linux-kernel - Re: [PATCH v4 5/8] sched/fair: Take into account latency priority at wakeup

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Message-ID: <0a437c9e-299d-574e-a393-f589c78ba2c7@arm.com>
Date:   Thu, 22 Sep 2022 18:50:00 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com,
        linux-kernel@...r.kernel.org, parth@...ux.ibm.com,
        qais.yousef@....com, chris.hyser@...cle.com,
        valentin.schneider@....com, patrick.bellasi@...bug.net,
        David.Laight@...lab.com, pjt@...gle.com, pavel@....cz,
        tj@...nel.org, qperret@...gle.com, tim.c.chen@...ux.intel.com,
        joshdon@...gle.com
Subject: Re: [PATCH v4 5/8] sched/fair: Take into account latency priority at
 wakeup

On 22/09/2022 09:12, Vincent Guittot wrote:
> On Thu, 22 Sept 2022 at 00:41, Dietmar Eggemann
> <dietmar.eggemann@....com> wrote:
>>
>> On 20/09/2022 17:49, Vincent Guittot wrote:
>>> On Tue, 20 Sept 2022 at 15:18, Dietmar Eggemann
>>> <dietmar.eggemann@....com> wrote:
>>>>
>>>> On 19/09/2022 17:39, Vincent Guittot wrote:
>>>>> On Mon, 19 Sept 2022 at 12:05, Dietmar Eggemann
>>>>> <dietmar.eggemann@....com> wrote:
>>>>>>
>>>>>> On 16/09/2022 10:03, Vincent Guittot wrote:

[...]

> I thought you were speaking about priority 0 vs [1..19] as you made a
> difference in your previous comment below
> 
>>
>> (1) p = 10 curr =  19 -> wakeup_latency_gran() returns 12ms
>>
>> (2) p = 10 curr = -10 -> wakeup_latency_gran() returns 24ms
>>
>> In (1) only p's own latency counts whereas in (2) we take the diff,
> 
> Yes because  curr is latency sensitive in (2) whereas it's not in (1)
> 
>>
>> In (A) we 'punish' p even though it competes against curr which has an
>> even lower latency requirement than p,
> 
> What is (A) ?  Assuming you meant (1), having a positive nice latency

Sorry, yes I meant (1).

> means that you don't have latency requirement but you are tolerant to
> scheduling delay so we don't 'punish' p. P will preempt curr is we are
> above the tolerance

wakeup_preempt_entity() {

    vdiff = curr->vruntime - se->vruntime

    vdiff -= wakeup_latency_gran(curr, se)   <-- (3)

    if (vdiff <= 0)
        return -1;

    ...
}

Wouldn't it be more suitable to return 0 from wakeup_latency_gran() if
both have latency_nice >=0 in this case instead of se->latency_offset?

By `punish` I mean that vdiff (3) gets smaller in case we return (the
positive) `se->latency_offset` even `latency nice of curr` > `latency
nice of p`.

[...]

>> With p = -19 and curr = -19 we would take the diff, so 0ms.
>>
>> With p = 19 and curr = 19, if we would use `latency_offset -=
>> curr->latency_offset` wakeup_latency_gran() would return 973/1024*24ms -
>> 973/1024*24ms = 0ms and nothing will shift.
>>
>> OTHA, in case (1) wakeup_latency_gran() would return 512/1024*24ms -
>> 973/1024*24ms = - 10.80ms. So p would gain an advantage here instead of
>> a penalty.
> 
> And that's all the point. A priority >= 0 means that you don't care
> about scheduling delays so there is no reason to be more aggressive
> with a task that is also latency tolerant. We only have to ensure that
> the delay stays in the acceptable range

OK, I understand you model here but I'm still not convinced. Might be
interesting to hear what others think.