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Subject: Re: [PATCH] platform/x86: intel_pmc_core: Add Package C-states
residency info
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In-Reply-To: <CAHp75Vd5Wnio-RCEBENtonYWOJF2+88FDvqkUv1HzV3CdcaaPA@mail.gmail.com>

On Fri, Aug 18, 2017 at 08:17:32PM +0300, Andy Shevchenko wrote:
> +PeterZ (since I mentioned his name)
>
> On Fri, Aug 18, 2017 at 5:58 PM, Rajneesh Bhardwaj
> <rajneesh.bhardwaj@intel.com> wrote:
> > On Fri, Aug 18, 2017 at 03:57:34PM +0300, Andy Shevchenko wrote:
> >> On Fri, Aug 18, 2017 at 3:37 PM, Rajneesh Bhardwaj
> >> <rajneesh.bhardwaj@intel.com> wrote:
> >> > This patch introduces a new debugfs entry to read current Package C-state
> >> > residency values and, one new kernel API to read the Package C-10 residency
> >> > counter.
> >> >
> >> > Package C-state residency MSRs provide useful debug information about system
> >> > idle states. In idle states system must enter deeper Package C-states.
>
> >> Why this patch is needed?
> >
> > Andy, I'll try to give some background for this.
> >
> > This is needed to enhance the S0ix failure debug capabilities from within
> > the kernel. On ChromeOS we have S0ix failsafe kernel framework that is used
> > to validate S0ix and report the blockers in case of a failure.
> > https://patchwork.kernel.org/patch/9148999/
>
> (It's not part of upstream)

Sorry i sent an older link. There are fresh attempts to get this into
mainline kernel and looks like there is a traction for it.
https://patchwork.kernel.org/patch/9831229/

Package C-state (PC10) validation is discussed there.

>
> > So far only intel_pmc_slp_s0_counter_read is called by this framework to
> > check whether the previous attempt to enter S0ix was success or not.
>
> I harder see even a single user of that API in current kernel. It
> should be unexported and removed I think.
>
> > Having
> > another PC10 counter related exported function enhances the S0ix debug since
> > PC10 state is a prerequisite to enter S0ix.
> >
> >> See, we have turbostat and cpupower user space tools which do this
> >> without any additional code to be written in kernel. What prevents
> >> your user space application do the same?
> >>
> >> Moreover, we have events for cstate, I assume perf or something alike
> >> can monitor those counters as well.
> >
> > You're right, perhaps the debugfs is redundant when we have those user space
> > tools but such tools are not available readily for all platforms/distros.
> > Interfaces like /dev/cpu/*/msr that turbostat uses are not available on all
> > the platforms.
> > PMC driver is a debug driver so i thought its better to show Package C-state
> > related info for low power debug here.
> >
> >>
> >> Sorry, NAK.
> >
> > This patch has two parts i.e. exported PC10 API and the debugfs. Based on
> > the above explanation, if the patch is not good as is, please let me know if
> > i should drop the debugfs part and respin a v2 with just the exported API or
> > drop this totally.
> >
> > Thanks for the feedback and thanks for taking time to review!
>
> Reading above makes me think that entire design of this is misguided.
> Since the most of values are counters they better to be accessed in a
> way how perf does.
>
> In case you need *in-kernel* facility, do some APIs (if it's not done
> yet) for events drivers first.
> cstate event driver is already in upstream.
>
> Sorry, NAK for entire patch until it would be blessed by people like Peter Z.
>
> --
> With Best Regards,
> Andy Shevchenko

--
Best Regards,
Rajneesh

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