Re: [PATCH v6 07/16] sched/core: uclamp: Add system default clamps

[Peter Zijlstra]

On Tue, Jan 22, 2019 at 02:43:29PM +0000, Patrick Bellasi wrote:
> On 22-Jan 14:56, Peter Zijlstra wrote:
> > On Tue, Jan 15, 2019 at 10:15:04AM +0000, Patrick Bellasi wrote:
> > 
> > > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > > index 84294925d006..c8f391d1cdc5 100644
> > > --- a/include/linux/sched.h
> > > +++ b/include/linux/sched.h
> > > @@ -625,6 +625,11 @@ struct uclamp_se {
> > >  	unsigned int bucket_id		: bits_per(UCLAMP_BUCKETS);
> > >  	unsigned int mapped		: 1;
> > >  	unsigned int active		: 1;
> > > +	/* Clamp bucket and value actually used by a RUNNABLE task */
> > > +	struct {
> > > +		unsigned int value	: bits_per(SCHED_CAPACITY_SCALE);
> > > +		unsigned int bucket_id	: bits_per(UCLAMP_BUCKETS);
> > > +	} effective;
> > 
> > I am confuzled by this thing..  so uclamp_se already has a value,bucket,
> > which per the prior code is the effective one.
> > 
> > Now; I think I see why you want another value; you need the second to
> > store the original value for when the system limits change and we must
> > re-evaluate.
> 
> Yes, that's one reason, the other one being to properly support
> CGroup when we add them in the following patches.
> 
> Effective will always track the value/bucket in which the task has
> been refcounted at enqueue time and it depends on the aggregated
> value.

> > Should you not update all tasks?
> 
> That's true, but that's also an expensive operation, that's why now
> I'm doing only lazy updates at next enqueue time.

Aaah, so you refcount on the original value, which allows you to skip
fixing up all tasks. I missed that bit.


> Do you think that could be acceptable?

Think so, it's a sysctl poke, 'nobody' ever does that.