Re: [PATCH v5 1/4] sched/fair: add util_est on top of PELT

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

On 07-Mar 10:39, Peter Zijlstra wrote:
> On Tue, Mar 06, 2018 at 07:58:51PM +0100, Peter Zijlstra wrote:
> > On Thu, Feb 22, 2018 at 05:01:50PM +0000, Patrick Bellasi wrote:
> > > +static inline void util_est_enqueue(struct cfs_rq *cfs_rq,
> > > +				    struct task_struct *p)
> > > +{
> > > +	unsigned int enqueued;
> > > +
> > > +	if (!sched_feat(UTIL_EST))
> > > +		return;
> > > +
> > > +	/* Update root cfs_rq's estimated utilization */
> > > +	enqueued  = READ_ONCE(cfs_rq->avg.util_est.enqueued);
> > > +	enqueued += _task_util_est(p);
> > > +	WRITE_ONCE(cfs_rq->avg.util_est.enqueued, enqueued);
> > > +}
> 
> > It appears to me this isn't a stable situation and completely relies on
> > the !nr_running case to recalibrate. If we ensure that doesn't happen
> > for a significant while the sum can run-away, right?
> > 
> > Should we put a max in enqueue to avoid this?
> 
> Thinking about this a bit more; would it make sense to adjust the
> running sum/avg on migration? Something along the lines of:
> 
>   util_avg = se->load_avg / (cfs_rq->load_avg + se->load_avg);
> 
> (which disregards cgroups), because that should more or less be the time
> it ends up running, given the WFQ rule.

I would say it makes sense from a purely mechanism stanpoing, but I'm
not entirely convinced it can be useful from a practical stanpoint.

First of all, that should be applied only when we migrate to a more
saturated CPU. Otherwise, when migrating on an empty CPU we would set
util_avg = 100%

Secondly, when we migrate to a saturated CPU, this adjustment will
contribute to under-estimate the task utilization.
Let say the task was running on a completely empty CPU, and thus we
was able to ramp up without being preempted. This value represents a
good estimation of the (most recent) task CPU demands.

Now, if on a following activation, we wakeup the task on an IDLE CPU
with a lot of blocked load, then we will scale down its util_avg
and assume the task will be smaller.
But:

a) if the blocked load does not turns into some task waking up again,
   underestimated the task introduces only further ramp-up latencies

b) if the load it due to really active tasks, the task will be
   preempted and it's utilization smaller... but we are already in a
   domain where utilization does not tell us anything useful for a
   task... and thus, why bothering to make it converging sooner?


> That way the disparity between tasks migrating into the CPU at u=1 and
> them going to sleep at u<1 is much smaller and the above sum doesn't run
> away nearly as wild (it still needs some upper bound though).

-- 
#include <best/regards.h>

Patrick Bellasi