From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753991Ab0IILa7 (ORCPT ); Thu, 9 Sep 2010 07:30:59 -0400 Received: from smtp207.alice.it ([82.57.200.103]:44496 "EHLO smtp207.alice.it" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752705Ab0IILa4 (ORCPT ); Thu, 9 Sep 2010 07:30:56 -0400 X-Greylist: delayed 332 seconds by postgrey-1.27 at vger.kernel.org; Thu, 09 Sep 2010 07:30:56 EDT From: fabio de francesco Organization: metanix.org To: Tim Blechmann , LKML Subject: Re: [process scheduler] Possible bug in context_swich()? Date: Thu, 9 Sep 2010 13:25:19 +0200 User-Agent: KMail/1.13.5 (Linux/2.6.35-tuxonice-r1-100826; KDE/4.4.5; x86_64; ; ) Cc: Peter Zijlstra , Mike Galbraith , Ingo Molnar References: <201009081728.18227.fabio@metanix.org> <1284028746.23762.54.camel@laptop> <201009091312.25424.tim@klingt.org> In-Reply-To: <201009091312.25424.tim@klingt.org> MIME-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-15" Content-Transfer-Encoding: 7bit Message-Id: <201009091325.20641.fabio@metanix.org> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thursday 09 September 2010 13:12:18 Tim Blechmann wrote: > On Thursday, September 09, 2010 12:39:06 pm Peter Zijlstra wrote: > > On Thu, 2010-09-09 at 04:32 +0200, Mike Galbraith wrote: > > > On Wed, 2010-09-08 at 17:54 +0200, Peter Zijlstra wrote: > > > > On Wed, 2010-09-08 at 17:28 +0200, fabio de francesco wrote: > > > > > In context_switch() (in linux/kernel/sched.c), starting with > > > > > release 2.6.33, two "unlikely" macro have been changed to > > > > > "likely". I think the previous logic was right while the latter is > > > > > wrong. > > > > > > > > > > In case I am missing something I, please, ask someone to explain > > > > > the above mentioned inversion of logic through releases. > > > > > > > > It helps if you CC people, LKML alone is a bit of a gamble. > > > > > > > > git blame kernel/sched.c, will tell you that the change you refer to > > > > comes from: > > > > > > > > commit 710390d90f143a9ebb87a475215140f426792efd > > > > Author: Tim Blechmann > > > > Date: Tue Nov 24 11:55:27 2009 +0100 > > > > > > > > sched: Optimize branch hint in context_switch() > > > > > > > > Branch hint profiling on my nehalem machine showed over 90% > > > > > > > incorrect branch hints: > > > That change never made any sense to me, seems Tim must have been > > > measuring a kthread load. I benched at the time, and saw absolutely > > > zero difference one way or the other wrt max ctx rate on my Q6600. > > > > One option is to simply remove the whole branch hint.. But lets ask Tim > > what kind of workload he used.. > > i was using a standard desktop workload, nothing special ... There could have been just kernel threads ready to run... Could a server or workstation workload behave differently? fabio