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

Andreas Herrmann
2018-07-17T10:21:36Z

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@suse.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@suse.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;
> }
>
>