Re: [PATCH v6 04/16] sched/core: uclamp: Add CPU's clamp buckets refcounting

Peter Zijlstra
2019-01-21T16:12:37Z

On Mon, Jan 21, 2019 at 03:23:11PM +0000, Patrick Bellasi wrote:
> On 21-Jan 15:59, Peter Zijlstra wrote:
> > On Tue, Jan 15, 2019 at 10:15:01AM +0000, Patrick Bellasi wrote:
> > > @@ -835,6 +954,28 @@ static void uclamp_bucket_inc(struct uclamp_se *uc_se, unsigned int clamp_id,
> > > } while (!atomic_long_try_cmpxchg(&uc_maps[bucket_id].adata,
> > > &uc_map_old.data, uc_map_new.data));
> > >
> > > + /*
> > > + * Ensure each CPU tracks the correct value for this clamp bucket.
> > > + * This initialization of per-CPU variables is required only when a
> > > + * clamp value is requested for the first time from a slow-path.
> > > + */
> >
> > I'm confused; why is this needed?
>
> That's a lazy initialization of the per-CPU uclamp data for a given
> bucket, i.e. the clamp value assigned to a bucket, which happens only
> when new clamp values are requested... usually only at system
> boot/configuration time.
>
> For example, let say we have these buckets mapped to given clamp
> values:
>
> bucket_#0: clamp value: 10% (mapped)
> bucket_#1: clamp value: 20% (mapped)
> bucket_#2: clamp value: 30% (mapped)
>
> and then let's assume all the users of bucket_#1 are "destroyed", i.e.
> there are no more tasks, system defaults or cgroups asking for a
> 20% clamp value. The corresponding bucket will become free:
>
> bucket_#0: clamp value: 10% (mapped)
> bucket_#1: clamp value: 20% (free)
> bucket_#2: clamp value: 30% (mapped)
>
> If, in the future, we ask for a new clamp value, let say a task ask
> for a 40% clamp value, then we need to map that value into a bucket.
> Since bucket_#1 is free we can use it to fill up the hold and keep all
> the buckets in use at the beginning of a cache line.
>
> However, since now bucket_#1 tracks a different clamp value (40
> instead of 20) we need to walk all the CPUs and updated the cached
> value:
>
> bucket_#0: clamp value: 10% (mapped)
> bucket_#1: clamp value: 40% (mapped)
> bucket_#2: clamp value: 30% (mapped)
>
> Is that more clear ?

Yes, and I realized this a little while after sending this; but I'm not
sure I have an answer to why though.

That is; why isn't the whole thing hard coded to have:

bucket_n: clamp value: n*UCLAMP_BUCKET_DELTA

We already do that division anyway (clamp_value / UCLAMP_BUCKET_DELTA),
and from that we instantly have the right bucket index. And that allows
us to initialize all this beforehand.

> and keep all
> the buckets in use at the beginning of a cache line.

That; is that the rationale for all this? Note that per the defaults
everything is in a single line already.