linux-kernel - Re: Commit 554c8aa8ecad causing severe performance degression with pcc-cpufreq

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Date:   Tue, 17 Jul 2018 12:21:36 +0200
From:   Andreas Herrmann <aherrmann@...e.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Commit 554c8aa8ecad causing severe performance degression with
 pcc-cpufreq

On Tue, Jul 17, 2018 at 12:09:21PM +0200, Rafael J. Wysocki wrote:
> On Tuesday, July 17, 2018 11:36:20 AM CEST Andreas Herrmann wrote:
> > On Tue, Jul 17, 2018 at 11:27:21AM +0200, Andreas Herrmann wrote:
> > > On Tue, Jul 17, 2018 at 11:23:25AM +0200, Rafael J. Wysocki wrote:
> > > > On Tue, Jul 17, 2018 at 11:11 AM, Andreas Herrmann <aherrmann@...e.com> wrote:
> > > > > On Tue, Jul 17, 2018 at 11:06:29AM +0200, Rafael J. Wysocki wrote:
> > > > >> On Tue, Jul 17, 2018 at 10:50 AM, Andreas Herrmann <aherrmann@...e.com> wrote:
> > > > >>
> > > > >> [cut]
> > > > >>
> > > > >> >
> > > > >> > On balance before this commit users could use pcc-cpufreq but had
> > > > >> > already suboptimal performance (compared to say intel_pstate driver
> > > > >> > which can be used changing BIOS options).
> > > > >>
> > > > >> BTW, I wonder why you need to change the BIOS options for intel_pstate to load.
> > > > >
> > > > > I think this is because of (in intel_pstate_init()):
> > > > >
> > > > >         /*
> > > > >          * The Intel pstate driver will be ignored if the platform
> > > > >          * firmware has its own power management modes.
> > > > >          */
> > > > >         if (intel_pstate_platform_pwr_mgmt_exists())
> > > > >                 return -ENODEV;
> > > > >
> > > > 
> > > > OK, because of the "Proliant" entry, right?
> > > > 
> > > > So it looks like we have an issue there.  We find the entry and we
> > > > look for _PSS.  It is not there, so we assume that the firmware is
> > > > expected to control performance, which is not the case.
> > 
> > FYI, there is another BIOS setting on those systems. It's called
> > "Collaborative Power Control" (AFAIK enabled by default).
> > 
> > Only if this is disabled, firmware is (alone) in control of
> > performance. (And of course in this case neither pcc-cpufreq nor
> > intel_pstate will be loaded).
> 
> OK, the patch is below.
> 
> First, I hope that if "Collaborative Power Control" is disabled, it will
> simply hide the PCCH object and so intel_pstate will still not load then.

PCCH is hidden in that case.

> The main question basically is what the OS is expected to do if
> "Dynamic Power Savings Mode" is set.  If we are *expected* to use
> the PCC interface then, intel_pstate may not work in that case, but
> I suspect that the PCC interface allows extra energy to be saved
> over what is possible without it.

I'll test it and see what happens.


Andreas

> ---
>  drivers/cpufreq/intel_pstate.c |   17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
> 
> Index: linux-pm/drivers/cpufreq/intel_pstate.c
> ===================================================================
> --- linux-pm.orig/drivers/cpufreq/intel_pstate.c
> +++ linux-pm/drivers/cpufreq/intel_pstate.c
> @@ -2391,6 +2391,18 @@ static bool __init intel_pstate_no_acpi_
>  	return true;
>  }
>  
> +static bool __init intel_pstate_no_acpi_pcch(void)
> +{
> +	acpi_status status;
> +	acpi_handle handle;
> +
> +	status = acpi_get_handle(NULL, "\\_SB", &handle);
> +	if (ACPI_FAILURE(status))
> +		return true;
> +
> +	return !acpi_has_method(handle, "PCCH");
> +}
> +
>  static bool __init intel_pstate_has_acpi_ppc(void)
>  {
>  	int i;
> @@ -2450,7 +2462,10 @@ static bool __init intel_pstate_platform
>  
>  	switch (plat_info[idx].data) {
>  	case PSS:
> -		return intel_pstate_no_acpi_pss();
> +		if (!intel_pstate_no_acpi_pss())
> +			return false;
> +
> +		return intel_pstate_no_acpi_pcch();
>  	case PPC:
>  		return intel_pstate_has_acpi_ppc() && !force_load;
>  	}
> 
>