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

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Message-ID: <003801d18c44$ab9134e0$02b39ea0$@net>
Date:	Fri, 1 Apr 2016 11:31:12 -0700
From:	"Doug Smythies" <dsmythies@...us.net>
To:	"'Srinivas Pandruvada'" <srinivas.pandruvada@...ux.intel.com>,
	'Jörg Otte' <jrg.otte@...il.com>,
	"'Rafael J. Wysocki'" <rjw@...ysocki.net>
Cc:	"'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 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. 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.
I don't have enough reverted data, but so far this seems very much like
that bug report I referenced earlier.

... Doug