Re: v5.7: new core kernel option missing help text

[Thara Gopinath]

On 6/3/20 4:25 PM, Valentin Schneider wrote:
> On 03/06/20 20:58, Russell King - ARM Linux admin wrote:
> > On Wed, Jun 03, 2020 at 09:24:56PM +0200, Vincent Guittot wrote:
> > > On Wed, 3 Jun 2020 at 20:45, Russell King - ARM Linux admin
> > > <linux@xxxxxxxxxxxxxxx> wrote:
> > > > It's a start.  I'm still wondering whether I should answer yes or no
> > > > for the platforms I'm building for.
> > > >
> > > > So far, all I've found is:
> > > >
> > > > arch/arm/include/asm/topology.h:#define arch_scale_thermal_pressure topology_get_thermal_pressure
> > > >
> > > > which really doesn't tell me anything about this.  So I'm still in
> > > > the dark.
> > > >
> > > > I guess topology_get_thermal_pressure is provided by something in
> > > > drivers/ which will be conditional on some driver or something.
> > >
> > > You need cpufreq_cooling device to make it useful and only for SMP
> > > I don't think that this should not be user configurable because even
> > > with the description above, it is not easy to choose.
> > > This should be set by the driver that implement the feature which is
> > > only cpufreq cooling device for now it
> >
> > As I have CONFIG_CPU_FREQ_THERMAL=y in my config, I'm guessing (and it's
> > only a guess) that I should say y to SCHED_THERMAL_PRESSURE ?
>
> arm and arm64 implement arch_scale_thermal_pressure(); the actual
> implementation is in the arch_topology "driver" (GENERIC_ARCH_TOPOLOGY).
>
> Then, the caller of arch_set_thermal_pressure() is cpufreq_cooling (see
> below); that'll only get called if you have thermal zones using CPU
> cooling devices.
>
> AFAICT the current state of things imply we should have something like
>
>          depends on (ARM || ARM64) && GENERIC_ARCH_TOPOLOGY
>
> for that option.

Hi Russel/Valentin

The feature itself like Valentin explained below allows scheduler to be 
aware of cpu capacity reduced due to thermal throttling. 
arch_set_thermal_pressure feeds the capped capacity to the scheduler and 
hence the feature makes sense only if arch_set_thermal_pressure is 
implemented. Having said that arch_set_thermal_pressure is implemented 
in arch_topology driver for arm and arm64 platforms. But the feature 
itself is not bound to arm/arm64 platforms. So it would make it wrong to 
add a "depends on (ARM || ARM64) option."

I agree with Vincent that allowing user to choose this option is 
probably not the best. IMO, this should be enabled by default in arm64 
defconfig considering both GENERIC_ARCH_TOPOLOGY and CPU_FREQ_THERMAL 
are enabled by default.
So if it is acceptable three things to be done are:
1. Add the help text.
2. Don't allow SCHED_THERMAL_PRESSURE configurable by user
3. Enable it by default in arm64 defconfig

> > > > > +     help
> > > > > +       This option allows the scheduler to be aware of CPU thermal throttling
> > > > > +       (i.e. thermal pressure), providing arch_scale_thermal_pressure() is
> > > > > +       implemented.
> >
> > Is this feature documented in terms of what it does?  Do I assume that
> > as the thermal trip points start tripping, that has an influence on
> > the scheduler?  Or is it the case that the scheduler is wanting to
> > know when the cpu frequency changes?
> >
> > Grepping for "thermal" in Documentation/scheduler brings up nothing.
>
> The former; changing a CPU cooling device's state (IOW changing its max
> allowed frequency for thermal reasons) leads to a call to
> arch_set_thermal_pressure() (see
> cpufreq_cooling.c::cpufreq_set_cur_state()).
>
> It's somewhat interesting to have, at least in theory. On plain SMP that
> would let the scheduler see if some CPUs are more throttled that others,
> which would be leveraged when doing load balancing. It's more
> interesting for big.LITTLE & co, where in the worst cases we can have
> things like capacity inversion, i.e. the bigs are so thermally throttled
> that they give less oomf than a LITTLE.

--
Warm Regards
Thara