[PATCH 2/3] fix a bug in the -deadline utilization tracking mechanism

[Juri Lelli]

Currently, a task doing
	while(1) {
		switch to SCHED_DEADLINE
		switch to SCHED_OTHER
	}
brings dl_b->total_bw below 0.
This happens because when the task switches back from SCHED_DEADLINE
to SCHED_OTHER, switched_from_dl() does not clear its deadline
parameters (they will be cleared by the deadline timer when it fires).
But dl_overflow() removes its utilization from dl_b->total_bw.
When the task switches back to SCHED_DEADLINE, the
	if (new_bw == p->dl.dl_bw)
check in dl_overflow() prevents __dl_add() from being called, and
so when the task switches back to SCHED_OTHER dl_b->total_bw becomes
negative.
This patch changes the check so that if the task is switching from
SCHED_OTHER to SCHED_DEADLINE __dl_add() is correctly invoked.
---
 kernel/sched/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 9503d59..d59fa20 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2432,7 +2432,7 @@ static int dl_overflow(struct task_struct *p, int policy,
 	u64 new_bw = dl_policy(policy) ? to_ratio(period, runtime) : 0;
 	int cpus, err = -1;

-	if (new_bw == p->dl.dl_bw)
+	if (task_has_dl_policy(p) && new_bw == p->dl.dl_bw)
 		return 0;

 	/*
-- 
2.5.0