Re: [PATCH v6 09/16] sched/cpufreq: uclamp: Add utilization clamping for RT tasks

Peter Zijlstra
2019-01-24T15:12:12Z

On Thu, Jan 24, 2019 at 12:38:35PM +0000, Patrick Bellasi wrote:
> On 24-Jan 12:30, Patrick Bellasi wrote:
> > On 23-Jan 21:11, Peter Zijlstra wrote:
> > > On Wed, Jan 23, 2019 at 02:40:11PM +0000, Patrick Bellasi wrote:
> > > > On 23-Jan 11:49, Peter Zijlstra wrote:
> > > > > On Tue, Jan 15, 2019 at 10:15:06AM +0000, Patrick Bellasi wrote:
>
> [...]
>
> > > I'm thikning that if we haz a single bit, say:
> > >
> > > struct uclamp_se {
> > > ...
> > > unsigned int changed : 1;
> > > };
> > >
> > > We can update uclamp_se::value and set uclamp_se::changed, and then the
> > > next enqueue will (unlikely) test-and-clear changed and recompute the
> > > bucket_id.
> >
> > This mean will lazy update the "requested" bucket_id by deferring its
> > computation at enqueue time. Which saves us a copy of the bucket_id,
> > i.e. we will have only the "effective" value updated at enqueue time.
> >
> > But...
> >
> > > Would that not be simpler?
> >
> > ... although being simpler it does not fully exploit the slow-path,
> > a syscall which is usually running from a different process context
> > (system management software).
> >
> > It also fits better for lazy updates but, in the cgroup case, where we
> > wanna enforce an update ASAP for RUNNABLE tasks, we will still have to
> > do the updates from the slow-path.
> >
> > Will look better into this simplification while working on v7, perhaps
> > the linear mapping can really help in that too.
>
> Actually, I forgot to mention that:
>
> uclamp_se::effective::{
> value, bucket_id
> }
>
> will be still required to proper support the cgroup delegation model,
> where a child group could be restricted by the parent but we want to
> keep track of the original "requested" value for when the parent
> should relax the restriction.
>
> Thus, since effective values are already there, why not using them
> also to pre-compute the new requested bucket_id from the slow path?

Well, we need the orig_value; but I'm still not sure why you need more
bucket_id's. Also, retaining orig_value is already required for the
system limits, there's nothing cgroup-y about this afaict.