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

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Message-ID: <2be25c2e-8c10-e62a-3a66-19da8143f2b0@lwfinger.net>
Date:   Fri, 23 Sep 2016 21:45:09 -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 09/18/2016 09:54 PM, Larry Finger wrote:
> On 09/14/2016 11:00 AM, Larry Finger wrote:
>> 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.
>
> And still does. My bisection seemed to be trending toward an improbable set of
> commits, and I needed to do some other work with the machine, thus I started
> running 4.8-rc6. It failed nearly 48 hours after the reboot, which indicated
> that using 3 days to indicate a "good" trial was likely too short. I am
> currently testing the first of the trial and will run it for at least a week. It
> is unlikely that these tests will be complete before 4,8 is released, even if
> -rc8 is needed. I will keep attempting to find the faulty commit.

My debugging continues. After 7 days of beating on commit f7816ad, I have 
concluded that it is likely good. Thus I think the bug lies between commit 
581e0cd (bad) and f7816ad (good). I will need to do a long test on commit 
1b05cf6, which did not fail with a shorter run.

Larry