linux-kernel - Re: [PATCH] cpufreq, intel_pstate, Fix intel_pstate powersave min_perf

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Date:	Wed, 14 Oct 2015 20:02:02 -0400
From:	Prarit Bhargava <prarit@...hat.com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
CC:	Kristen Carlson Accardi <kristen@...ux.intel.com>,
	linux-kernel@...r.kernel.org,
	Viresh Kumar <viresh.kumar@...aro.org>,
	linux-pm@...r.kernel.org, Doug Smythies <dsmythies@...us.net>
Subject: Re: [PATCH] cpufreq, intel_pstate, Fix intel_pstate powersave min_perf_pct
 value



On 10/14/2015 08:29 PM, Rafael J. Wysocki wrote:
> On Wednesday, October 14, 2015 07:59:38 PM Prarit Bhargava wrote:
>>
>> On 10/14/2015 05:09 PM, Kristen Carlson Accardi wrote:
>>> On Wed, 14 Oct 2015 07:41:59 -0400
>>> Prarit Bhargava <prarit@...hat.com> wrote:
>>>
>>>> On systems that initialize the intel_pstate driver with the performance
>>>> governor, and then switch to the powersave governor will not transition to
>>>> lower cpu frequencies until /sys/devices/system/cpu/intel_pstate/min_perf_pct
>>>> is set to a low value.
>>>>
>>>> The behavior of governor switching changed after commit a04759924e25
>>>> ("[cpufreq] intel_pstate: honor user space min_perf_pct override on
>>>>  resume").  The commit introduced tracking of performance percentage
>>>> changes via sysfs in order to restore userspace changes during
>>>> suspend/resume.  The problem occurs because the global values of the newly
>>>> introduced max_sysfs_pct and min_sysfs_pct are not lowered on the governor
>>>> change and this causes the powersave governor to inherit the performance
>>>> governor's settings.
>>>>
>>>> A simple change would have been to reset max_sysfs_pct to 100 and
>>>> min_sysfs_pct to 0 on a governor change, which fixes the problem with
>>>> governor switching.  However, since we cannot break userspace[1] the fix
>>>> is now to give each governor its own limits storage area so that governor
>>>> specific changes are tracked.
>>>>
>>>> I successfully tested this by booting with both the performance governor
>>>> and the powersave governor by default, and switching between the two
>>>> governors (while monitoring /sys/devices/system/cpu/intel_pstate/ values,
>>>> and looking at the output of cpupower frequency-info).  Suspend/Resume
>>>> testing was performed by Doug Smythies.
>>>>
>>>> [1] Systems which suspend/resume using the unmaintained pm-utils package
>>>> will always transition to the performance governor before the suspend and
>>>> after the resume.  This means a system using the powersave governor will
>>>> go from powersave to performance, then suspend/resume, performance to
>>>> powersave.  The simple change during governor changes would have been
>>>> overwritten when the governor changed before and after the suspend/resume.
>>>> I have submitted https://bugzilla.redhat.com/show_bug.cgi?id=1271225
>>>> against Fedora to remove the 94cpufreq file that causes the problem.  It
>>>> should be noted that pm-utils is obsoleted with newer versions of systemd.
>>>>
>>>> Cc: Kristen Carlson Accardi <kristen@...ux.intel.com>
>>>> Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>
>>>> Cc: Viresh Kumar <viresh.kumar@...aro.org>
>>>> Cc: linux-pm@...r.kernel.org
>>>> Cc: Doug Smythies <dsmythies@...us.net>
>>>> Signed-off-by: Prarit Bhargava <prarit@...hat.com>
>>>
>>> Acked-by: Kristen Carlson Accardi <kristen@...ux.intel.com>
>>>
>>> BTW - I think I can see an issue here with HWP enabled systems.  It
>>> looks to me like the hwp settings will not be programmed correctly
>>> during a governor switch.  This probably needs to be addressed in a
>>> separate patch.
>>>
>>
>> Oh, I see it now too.  I'll get to that in another patch.  Thanks for pointing
>> that out Kristen.
> 
> The $subject patch doesn't apply any more after the series from Srinivas that
> I've just applied.
> 
> Can you please rebase it on top of my bleeding-edge branch?
> 

Sure -- can you send me a pointer to the branch?

P.

> Thanks,
> Rafael
> 
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