Re: [git] CFS-devel, latest code

[Dmitry Adamushko <dmitry.adamushko@gmail.com>]

humm... I think, it'd be safer to have something like the following
change in place.

The thing is that __pick_next_entity() must never be called when
first_fair(cfs_rq) == NULL. It wouldn't be a problem, should 'run_node'
be the very first field of 'struct sched_entity' (and it's the second).

The 'nr_running != 0' check is _not_ enough, due to the fact that
'current' is not within the tree. Generic paths are ok (e.g. schedule()
as put_prev_task() is called previously)... I'm more worried about e.g.
migration_call() -> CPU_DEAD_FROZEN -> migrate_dead_tasks()... if
'current' == rq->idle, no problems.. if it's one of the SCHED_NORMAL
tasks (or imagine, some other use-cases in the future -- i.e. we should
not make outer world dependent on internal details of sched_fair class)
-- it may be "Houston, we've got a problem" case.

it's +16 bytes to the ".text". Another variant is to make 'run_node' the
first data member of 'struct sched_entity' but an additional check (se !
= NULL) is still needed in pick_next_entity().

what do you think?


---
 diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index dae714a..33b2376 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -563,9 +563,12 @@ set_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
 
 static struct sched_entity *pick_next_entity(struct cfs_rq *cfs_rq)
 {
-	struct sched_entity *se = __pick_next_entity(cfs_rq);
-
-	set_next_entity(cfs_rq, se);
+	struct sched_entity *se = NULL;
+	
+	if (first_fair(cfs_rq)) {
+		se = __pick_next_entity(cfs_rq);
+		set_next_entity(cfs_rq, se);
+	}
 
 	return se;
 }

---