From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751815Ab0IICcY (ORCPT ); Wed, 8 Sep 2010 22:32:24 -0400 Received: from mailout-de.gmx.net ([213.165.64.22]:41980 "HELO mail.gmx.net" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with SMTP id S1751228Ab0IICcS (ORCPT ); Wed, 8 Sep 2010 22:32:18 -0400 X-Authenticated: #14349625 X-Provags-ID: V01U2FsdGVkX1+YRQERGgCYGTbHXsg+eHE0gEGcvkkPW3c4zrbMXE HYjtgRJ/S0inWm Subject: Re: [process scheduler] Possible bug in context_swich()? From: Mike Galbraith To: Peter Zijlstra Cc: fabio de francesco , LKML In-Reply-To: <1283961258.23762.33.camel@laptop> References: <201009081728.18227.fabio@metanix.org> <1283961258.23762.33.camel@laptop> Content-Type: text/plain Date: Thu, 09 Sep 2010 04:32:19 +0200 Message-Id: <1283999539.9103.54.camel@marge.simson.net> Mime-Version: 1.0 X-Mailer: Evolution 2.24.1.1 Content-Transfer-Encoding: 7bit X-Y-GMX-Trusted: 0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. -Mike