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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>
Message-ID: <3a1047fc-a86a-014a-b17a-eae71f669da1@arm.com>
Date:   Wed, 12 Aug 2020 20:39:08 +0200
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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>

[...]

>> Trace a run of 2 SCHED_OTHER (nice 0) tasks and 1 SCHED_IDLE task on a
>> single CPU and trace_printk the conditions  'if (delta < 0)' and ' if
>> (delta > ideal_runtime)' in check_preempt_tick().
>>
>> Then do the same with 3 SCHED_OTHER (nice 0) tasks. You can also change
>> the niceness of the 2 SCHED_OTHER task to 19 to see some differences in
>> the kernelshark's task layout.
>>
>> rt-app (https://github.com/scheduler-tools/rt-app) is a nice tool to
>> craft those artificial use cases.
> With rt-app tool, sched_switch traced by ftrace, the result is as what I expected,

I use: 

{
        "tasks" : {
                "task_other" : {
                        "instance" : 2,
                        "loop" : 200,
                        "policy" : "SCHED_OTHER",
                        "run" : 8000,
                        "timer" : { "ref" : "unique1" , "period" : 16000, "mode" : "absolute" },
                        "priority" : 0
                },
                "task_idle" : {
                        "instance" : 1,
                        "loop" : 200,
                        "policy" : "SCHED_IDLE",
                        "run" : 8000,
                        "timer" : { "ref" : "unique2" , "period" : 16000, "mode" : "absolute" }
                }
        },
        "global" : {
                "calibration" : 243, <-- Has to be calibrated against the CPU you run on !!!
                "default_policy" : "SCHED_OTHER",
                "duration" : -1
        }
}

to have 2 (periodic) SCHED_OTHER and 1 SCHED_IDLE task. 

> ** 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.

> ** 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.

> ** 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.

[...]