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

[Peter Zijlstra <peterz@infradead.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 <ego@in.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@in.ibm.com>
> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Cc: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
> Cc: Ingo Molnar <mingo@elte.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;