Re: [PATCH v7 01/15] sched/core: uclamp: Add CPU's clamp buckets refcounting

Peter Zijlstra
2019-03-13T19:39:16Z

On Wed, Mar 13, 2019 at 03:59:54PM +0000, Patrick Bellasi wrote:
> On 13-Mar 14:52, Peter Zijlstra wrote:

> Because of backetization, we potentially end up tracking tasks with
> different requested clamp values in the same bucket.
>
> For example, with 20% bucket size, we can have:
> Task1: util_min=25%
> Task2: util_min=35%
> accounted in the same bucket.

> > Given all that, what is to stop the bucket value to climbing to
> > uclamp_bucket_value(+1)-1 and staying there (provided there's someone
> > runnable)?
>
> Nothing... but that's an expected consequence of bucketization.

No, it is not.

> > Why are we doing this... ?
>
> You can either decide to:
>
> a) always boost tasks to just the bucket nominal value
> thus always penalizing both Task1 and Task2 of the example above

This is the expected behaviour. When was the last time your histogram
did something like b?

> b) always boost tasks to the bucket "max" value
> thus always overboosting both Task1 and Task2 of the example above
>
> The solution above instead has a very good property: in systems
> where you have only few and well defined clamp values we always
> provide the exact boost.
>
> For example, if your system requires only 23% and 47% boost values
> (totally random numbers), then you can always get the exact boost
> required using just 3 bucksts or ~33% size each.
>
> In systems where you don't know which boost values you will have, you
> can still defined the maximum overboost granularity you accept for
> each task by just tuning the number of clamp groups. For example, with
> 20 groups you can have a 5% max overboost.

Maybe, but this is not a direct concequence of buckets, but an
additional heuristic that might work well in this case.

Maybe split this out in a separate patch? So start with the trivial
bucket, and then do this change on top with the above few paragraphs as
changelog?