From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1761085AbcIXCpO (ORCPT ); Fri, 23 Sep 2016 22:45:14 -0400 Received: from mail-oi0-f48.google.com ([209.85.218.48]:35912 "EHLO mail-oi0-f48.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757363AbcIXCpM (ORCPT ); Fri, 23 Sep 2016 22:45:12 -0400 Subject: Re: Regression in 4.8 - CPU speed set very low To: LKML References: <28fa8706-4251-4d07-6a6a-4ee8d9d1267b@lwfinger.net> From: Larry Finger Message-ID: <2be25c2e-8c10-e62a-3a66-19da8143f2b0@lwfinger.net> Date: Fri, 23 Sep 2016 21:45:09 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.3.0 MIME-Version: 1.0 In-Reply-To: <28fa8706-4251-4d07-6a6a-4ee8d9d1267b@lwfinger.net> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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