621 next_prio=120
>           <...>-37620 [002] d... 47958.423734: sched_switch: prev_comm=task_other-1 prev_pid=37620 prev_prio=120 prev_state=R ==> next_comm=task_idle-2 next_pid=37621 next_prio=120
>            <...>-37620 [002] d... 47959.119740: sched_switch: prev_comm=task_other-1 prev_pid=37620 prev_prio=120 prev_state=R ==> next_comm=task_idle-2 next_pid=37621 next_prio=120
> // After the first preemption, the rest preemption intervals are all about 600ms+. :)

Are you sure about this?

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)

>>> ** 3normal+idle: idle preempt normal every 1000+ms **
>>
>> Ah, this was meant to be 3 SCHED_OTHER tasks only! To see the difference
>> in behavior.
> With 3 SCHED_OHTER tasks only, the SCHED_OHTER task is switched in
> Every 27ms.

normal task: (1024 / (1024 + 1024 + 1024))^(-1) * 4ms = 12ms

>>> ** 2normal(nice 19)+1idle(nice 0): idle preempt normal every 30+ms **
>>
>> During the 3.2s the 2 SCHED_OTHER tasks run, the SCHED_IDLE task is
>> switched in every ~45ms.
> That’s as what I expected. :)

idle task:    (3 / (15 + 15 + 3))^(-1) * 4ms = 44ms

normal task: (15 / (15 + 15 + 3))^(-1) * 4ms =  9ms