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

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Date:   Mon, 22 Aug 2016 21:08:05 -0700
From:   Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
To:     Wanpeng Li <kernellwp@...il.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Linux PM list <linux-pm@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        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

On Tue, 2016-08-23 at 11:48 +0800, Wanpeng Li wrote:
> 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?
Any request to request a P-state even upper limit can be totally
ignored in turbo range. 

Thanks,
Srinivas

> 
> Regards,
> Wanpeng Li
> --
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