From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S964885AbcISCzH (ORCPT ); Sun, 18 Sep 2016 22:55:07 -0400 Received: from mail-oi0-f48.google.com ([209.85.218.48]:33613 "EHLO mail-oi0-f48.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S936454AbcISCy6 (ORCPT ); Sun, 18 Sep 2016 22:54:58 -0400 Subject: Re: Regression in 4.8 - CPU speed set very low To: LKML References: From: Larry Finger Message-ID: <28fa8706-4251-4d07-6a6a-4ee8d9d1267b@lwfinger.net> Date: Sun, 18 Sep 2016 21:54:55 -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: 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 09/14/2016 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. Larry