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

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Date:   Sat, 3 Aug 2019 08:00:23 -0700
From:   "Doug Smythies" <dsmythies@...us.net>
To:     "'Rafael J. Wysocki'" <rjw@...ysocki.net>,
        "'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 2019.08.02 02:28 Rafael J. Wysocki wrote:
> 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.

All governors for the intel_cpufreq (intel_pstate in passive mode) CPU frequency
scaling driver are broken with respect to this issue, not just the schedutil
governor. My initial escalation had been focused on acpi-cpufreq/schedutil
and intel_cpufreq/schedutil, as they were both broken, and both fixed by my initially
submitted reversion. What can I say, I missed that other intel_cpufreq governors
were also involved.

I tested all of them: conservative ondemand userspace powersave performance schedutil
Note that no other governor uses resolve_freq().

... Doug