linux-kernel - Re: [PATCH V3 2/2] cpufreq: intel_pstate: Implement ->resolve

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Message-ID: <2676200.jfxhmTd764@kreacher>
Date:   Fri, 02 Aug 2019 11:28:24 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Len Brown <lenb@...nel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        "v4 . 18+" <stable@...r.kernel.org>,
        Doug Smythies <doug.smythies@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V3 2/2] cpufreq: intel_pstate: Implement ->resolve_freq()

On Friday, August 2, 2019 11:17:55 AM CEST Rafael J. Wysocki wrote:
> On Fri, Aug 2, 2019 at 7:44 AM Viresh Kumar <viresh.kumar@...aro.org> wrote:
> >
> > Intel pstate driver exposes min_perf_pct and max_perf_pct sysfs files,
> > which can be used to force a limit on the min/max P state of the driver.
> > Though these files eventually control the min/max frequencies that the
> > CPUs will run at, they don't make a change to policy->min/max values.
> 
> That's correct.
> 
> > When the values of these files are changed (in passive mode of the
> > driver), it leads to calling ->limits() callback of the cpufreq
> > governors, like schedutil. On a call to it the governors shall
> > forcefully update the frequency to come within the limits.
> 
> OK, so the problem is that it is a bug to invoke the governor's ->limits()
> callback without updating policy->min/max, because that's what
> "limits" mean to the governors.
> 
> Fair enough.

AFAICS this can be addressed by adding PM QoS freq limits requests of each CPU to
intel_pstate in the passive mode such that changing min_perf_pct or max_perf_pct
will cause these requests to be updated.