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

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Date:   Tue, 11 Aug 2020 17:54:29 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     benbjiang(蒋彪) <benbjiang@...cent.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 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>

[...]

>> Because of this very small weight (weight=3), compared to a SCHED_NORMAL
>> nice 0 task (weight=1024), a SCHED_IDLE task is penalized by a huge
>> se->vruntime value (1024/3 higher than for a SCHED_NORMAL nice 0 task).
>> This should make sure it doesn't tick preempt a SCHED_NORMAL nice 0 task.
> Could you please explain how the huge penalization of vruntime(1024/3) could
> make sure SCHED_IDLE not tick preempting SCHED_NORMAL nice 0 task?
> 
> Thanks a lot.

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.

[...]