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SubjectRe: [PATCH v4 1/2] sched: Fix UCLAMP_FLAG_IDLE setting
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?

> > 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.

Cheers,
Quentin

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