From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932226AbWDYXXf (ORCPT ); Tue, 25 Apr 2006 19:23:35 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S932294AbWDYXXf (ORCPT ); Tue, 25 Apr 2006 19:23:35 -0400 Received: from omta03ps.mx.bigpond.com ([144.140.82.155]:51346 "EHLO omta03ps.mx.bigpond.com") by vger.kernel.org with ESMTP id S932226AbWDYXXe (ORCPT ); Tue, 25 Apr 2006 19:23:34 -0400 Message-ID: <444EAF74.8090705@bigpond.net.au> Date: Wed, 26 Apr 2006 09:23:32 +1000 From: Peter Williams User-Agent: Thunderbird 1.5 (X11/20060313) MIME-Version: 1.0 To: "Siddha, Suresh B" CC: Andrew Morton , "Chen, Kenneth W" , Con Kolivas , Ingo Molnar , Linux Kernel Mailing List , Mike Galbraith , Nick Piggin Subject: Re: [PATCH] sched: fix evaluation of skip_for_load in move_tasks() References: <444D9290.6070706@bigpond.net.au> <20060425092840.A23188@unix-os.sc.intel.com> In-Reply-To: <20060425092840.A23188@unix-os.sc.intel.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-Authentication-Info: Submitted using SMTP AUTH PLAIN at omta03ps.mx.bigpond.com from [147.10.133.38] using ID pwil3058@bigpond.net.au at Tue, 25 Apr 2006 23:23:33 +0000 Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Siddha, Suresh B wrote: > On Tue, Apr 25, 2006 at 01:08:00PM +1000, Peter Williams wrote: >> Problem: >> >> In the patch >> sched-avoid-unnecessarily-moving-highest-priority-task-move_tasks.patch >> I got a the sense of a boolean expression wrong when assigning a value >> to skip_for_load. The expression should have been negated before being >> assigned. >> >> Additionally, busiest_best_prio_seen is being set when tasks are moved >> instead of when they are skipped which will cause problems when the >> current task does not have prio=busiest_best_prio. >> >> Solution: >> >> Negate the expression and apply de Marcos rule to simplify it and move >> the setting of busiest_best_prio_seen. >> >> This patch is on top of >> sched-avoid-unnecessarily-moving-highest-priority-task-move_tasks.patch >> >> Signed-off-by: Peter Williams >> >> -- >> Peter Williams pwil3058@bigpond.net.au >> >> "Learning, n. The kind of ignorance distinguishing the studious." >> -- Ambrose Bierce > >> Index: MM-2.6.17-rc1-mm3/kernel/sched.c >> =================================================================== >> --- MM-2.6.17-rc1-mm3.orig/kernel/sched.c 2006-04-25 12:53:39.000000000 +1000 >> +++ MM-2.6.17-rc1-mm3/kernel/sched.c 2006-04-25 12:56:14.000000000 +1000 >> @@ -2059,7 +2059,10 @@ static int move_tasks(runqueue_t *this_r >> busiest_best_prio = rq_best_prio(busiest); >> /* >> * Enable handling of the case where there is more than one task >> - * with the best priority. >> + * with the best priority. If the current running task is one >> + * of those with prio==busiest_best_prio we know it won't be moved >> + * and therefore it's safe to override the skip (based on load) of >> + * any task we find with that prio. >> */ >> busiest_best_prio_seen = busiest_best_prio == busiest->curr->prio; >> >> @@ -2108,9 +2111,10 @@ skip_queue: >> */ >> skip_for_load = tmp->load_weight > rem_load_move; >> if (skip_for_load && idx < this_best_prio) >> - skip_for_load = busiest_best_prio_seen || idx != busiest_best_prio; >> + skip_for_load = !busiest_best_prio_seen && idx == busiest_best_prio; > > I think we need to change this to > if (skip_for_load && idx < this_best_prio && idx == busiest_best_prio) > skip_for_load = !busiest_best_prio_seen; > > Otherwise we will reset skip_for_load to '0' even for the tasks whose prio is > less than this_best_prio but not equal to busiest_best_prio. And why is that a problem? The intention of this code is to make sure at least one busiest_best_prio task doesn't get moved as a result of the "skip for reasons of load weight" mechanism being overridden by the "idx < this_best_prio" exception. I can't see how this intention is being subverted. Peter -- Peter Williams pwil3058@bigpond.net.au "Learning, n. The kind of ignorance distinguishing the studious." -- Ambrose Bierce