Messages in this thread Patch in this message | | | Date | Mon, 09 Mar 2009 21:49:19 +1030 | From | David Newall <> | Subject | Re: scheduler oddity [bug?] |
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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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