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Date:   Thu, 13 Aug 2020 23:55:14 +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)

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>
> 
> [...]
> 
>>> 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.
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+. :)

> 
>> ** 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.

> 
>> ** 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. :)

Thx.
Regards,
Jiang
> 
> [...]

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