linux-kernel - Re: [intel-pstate driver regression] processor frequency very high even if in idle

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Date:	Fri, 1 Apr 2016 18:46:13 +0200
From:	Jörg Otte <jrg.otte@...il.com>
To:	Doug Smythies <dsmythies@...us.net>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Subject: Re: [intel-pstate driver regression] processor frequency very high
 even if in idle

2016-04-01 17:20 GMT+02:00 Doug Smythies <dsmythies@...us.net>:
> On 2016.04.01 05:40 Rafael J. Wysocki wrote:
>> On Friday, April 01, 2016 11:20:42 AM Jörg Otte wrote:
>>> 2016-03-31 17:43 GMT+02:00 Rafael J. Wysocki <rjw@...ysocki.net>:
>>>> On Thursday, March 31, 2016 05:25:18 PM Jörg Otte wrote:
>>>>> 2016-03-31 13:42 GMT+02:00 Rafael J. Wysocki <rjw@...ysocki.net>:
>>>>>> On Thursday, March 31, 2016 11:05:56 AM Jörg Otte wrote:
>
>>>
>>> here they are.
>>>
>
>> First of all, the sampling mechanics works as expected
>> in the failing case, which is the most important thing
>> I wanted to know.
>
> Yes, but that might be part of the problem, as for some CPUs
> there is never a long duration, and thus the long duration
> check never kicks in driving the target pstate down.
>
>> The core_busy column is clearly suspicious and it
>> looks like CPUs 2 and 3 never really go idle.
>
> This has been observed several times before [1].
> Due to beat frequencies between desktop type frame rates
> and such, the worst manifestation of the issue seems to be
> for 300 Hz kernels, but Ubuntu uses uses 250 Hz.
>
> Oh look, Jörg is using 300 Hz!!
>
> $ grep CONFIG_HZ .config_jorg
> # CONFIG_HZ_PERIODIC is not set
> # CONFIG_HZ_100 is not set
> # CONFIG_HZ_250 is not set
> CONFIG_HZ_300=y
> # CONFIG_HZ_1000 is not set
> CONFIG_HZ=300
>

I use 300Hz because of:
"250 Hz is a good compromise choice allowing server performance
while also showing good interactive responsiveness even
on SMP and NUMA systems. If you are going to be using NTSC video
or multimedia, selected 300Hz instead." (from KBuild helptext)

-> I often use multimedia so according this text 300 Hz is the better
choice.

>> I guess we'll need to find out
>> why they don't go idle to get to the bottom of this, but it firmly falls into
>> the weird stuff territory already.

> I'm compiling a 300 Hz kernel now, also with "# CONFIG_NO_HZ is not set",

Again from KBuild helptext:
"CONFIG_NO_HZ:
This is the old config entry that enables dynticks idle.
We keep it around for a little while to enforce backward
compatibility with older config files."

-> NO_HZ outdated.

> but I have never been able to re-create these type of findings before.
>
> I have also tried several other things in an attempt re-create Jörg's
> Case, so far without success.
>
> References:
> [1] https://bugzilla.kernel.org/show_bug.cgi?id=93521
> In particular:
> https://bugzilla.kernel.org/show_bug.cgi?id=93521#c35
> https://bugzilla.kernel.org/show_bug.cgi?id=93521#c42
> https://bugzilla.kernel.org/show_bug.cgi?id=93521#c77
>
> ... Doug


Nevertheless, I'll try setting 250Hz + NO_HZ

Thanks, Jörg