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

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Date:   Mon, 17 Aug 2020 12:05:09 +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 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>
> 
> [...]
> 
>>>> ** 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?
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? :)

Thx.
Regards,
Jiang
> 
>>>> ** 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