Re: [PATCH 6/7] thermal: netlink: Add a new event to notify CPU capabilities change

[Lukasz Luba]

On 11/9/21 2:15 PM, Srinivas Pandruvada wrote:
> On Tue, 2021-11-09 at 13:53 +0000, Lukasz Luba wrote:
> > Hi Srinivas,
> >
> > On 11/9/21 1:23 PM, Srinivas Pandruvada wrote:
> > > Hi Lukasz,
> > >
> > > On Tue, 2021-11-09 at 12:39 +0000, Lukasz Luba wrote:
> > > > Hi Ricardo,
> > > >
> > > >
> > > > On 11/6/21 1:33 AM, Ricardo Neri wrote:
> > > > > From: Srinivas Pandruvada <srinivas.pandruvada@xxxxxxxxxxxxxxx>
> > > > >
> > > > > Add a new netlink event to notify change in CPU capabilities in
> > > > > terms of
> > > > > performance and efficiency.
> > > >
> > > > Is this going to be handled by some 'generic' tools? If yes,
> > > > maybe
> > > > the values for 'performance' might be aligned with capacity
> > > > [0,1024] ? Or are they completely not related so the mapping is
> > > > simply impossible?
> > >
> > > That would have been very useful.
> > >
> > > The problem is that we may not know the maximum performance as
> > > system
> > > may be booting with few CPUs (using maxcpus kernel command line)
> > > and
> > > then user hot adding them. So we may need to rescale when we get a
> > > new
> > > maximum performance CPU and send to user space.
> > >
> > > We can't just use max from HFI table at in instance as it is not
> > > necessary that HFI table contains data for all CPUs.
> > >
> > > If HFI max performance value of 255 is a scaled value to max
> > > performance CPU value in the system, then this conversion would
> > > have
> > > been easy. But that is not.
> >
> > I see. I was asking because I'm working on similar interface and
> > just wanted to understand your approach better. In my case we
> > would probably simply use 'capacity' scale, or more
> > precisely available capacity after subtracting 'thermal pressure'
> > value.
> > That might confuse a generic tool which listens to these socket
> > messages, though. So probably I would have to add a new
> > THERMAL_GENL_ATTR_CPU_CAPABILITY_* id
> > to handle this different normalized across CPUs scale.
> I can add a field capacity_scale. In HFI case it will always be 255. In
> your cases it will 1024.

Sounds good, with that upper limit those tools would not build
up assumptions (they would have to parse that scale value).
Although, I would prefer to call it 'performance_scale' if you don't
mind.
I've done similar renaming  s/capacity/performance/ in the Energy Model
(EM) some time ago [1]. Some reasons:
- in the scheduler we have 'Performance Domains (PDs)'
- for GPUs we talk about 'performance', because 'capacity' sounds odd
  in that case

[1] 
https://lore.kernel.org/linux-pm/20200527095854.21714-2-lukasz.luba@xxxxxxx/