Date:   Thu, 22 Sep 2022 00:41:40 +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 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:

[...]

>>>>> +      * the idle thread and don't set next buddy as a candidate for being
>>>>> +      * picked in priority.
>>>>> +      * In case of simultaneous wakeup from idle, the latency sensitive tasks
>>>>> +      * lost opportunity to preempt non sensitive tasks which woke up
>>>>> +      * simultaneously.
>>>>> +      */
>>>>
>>>> The position of this comment block within this function is somehow
>>>> misleading since it describes the reason for the function rather then a
>>>> particular condition within this function. Wouldn't it be more readable
>>>> when it would be a function header comment instead?
>>>
>>> I put it after the usual early return tests to put the comment close
>>> to the useful part: the use of next buddy and __pick_first_entity()
>>
>> So you want to have the `wakeup_preempt_entity(se, pse) == 1` condition
>> from check_preempt_wakeup() also for cfs_task woken up by others.
> 
> I wake the wakeup_preempt_entity(cfs_rq->next, left) < 1 in
> pick_next_entity() to pick the task with highest latency constraint
> when another class is running while waking up

That's correct. This is where you potentially pick this task since it is
the next_buddy.
All I wanted to say is that check_preempt_from_others() and its `next &&
wakeup_preempt_entity(next, se) == 1` is the counterpart of the
`wakeup_preempt_entity(se, pse) == 1` in check_preempt_wakeup() to be
able to set next_buddy even curr is from an other class than CFS.

[...]

>>>> I still don't get the rationale behind why when either one (se or curr)
>>>> of the latency_nice values is negative, we use the diff between them but
>>>> if not, we only care about se's value. Why don't you always use the diff
>>>> between se and curr? Since we have a range [-20 ... 19] why shouldn't we
>>>> use the difference between let's say se = 19 and curr = 5?
>>>> You discussed this with Tao Zhou on the v1 but I didn't understand it fully.
>>>
>>> Let say that current has a latency nice prio of 19 and a task A with a
>>> latency nice of 10 wakes up. Both tasks don't care about scheduling
>>> latency (current more than task A). If we use the diff, the output of
>>> wakeup_latency_gran() would be negative (-10ms) which reflects the
>>> fact that the waking task is sensitive to the latency and wants to
>>> preempt current even if its vruntime is after. But obviously both
>>> current and task A don't care to preempt at wakeup.
>>
>> OK, I understand but there is a certain level of unsteadiness here.
>>
>> If p has >0 it gets treated differently in case current has >=0 and case
> 
> "If p >=0"; 0 has same behavior than [1..19]
> 
>> current has <0.

Not quite. It depends on curr. With sysctl_sched_latency = 24ms:

(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,

In (A) we 'punish' p even though it competes against curr which has an
even lower latency requirement than p,

>> Do we expect that tasks set their value to [1..19] in this case, when
>> the default 0 already indicates a 'don't care'?
> 
> I'm not sure that I understand your concern as [0..19] are treated in
> the same way. Only tasks (curr or se) with offset <0 need a relative
> comparison to the other. If curr and se has both a latency nice of
> -19, se should not blindly preempt curr but only if curr already run
> for its amount of time

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.

Essentially using the full [-20 .. 19] nice scope for `se vs. curr`
comparison.

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