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

Peter Zijlstra
2008-09-05T17:24:15Z

On Fri, 2008-09-05 at 17:13 +0200, Peter Zijlstra wrote:
> 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.

Gregory pointed me to this thread:

http://lkml.org/lkml/2008/8/11/81

ego, can you run sysbench to confirm?

> 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;
>
> --
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