Re: [PATCH v3 07/14] sched/core: uclamp: enforce last task UCLAMP_MAX

[Dietmar Eggemann]

On 08/16/2018 06:47 PM, Patrick Bellasi wrote:
> On 16-Aug 17:43, Dietmar Eggemann wrote:
> > On 08/06/2018 06:39 PM, Patrick Bellasi wrote:
> > > When a util_max clamped task sleeps, its clamp constraints are removed
> > >from the CPU. However, the blocked utilization on that CPU can still be
> > > higher than the max clamp value enforced while that task was running.
> > > This max clamp removal when a CPU is going to be idle could thus allow
> > > unwanted CPU frequency increases, right while the task is not running.
> >
> > So 'rq->uclamp.flags == UCLAMP_FLAG_IDLE' means CPU is IDLE because
> > non-clamped tasks are tracked as well ((group_id = 0)).
>
> Right, but... with (group_id = 0) you mean that "non-clamped tasks are
> tracked" in the first clamp group?

Yes. I was asking myself what will happen if there are only non-clamped 
tasks runnable ...

> > Maybe this is worth mentioning here?
>
> Maybe I can explicitely say that we detect that there are not RUNNABLE
> tasks because all the clamp groups are in UCLAMP_NOT_VALID status.

Yes, would have helped me the grasp this earlier ...

[...]

> > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > index bc2beedec7bf..ff76b000bbe8 100644
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -906,7 +906,8 @@ uclamp_group_find(int clamp_id, unsigned int clamp_value)
> > >    * For the specified clamp index, this method computes the new CPU utilization
> > >    * clamp to use until the next change on the set of RUNNABLE tasks on that CPU.
> > >    */
> > > -static inline void uclamp_cpu_update(struct rq *rq, int clamp_id)
> > > +static inline void uclamp_cpu_update(struct rq *rq, int clamp_id,
> > > +				     unsigned int last_clamp_value)
> > >   {
> > >   	struct uclamp_group *uc_grp = &rq->uclamp.group[clamp_id][0];
> > >   	int max_value = UCLAMP_NOT_VALID;
> > > @@ -924,6 +925,19 @@ static inline void uclamp_cpu_update(struct rq *rq, int clamp_id)
> >
> > The condition:
> >
> >      if (!uclamp_group_active(uc_grp, group_id))
> >          continue;
> >
> > in 'for (group_id = 0; group_id <= CONFIG_UCLAMP_GROUPS_COUNT; ++group_id)
> > {}' makes sure that 'max_value == UCLAMP_NOT_VALID' is true for the if
> > condition (*):
> >
> >
> > >   		if (max_value >= SCHED_CAPACITY_SCALE)
> > >   			break;
> > >   	}
> > > +
> > > +	/*
> > > +	 * Just for the UCLAMP_MAX value, in case there are no RUNNABLE
> > > +	 * task, we keep the CPU clamped to the last task's clamp value.
> > > +	 * This avoids frequency spikes to MAX when one CPU, with an high
> > > +	 * blocked utilization, sleeps and another CPU, in the same frequency
> > > +	 * domain, do not see anymore the clamp on the first CPU.
> > > +	 */
> > > +	if (clamp_id == UCLAMP_MAX && max_value == UCLAMP_NOT_VALID) {
> > > +		rq->uclamp.flags |= UCLAMP_FLAG_IDLE;
> > > +		max_value = last_clamp_value;
> > > +	}
> > > +
> >
> > (*): So the uc_grp[group_id].value stays last_clamp_value?
>
> A bit confusing... but I think you've got the point.

OK.

> > What do you do when the blocked utilization decays below this enforced
> > last_clamp_value on that CPU?
>
> This is done _just_ for max_util:
> - it clamps a blocked utilization bigger then last_clamp_value
>    thus avoiding the selection of an OPP bigger then the one enforced
>    while the task was runnable
> - it has not effect on a blocked utilization smaller then last_clamp_value
>    thus allowing to reduce gracefully the OPP as long as the blocked
>    utilization is decayed

Ah correct, max_util is about capping, not boosting.

> > I assume there are plenty of this kind of corner cases because we have
> > blocked signals (including all tasks) and clamping (including runnable
> > tasks).
>
> This is a pretty compelling one I've noticed in my tests and thus
> worth a fix... I don't have on hand other similar corner cases, do
> you?

No not right now, will continue to watch out for them ...