linux-kernel - Re: [RFC][PATCH 7/7] cpufreq: intel_pstate: Change P-state selection algorithm for Core

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Message-ID: <CANRm+Cxzm9xoLpddNdkXYahQMHUwsvkn4d-CObvApRhxU-GjWg@mail.gmail.com>
Date:   Tue, 23 Aug 2016 11:48:04 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Linux PM list <linux-pm@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Steve Muckle <steve.muckle@...aro.org>,
        Juri Lelli <juri.lelli@....com>, Ingo Molnar <mingo@...nel.org>
Subject: Re: [RFC][PATCH 7/7] cpufreq: intel_pstate: Change P-state selection
 algorithm for Core

Hi Rafael,
2016-08-01 7:38 GMT+08:00 Rafael J. Wysocki <rjw@...ysocki.net>:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>
> The PID-base P-state selection algorithm used by intel_pstate for
> Core processors is based on very weak foundations.  Namely, its
> decisions are mostly based on the values of the APERF and MPERF
> feedback registers and it only estimates the actual utilization to
> check if it is not extremely low (in order to avoid getting stuck
> in the highest P-state in that case).
>
> Since it generally causes the CPU P-state to ramp up quickly, it
> leads to satisfactory performance, but the metric used by it is only
> really valid when the CPU changes P-states by itself (ie. in the turbo
> range) and if the P-state value set by the driver is treated by the
> CPU as the upper limit on turbo P-states selected by it.

Do you mean CPU will ignore the request value which is submitted by
intel_pstate driver in the turbo range, but respect the upper limit on
the turbo P-states which is submitted by intel_pstate driver?

Regards,
Wanpeng Li