Messages in this thread | | | Subject | Re: Regression in 4.8 - CPU speed set very low | From | Larry Finger <> | Date | Fri, 23 Sep 2016 21:45:09 -0500 |
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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
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