Re: [PATCH v5 00/10] track CPU utilization

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

On 05-Jun 16:18, Peter Zijlstra wrote:
> On Mon, Jun 04, 2018 at 08:08:58PM +0200, Vincent Guittot wrote:
> > On 4 June 2018 at 18:50, Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > > So this patch-set tracks the !cfs occupation using the same function,
> > > which is all good. But what, if instead of using that to compensate the
> > > OPP selection, we employ that to renormalize the util signal?
> > >
> > > If we normalize util against the dynamic (rt_avg affected) cpu_capacity,
> > > then I think your initial problem goes away. Because while the RT task
> > > will push the util to .5, it will at the same time push the CPU capacity
> > > to .5, and renormalized that gives 1.

And would not that mean also that a 50% task co-scheduled with the
same 50% RT task, will be reported as a 100% util_avg task?

> > >
> > >   NOTE: the renorm would then become something like:
> > >         scale_cpu = arch_scale_cpu_capacity() / rt_frac();
> 
> Should probably be:
> 
> 	scale_cpu = atch_scale_cpu_capacity() / (1 - rt_frac())
> 
> > >
> > >
> > > On IRC I mentioned stopping the CFS clock when preempted, and while that
> > > would result in fixed numbers, Vincent was right in pointing out the
> > > numbers will be difficult to interpret, since the meaning will be purely
> > > CPU local and I'm not sure you can actually fix it again with
> > > normalization.
> > >
> > > Imagine, running a .3 RT task, that would push the (always running) CFS
> > > down to .7, but because we discard all !cfs time, it actually has 1. If
> > > we try and normalize that we'll end up with ~1.43, which is of course
> > > completely broken.
> > >
> > >
> > > _However_, all that happens for util, also happens for load. So the above
> > > scenario will also make the CPU appear less loaded than it actually is.
> > 
> > The load will continue to increase because we track runnable state and
> > not running for the load
> 
> Duh yes. So renormalizing it once, like proposed for util would actually
> do the right thing there too.  Would not that allow us to get rid of
> much of the capacity magic in the load balance code?
> 
> /me thinks more..
> 
> Bah, no.. because you don't want this dynamic renormalization part of
> the sums. So you want to keep it after the fact. :/
> 
> > As you mentioned, scale_rt_capacity give the remaining capacity for
> > cfs and it will behave like cfs util_avg now that it uses PELT. So as
> > long as cfs util_avg <  scale_rt_capacity(we probably need a margin)
> > we keep using dl bandwidth + cfs util_avg + rt util_avg for selecting
> > OPP because we have remaining spare capacity but if  cfs util_avg ==
> > scale_rt_capacity, we make sure to use max OPP.

What will happen for the 50% task of the example above?

> Good point, when cfs-util < cfs-cap then there is idle time and the util
> number is 'right', when cfs-util == cfs-cap we're overcommitted and
> should go max.

Again I cannot easily read the example above...

Would that mean that a 50% CFS task, preempted by a 50% RT task (which
already set OPP to max while RUNNABLE) will end up running at the max
OPP too?

> Since the util and cap values are aligned that should track nicely.

True... the only potential issue I see is that we are steering PELT
behaviors towards better driving schedutil to run high-demand
workloads while _maybe_ affecting quite sensibly the capacity of PELT
to describe how much CPU a task uses.

Ultimately, utilization has always been a metric on "how much you
use"... while here it seems to me we are bending it to be something to
define "how fast you have to run".

-- 
#include <best/regards.h>

Patrick Bellasi