linux-kernel - Re: Regression in 4.8 - CPU speed set very low

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Date:   Wed, 28 Sep 2016 21:26:42 -0500
From:   Larry Finger <Larry.Finger@...inger.net>
To:     Lennart Sorensen <lsorense@...lub.uwaterloo.ca>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux PM list <linux-pm@...r.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Subject: Re: Regression in 4.8 - CPU speed set very low

On 09/27/2016 09:51 AM, Lennart Sorensen wrote:
> On Mon, Sep 26, 2016 at 04:28:29PM -0500, Larry Finger wrote:
>> Mostly I use a KDE applet named "System load" and look at the "average
>> clock", but the same info is also available in /proc/cpuinfo as "cpu MHz".
>> When the bug triggers, the system gets very slow, and the cpu fan stops even
>> though the cpu is still busy.
>>
>> Commit f7816ad, which had run for 7 days without showing the bug, failed
>> after about 2 hours today. All my testing since Sept. 9 has been wasted. Oh
>> well, that's the way it goes!
>
> Is it possible there is no bug and instead you have a hardware problem?
>
> What I am thinking:
>
> CPU fan stops, then CPU gets busy, CPU overheats, thermal throtling
> kicks in to protect CPU and it gets VERY slow.
>
> So maybe you have a bad CPU fan that is getting stuck.  Perhaps even if
> you have a motherboard that varies the CPU fan depending on need and the
> fan doesn't like the lowest speed and sometimes gets stuck when asked
> to go slow.
>
> Of course if the CPU fan is the problem that could explain why it takes
> varying amounts of time to see the problem.
>
> I suggest checking what the cpu temperature sensors are showing next
> time it gets slow.

By the time it gets slow, the CPU's cool, and one cannot see the temp just 
before that event happened.

The reason I suspect a bug is that it fails with 4.8-rcX, but not with 4.7. Of 
course, it could be something subtle that slightly changes the heat load, which 
causes the CPU temp to be a little higher so that the effect is triggered.

I am reasonably confident that it is not a hardware problem, but we may have to 
wait until 4.8 is released and gets wider usage. If no one else reports a 
problem, then I am certainly wrong.

Larry