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

Peter Zijlstra
2008-09-05T15:19:20Z

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;