Re: Regression in 4.8 - CPU speed set very low

[Srinivas Pandruvada]

On Mon, 2016-09-26 at 23:37 +0200, Rafael J. Wysocki wrote:
> On Mon, Sep 26, 2016 at 11:28 PM, Larry Finger
> <Larry.Finger@xxxxxxxxxxxx> wrote:
> > 
> > On 09/26/2016 04:06 PM, Rafael J. Wysocki wrote:
> > > 
> > > 
> > > On Monday, September 26, 2016 11:15:45 AM Larry Finger wrote:
> > > > 
> > > > 
> > > > 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.
> > > 
> > > 
> > > If the governor is ondemand, the driver is acpi-cpufreq, most
> > > likely.
> > > 
> > > How do you measure the frequency?
> > 
> > 
> > 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.
> 
> That sounds like thermal throttling kicking in.
> 
This will help to know, if there is thermal throttle from OS.
# cat /sys/devices/system/cpu/cpufreq/policy?/scaling_max_freq
# grep -r . /sys/class/thermal/thermal_zone*

Thanks,
Srinivas