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

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Message-ID: <CAJZ5v0gVvMg9FRGOkHCPJusCVG3JvEbF6kmnAtFziN+inOGiHA@mail.gmail.com>
Date:	Fri, 1 Apr 2016 21:54:08 +0200
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Doug Smythies <dsmythies@...us.net>
Cc:	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Jörg Otte <jrg.otte@...il.com>,
	"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>
Subject: Re: [intel-pstate driver regression] processor frequency very high
 even if in idle

On Fri, Apr 1, 2016 at 8:31 PM, Doug Smythies <dsmythies@...us.net> wrote:
> On 2106.034.01 10:45 Srinivas Pandruvada wrote:
>> On Fri, 2016-04-01 at 16:06 +0200, Jörg Otte wrote:
>> > > > > >
>>> Done. Attached the tracer.
>>> For me it looks like the previous one of the failing case.
>>
>> The traces show that idle task is constantly running without sleep.
>
> No, they (at least the first one, I didn't look at the next one yet)
> show that CPUs 2 and 3 are spending around 99% of their time not in state
> C0.

How do you figure that out if I may ask?  It is not so obvious to me
to be honest.

> That the sample rate is ending up at ~10 Milliseconds, indicates some
> high frequency (>= 100Hz) events on those CPUs. Those events, apparently,
> take very little CPU time to complete, hence a load of about 1% on average.
>
> By the way, I can recreate the high sample rate with virtually no load
> on my system easy, but so far have been unable to get the high CPU
> frequencies observed by Jörg. I can get my system to about a target pstate of
> 20 where it should have remained at 16, but that is about it.
>
>> The driver is processing samples for idle task for every 10ms and
>> aperf/mperf are showing that we are always in turbo mode for idle task.
>
> That column pretty much always says "idle" (or swapper for my way of doing
> things). I have not found it to very useful as an indicator, and considerably
> more so since the utilization changes.
>
>>
>> Need to find out why idle task is not sleeping.
>
> I contend that is it.

Why?

Thanks,
Rafael