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

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Message-ID: <075788ab-34e0-803c-f2b4-3f370ecc6b14@lwfinger.net>
Date:   Mon, 26 Sep 2016 11:15:45 -0500
From:   Larry Finger <Larry.Finger@...inger.net>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux PM list <linux-pm@...r.kernel.org>
Subject: Re: Regression in 4.8 - CPU speed set very low

On 09/26/2016 06:37 AM, Rafael J. Wysocki wrote:
> On Friday, September 23, 2016 09:45:09 PM Larry Finger wrote:
>> 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.
>
> 581e0cd is not a valid mainline commit hash AFAICS.

That was a typo. The correct value is 581e0c7.
>
> What cpufreq driver do you use?

My "Default CPUFreq governor" is on demand.

Running the command 'egrep -r "CPU_FREQ|CPUFREQ" .config' results in

CONFIG_ACPI_CPU_FREQ_PSS=y
CONFIG_CPU_FREQ=y
CONFIG_CPU_FREQ_GOV_ATTR_SET=y
CONFIG_CPU_FREQ_GOV_COMMON=y
# CONFIG_CPU_FREQ_STAT is not set
# CONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE is not set
# CONFIG_CPU_FREQ_DEFAULT_GOV_POWERSAVE is not set
# CONFIG_CPU_FREQ_DEFAULT_GOV_USERSPACE is not set
CONFIG_CPU_FREQ_DEFAULT_GOV_ONDEMAND=y
# CONFIG_CPU_FREQ_DEFAULT_GOV_CONSERVATIVE is not set
# CONFIG_CPU_FREQ_DEFAULT_GOV_SCHEDUTIL is not set
CONFIG_CPU_FREQ_GOV_PERFORMANCE=y
CONFIG_CPU_FREQ_GOV_POWERSAVE=m
CONFIG_CPU_FREQ_GOV_USERSPACE=m
CONFIG_CPU_FREQ_GOV_ONDEMAND=y
CONFIG_CPU_FREQ_GOV_CONSERVATIVE=m
# CONFIG_CPU_FREQ_GOV_SCHEDUTIL is not set
CONFIG_X86_PCC_CPUFREQ=m
CONFIG_X86_ACPI_CPUFREQ=m
CONFIG_X86_ACPI_CPUFREQ_CPB=y

Commit 1b05cf6 did fail on longer testing, thus my bisection had ended up going 
wrong. Further tests have shown that commit 351a4ded is bad. Once again, by 
bisection seems to be converging to a set of commits that seem unlikely to cause 
this problem. Perhaps commit f7816ad is not really good even though it survived 
7 days of heavy CPU usage.

I have been reluctant to post my entire .config on the list. It is available at 
http://pastebin.com/aMZaAKwL.

Larry