From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1764115AbcINQA2 (ORCPT ); Wed, 14 Sep 2016 12:00:28 -0400 Received: from mail-oi0-f49.google.com ([209.85.218.49]:34621 "EHLO mail-oi0-f49.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756501AbcINQA0 (ORCPT ); Wed, 14 Sep 2016 12:00:26 -0400 Subject: Re: Regression in 4.8 - CPU speed set very low To: LKML References: From: Larry Finger Message-ID: Date: Wed, 14 Sep 2016 11:00:23 -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 09/09/2016 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. Larry