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

[Vincent Guittot]

On 15 December 2015 at 17:41, Mark Rutland <mark.rutland@xxxxxxx> 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.
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 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

Vincent
>
> I am not opposed to that mechanism being "assume equal".
>
> Mark.
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