Re: [PATCH 4/4] cpuidle/menu: add per cpu pm_qos_resume_latency consideration

[Rafael J. Wysocki]

On 9/14/2016 10:28 AM, Vincent Guittot wrote:
> Hi Rafael,
>
> On 14 September 2016 at 00:10, Rafael J. Wysocki <rjw@xxxxxxxxxxxxx> wrote:
> > On Tuesday, September 13, 2016 10:02:49 PM Alex Shi wrote:
> > > Hi Daniel & Rafael,
> > >
> > > Any comments on this patch?
> > I actually am not sure about the whole series.
> >
> > I know your motivation, but honestly the changes here may not be the best way
> > to achieve what you need.
> >
> > You may think that the changes are trivial, but in fact they are not.  There
> > are side effects and I'm not sure about the resulting user space interface
> > at all.
> This patchset has got 2 parts:
> - one part is about taking into account per-device resume latency
> constraint when selecting the idle state of a CPU. This value can
> already be set by kernel (even if it's probably not done yet) but this
> constraint is never taken into account
> - the other part is about exposing the resume latency to userspace.
> This part might raise more discussion but I see one example that could
> take advantage of this. When you have several clusters of CPUs and you
> want to dedicate some CPUs  to latency sensitive activity and prevent
> deep sleep  state on these CPUs but you want to let the other CPUs
> using all C-state

The first very basic question about this I have is whether or not the 
device PM QoS mechanism is suitable for the task at hand at all.

It certainly hasn't been invented with it in mind.

Thanks,
Rafael