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SubjectRe: scheduler oddity [bug?]
Balazs Scheidler wrote:
> Some more test results:
>
> Latest tree from Linus seems to work, at least the program runs on both
> cores as it should. I bisected the patch that changed behaviour, and
> I've found this:
>
> commit 38736f475071b80b66be28af7b44c854073699cc
> Author: Gautham R Shenoy <ego@in.ibm.com>
> Date: Sat Sep 6 14:50:23 2008 +0530
>
> sched: fix __load_balance_iterator() for cfq with only one task
>
> 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.
>
> Signed-off-by: Gautham R Shenoy <ego@in.ibm.com>
> Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Signed-off-by: Ingo Molnar <mingo@elte.hu>

Woops! That fails when the task is the last entry on the list. This
fixes that:

--- sched_fair.c 2009-02-21 09:09:34.000000000 +1030
+++ sched_fair.c.dn 2009-03-09 20:48:36.000000000 +1030
@@ -1440,7 +1440,7 @@
__load_balance_iterator(struct cfs_rq *cfs_rq, struct list_head *next)
{
struct task_struct *p = NULL;
- struct sched_entity *se;
+ struct sched_entity *se = NULL;

if (next == &cfs_rq->tasks)
return NULL;
@@ -1451,7 +1451,7 @@
next = next->next;
} while (next != &cfs_rq->tasks && !entity_is_task(se));

- if (next == &cfs_rq->tasks)
+ if (se == NULL || !entity_is_task(se))
return NULL;

cfs_rq->balance_iterator = next;

Really, though, the function could stand a spring-cleaning, for example
either of the following, depending on how much you hate returning from
within a loop:

__load_balance_iterator(struct cfs_rq *cfs_rq, struct list_head *next)
{
do {
struct sched_entity *se = list_entry(next, struct sched_entity, group_node);
next = next->next;
if (entity_is_task(se))
{
cfs_rq->balance_iterator = next;
return task_of(se);
}
} while (next != &cfs_rq->tasks);
return NULL;
}

__load_balance_iterator(struct cfs_rq *cfs_rq, struct list_head *next)
{
struct sched_entity *se;
for ( ; next != &cfs_rq->tasks; next = next->next)
{
se = list_entry(next, struct sched_entity, group_node);
if (entity_is_task(se))
break;
}
if (next == &cfs_rq->tasks)
return NULL;
cfs_rq->balance_iterator = next->next;
return task_of(se);
}

I wonder if it was intended to set balance_iterator to the task's list
entry instead of the following one.



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