linux-kernel - Re: [PATCH] cpufreq: intel_pstate: Use most recent guaranteed performance values

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Date:   Sun, 20 Dec 2020 17:41:30 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     srinivas pandruvada <srinivas.pandruvada@...ux.intel.com>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Len Brown <len.brown@...el.com>
Subject: Re: [PATCH] cpufreq: intel_pstate: Use most recent guaranteed
 performance values

On Sat, Dec 19, 2020 at 6:21 AM srinivas pandruvada
<srinivas.pandruvada@...ux.intel.com> wrote:
>
> On Thu, 2020-12-17 at 20:17 +0100, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >
> > When turbo has been disabled by the BIOS, but HWP_CAP.GUARANTEED is
> > changed later, user space may want to take advantage of this
> > increased
> > guaranteed performance.
> >
> > HWP_CAP.GUARANTEED is not a static value.  It can be adjusted by an
> > out-of-band agent or during an Intel Speed Select performance level
> > change.  The HWP_CAP.MAX is still the maximum achievable performance
> > with turbo disabled by the BIOS, so HWP_CAP.GUARANTEED can still
> > change as long as it remains less than or equal to HWP_CAP.MAX.
> >
> > When HWP_CAP.GUARANTEED is changed, the sysfs base_frequency
> > attribute shows the most recent guaranteed frequency value. This
> > attribute can be used by user space software to update the scaling
> > min/max limits of the CPU.
> >
> > Currently, the ->setpolicy() callback already uses the latest
> > HWP_CAP values when setting HWP_REQ, but the ->verify() callback will
> > restrict the user settings to the to old guaranteed performance value
> > which prevents user space from making use of the extra CPU capacity
> > theoretically available to it after increasing HWP_CAP.GUARANTEED.
> >
> > To address this, read HWP_CAP in intel_pstate_verify_cpu_policy()
> > to obtain the maximum P-state that can be used and use that to
> > confine the policy max limit instead of using the cached and
> > possibly stale pstate.max_freq value for this purpose.
> >
> > For consistency, update intel_pstate_update_perf_limits() to use the
> > maximum available P-state returned by intel_pstate_get_hwp_max() to
> > compute the maximum frequency instead of using the return value of
> > intel_pstate_get_max_freq() which, again, may be stale.
> >
> > This issue is a side-effect of fixing the scaling frequency limits in
> > commit eacc9c5a927e ("cpufreq: intel_pstate: Fix
> > intel_pstate_get_hwp_max()
> > for turbo disabled") which currected
> corrected

Right, thanks!