linux-kernel - Re: [PATCH v4 1/2] sched: Fix UCLAMP_FLAG

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Date:   Thu, 22 Jul 2021 10:47:28 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Quentin Perret <qperret@...gle.com>
Cc:     mingo@...hat.com, peterz@...radead.org, vincent.guittot@...aro.org,
        qais.yousef@....com, rickyiu@...gle.com, wvw@...gle.com,
        patrick.bellasi@...bug.net, xuewen.yan94@...il.com,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v4 1/2] sched: Fix UCLAMP_FLAG_IDLE setting

On 21/07/2021 15:09, Quentin Perret wrote:
> Hi Dietmar,
> 
> On Wednesday 21 Jul 2021 at 12:07:04 (+0200), Dietmar Eggemann wrote:
>> On 19/07/2021 18:16, Quentin Perret wrote:
>>> The UCLAMP_FLAG_IDLE flag is set on a runqueue when dequeueing the last
>>> active task to maintain the last uclamp.max and prevent blocked util
>>
>> s/active/runnable ?
> 
> 'active' should still be correct here no? We enter uclamp_rq_max_value()
> -> uclamp_idle_value() when the last _active_ uclamp_se is decremented,
> and when all the buckets are empty, so I think that works?

Ah, it this uclamp ative `p->uclamp[clamp_id].active` which is set with
`bucket->tasks` in uclamp_rq_[inc/dec]_id.

Maybe add: last (uclamp) active task, i.e. (bucket.tasks == 0 for all
bucket_id's) ... ?

>>> from suddenly becoming visible.
>>>
>>
>> [...]
>>
>> IMHO, the main argument in v3 to do the clearing outside
>> uclamp_rq_inc_id() was a possible order change in `for_each_clamp_id()`.
>> So setting/clearing `rq->uclamp_flags` (UCLAMP_FLAG_IDLE) on UCLAMP_MAX
>> (currently the highest Uclamp constraint (UCLAMP_CNT-1)) could be
>> incorrect when UCLAMP_MIN and UCLAMP_MAX change place because the
>> same `rq->uclamp_flags` value is needed for both Uclamp constraint
>> values.
>>
>> What about decoupling rq->uclamp_flags` handling from UCLAMP_MAX and
>> doing this for 'UCLAMP_CNT - 1', i.e. always on the highest Uclamp
>> constraint?
>>
>> #define for_each_clamp_id(clamp_id) \
>>     for ((clamp_id) = 0; (clamp_id) < UCLAMP_CNT; (clamp_id)++)
>>
>> In this case the code change can be as easy as in your original v3.
>>
>> Setting UCLAMP_FLAG_IDLE in uclamp_idle_value():
>>
>>   uclamp_rq_dec_id() -> uclamp_rq_max_value() -> *uclamp_idle_value()*
>>
>> Resetting UCLAMP_FLAG_IDLE in uclamp_idle_reset():
>>
>>   uclamp_rq_inc_id()                          -> *uclamp_idle_reset()*  
>>
>> This would be more symmetrical then uclamp_idle_value() and
>> uclamp_rq_inc()/uclamp_rq_reinc_id().
> 
> Right, thanks for the suggestion but to be fair I feel like this is a
> matter of personal preference at this point. I personally like the way
> it is in this patch -- I find it easier to reason about, but maybe
> that's because I wrote it ...
> 
> Do you feel strongly about it? If not I'd prefer to not re-spin this
> another time if possible. Let me know what you think.

No, not at all ;-) Just like it better since it would mean less code
changes and only one place to reset UCLAMP_FLAG_IDLE.

You can add a:

Tested-by: Dietmar Eggemann <dietmar.eggemann@....com>

to your version in case you want to keep it.