Re: [RFC PATCH 2/8] Documentation: arm: define DT cpu capacity bindings

[Mark Rutland <mark.rutland-5wv7dgnIgG8@public.gmane.org>]

On Tue, Dec 15, 2015 at 05:59:34PM +0100, Vincent Guittot wrote:
> On 15 December 2015 at 17:41, Mark Rutland <mark.rutland-5wv7dgnIgG8@public.gmane.org> wrote:
> > On Tue, Dec 15, 2015 at 04:23:18PM +0000, Catalin Marinas wrote:
> >> On Tue, Dec 15, 2015 at 03:57:37PM +0000, Mark Rutland wrote:
> >> > On Tue, Dec 15, 2015 at 03:46:51PM +0000, Juri Lelli wrote:
> >> > > On 15/12/15 15:32, Mark Rutland wrote:
> >> > > > On Tue, Dec 15, 2015 at 03:08:13PM +0000, Mark Brown wrote:
> >> > > > > My expectation is that we just need good enough, not perfect, and that
> >> > > > > seems to match what Juri is saying about the expectation that most of
> >> > > > > the fine tuning is done via other knobs.
> >> > > >
> >> > > > My expectation is that if a ballpark figure is good enough, it should be
> >> > > > possible to implement something trivial like bogomips / loop_per_jiffy
> >> > > > calculation.
> >> > >
> >> > > I didn't really followed that, so I might be wrong here, but isn't
> >> > > already happened a discussion about how we want/like to stop exposing
> >> > > bogomips info or rely on it for anything but in kernel delay loops?
> >> >
> >> > I meant that we could have a benchmark of that level of complexity,
> >> > rather than those specific values.
> >>
> >> Or we could simply let user space use whatever benchmarks or hard-coded
> >> values it wants and set the capacity via sysfs (during boot). By
> >> default, the kernel would assume all CPUs equal.
> >
> > I assume that a userspace override would be available regardless of
> > whatever mechanism the kernel uses to determine relative
> > performance/effinciency.
> 
> Don't you think that if we let a complete latitude to the userspace
> to set whatever it wants, it will be used to abuse the kernel (and the
> scheduler in particular ) and that this will finish in a real mess to
> understand what is wrong when a task is not placed where it should be.

I'm not sure I follow what you mean by "abuse" here. Userspace currently
can force the scheduler to make sub-optimal decisions in a number of
ways, e.g.

* Hot-unplugging the preferred CPUs
* Changing a task's affinity mask
* Setting the nice value of a task
* Using rlimits and/or cgroups
* Using a cpufreq governor
* Fork-bombing

Practically all of these are prvileged operations. I would envisage the
userspace interface for "capacity" management to be similar.

> We can probably provide a debug mode to help soc manufacturer to
> define their capacity value but IMHO we should not let complete
> latitude in normal operation

In normal operation userspace wouldn't mess with this, as with most of
the cases above. Userspace can already shooti tself in the foot.

> In normal operation we need to give some methods to tweak the value to
> reflect a memory bounded or integer calculation work  or other kind of
> work that currently runs on the cpu but not more

You can already do that with the mechanisms above, to some extent. I'm
not sure I follow.

Mark.
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