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

[Patrick Bellasi <patrick.bellasi@arm.com>]

On 24-Jan 16:12, Peter Zijlstra wrote:
> 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.

Sure, the "effective" values are just a very convenient way (IMHO) to
know exactly which value/bucket_id is currently in use by a task while
keeping them well separated from the "requested" values.

So, you propose to add "orig_value"... but the end effect will be the
same... it's just that if we look at uclamp_se you have two dual
information:

 A) whatever a task or cgroup "request" is always in:

   uclamp_se::value
   uclamp_se::bucket_id

 B) whatever a task or cgroup "gets" is always in:

   uclamp_se::effective::value
   uclamp_se::effective::bucket_id

I find this partitioning useful and easy to use:

 1) the slow-path updates only data in A

 2) the fast-path updates only data in B
    by composing A data in uclamp_effective_get() @enqueue time.

-- 
#include <best/regards.h>

Patrick Bellasi