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

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Message-ID: <f29a73e6-7cf2-ee7b-ca28-a38a6ca20b40@lwfinger.net>
Date:   Wed, 14 Sep 2016 11:00:23 -0500
From:   Larry Finger <Larry.Finger@...inger.net>
To:     LKML <linux-kernel@...r.kernel.org>
Subject: Re: Regression in 4.8 - CPU speed set very low

On 09/09/2016 12:39 PM, Larry Finger wrote:
> I have found a regression in kernel 4.8-rc2 that causes the speed of my laptop
> with an Intel(R) Core(TM) i7-4600M CPU @ 2.90GHz to suddenly have a maximum cpu
> frequency of ~400 MHz. Unfortunately, I do not know how to trigger this problem,
> thus a bisection is not possible. It usually happens under heavy load, such as a
> kernel build or the RPM build of VirtualBox, but it does not always fail with
> these loads. In my most recent failure, 'hwinfo --cpu' reports cpu MHz of
> 396.130 for #3. The bogomips value is 5787.73, and the cpu clock before the
> fault is 3437 MHz. Nothing is logged when this happens.
>
> If I were to get a patch that would show a backtrace when the maximum CPU
> frequency is changed, perhaps it would be possible to track this bug.

I have not yet found the bad commit, but I have reduced the range of commits a 
bit. This bug has been difficult to trigger. So far, it has not taken over 1/2 
day to appear in bad kernels, thus I am allowing three days before deciding that 
a given trial is good. I never saw the problem with 4.7 kernels, but I did in 
4.8-rc1. I also know that it appeared before commit 581e0cd. Commit 1b05cf6 did 
not show the bug.

Testing continues.

Larry