linux-kernel - Re: [PATCH] sched: Fix __load_balance_iterator() for cfq with only one task

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Message-Id: <1220627628.11202.16.camel@twins.programming.kicks-ass.net>
Date:	Fri, 05 Sep 2008 17:13:48 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	ego@...ibm.com
Cc:	Gregory Haskins <ghaskins@...ell.com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Balbir Singh <balbir@...ibm.com>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org, Dipankar Sarma <dipankar@...ibm.com>
Subject: Re: [PATCH] sched: Fix __load_balance_iterator() for cfq with only
 one task

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 <ego@...ibm.com>
> 
> 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 <ego@...ibm.com>
> Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Cc: Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
> Cc: Ingo Molnar <mingo@...e.hu>
> ---
> 
>  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;

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