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sched: fix __pick_next_entity()

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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().

Signed-off-by: Dmitry Adamushko <dmitry.adamushko@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
This commit is contained in:
Dmitry Adamushko 2007-10-15 17:00:13 +02:00 committed by Ingo Molnar
parent 647e7cac2d
commit 08ec3df510
1 changed files with 5 additions and 2 deletions
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7
kernel/sched_fair.c
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@ -600,9 +600,12 @@ 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);
struct sched_entity *se = NULL;
set_next_entity(cfs_rq, se);
if (first_fair(cfs_rq)) {
se = __pick_next_entity(cfs_rq);
set_next_entity(cfs_rq, se);
}
return se;
}