linux-kernel - Re: [RFT][Update][PATCH 2/2] cpufreq: intel_pstate: Update max CPU frequency on global turbo changes

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Date:   Thu, 7 Mar 2019 12:23:44 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Quentin Perret <quentin.perret@....com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Chen Yu <yu.c.chen@...el.com>,
        Gabriele Mazzotta <gabriele.mzt@...il.com>
Subject: Re: [RFT][Update][PATCH 2/2] cpufreq: intel_pstate: Update max CPU
 frequency on global turbo changes

On Thu, Mar 07, 2019 at 11:02:58AM +0000, Quentin Perret wrote:
> Also on recent big little SoCs, the capacity ratio can be pretty high.
> You can get little CPUs with 300 of capacity or so. The arbitrary 128
> thing is basically gonna go near max freq in one step, although the CPUs
> actually 20 available OPPs or so. And I guess that's a shame.

Ah, but remember; with the latest patch the iowait_boost is scaled on
max. So in order to hit that 300, you first have to hit 1024.

+       boost = (sg_cpu->iowait_boost * max) >> SCHED_CAPACITY_SHIFT;
+       return max(boost, util);

So iowait_boost goes from 128 to 1024 in 4 wakeups (the first sets 128,
the next 3 get it to 1024), but the effective value is, given @max ==
300:

  128 -> 37
  256 -> 75
  512 -> 150
 1024 -> 300

so irrespective of the number of OPPs and their spread, it takes 4
wakeups to get to max.