From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754904AbYIEPTU (ORCPT ); Fri, 5 Sep 2008 11:19:20 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753415AbYIEPTN (ORCPT ); Fri, 5 Sep 2008 11:19:13 -0400 Received: from bombadil.infradead.org ([18.85.46.34]:33675 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750810AbYIEPTM (ORCPT ); Fri, 5 Sep 2008 11:19:12 -0400 Subject: Re: [PATCH] sched: Fix __load_balance_iterator() for cfq with only one task From: Peter Zijlstra To: ego@in.ibm.com Cc: Gregory Haskins , Vaidyanathan Srinivasan , Balbir Singh , Ingo Molnar , linux-kernel@vger.kernel.org, Dipankar Sarma In-Reply-To: <20080905123004.GD6238@in.ibm.com> References: <20080905123004.GD6238@in.ibm.com> Content-Type: text/plain Date: Fri, 05 Sep 2008 17:13:48 +0200 Message-Id: <1220627628.11202.16.camel@twins.programming.kicks-ass.net> Mime-Version: 1.0 X-Mailer: Evolution 2.23.91 (2.23.91-1.fc10) Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 2008-09-05 at 18:00 +0530, Gautham R Shenoy wrote: > sched: Fix __load_balance_iterator() for cfq with only one task. > > From: Gautham R Shenoy > > The __load_balance_iterator() returns a NULL when there's only one > sched_entity which is a task. It is caused by the following code-path. > > > /* Skip over entities that are not tasks */ > do { > se = list_entry(next, struct sched_entity, group_node); > next = next->next; > } while (next != &cfs_rq->tasks && !entity_is_task(se)); > > if (next == &cfs_rq->tasks) > return NULL; > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > This will return NULL even when se is a task. > > As a side-effect, there was a regression in sched_mc behavior since 2.6.25, > since iter_move_one_task() when it calls load_balance_start_fair(), > would not get any tasks to move! > > Fix this by checking if the last entity was a task or not. Gregory did a similar fix a while ago, but that caused grief of some kind.. Greg, can you recollect why we pulled it? I can't seem to find it. Aside from that this patch looks fine.. > Signed-off-by: Gautham R Shenoy > Cc: Peter Zijlstra > Cc: Vaidyanathan Srinivasan > Cc: Ingo Molnar > --- > > kernel/sched_fair.c | 2 +- > 1 files changed, 1 insertions(+), 1 deletions(-) > > > diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c > index fb8994c..f1c96e3 100644 > --- a/kernel/sched_fair.c > +++ b/kernel/sched_fair.c > @@ -1451,7 +1451,7 @@ __load_balance_iterator(struct cfs_rq *cfs_rq, struct list_head *next) > next = next->next; > } while (next != &cfs_rq->tasks && !entity_is_task(se)); > > - if (next == &cfs_rq->tasks) > + if (next == &cfs_rq->tasks && !entity_is_task(se)) > return NULL; > > cfs_rq->balance_iterator = next;