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

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Date:   Tue, 17 Jul 2018 11:27:21 +0200
From:   Andreas Herrmann <aherrmann@...e.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
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 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.  It looks like
> we also should look for the presence of the PCC interface in there.
> 
> I can provide a patch for that, will you be able to test it?

Yes, I can test it.

> >> It should be initialized before pcc-cpufreq (according to their
> >> respective initcall levels), so in theory intel_pstate should be used
> >> by default on the affected systems anyway.
> >
> >> What BIOS settings need to be changed for that?
> >
> > Already answered in other mail.
> 
> Indeed.


Andreas