[PATCH] Fix migration of SCHED_DEADLINE tasks

Luca Abeni
2015-10-15T11:09:47Z

Commit 9d5142624256 ("sched/deadline: Reduce rq lock contention by
eliminating locking of non-feasible target") broke select_task_rq_dl()
and find_lock_later_rq(), because it introduced a comparison between
the local task's deadline and dl.earliest_dl.curr of the remote queue.
However, if the remote runqueue does not contain any SCHED_DEADLINE
task its earliest_dl.curr is 0 (always smaller than the deadline of
the local task) and the remote runqueue is not selected for pushing.
As a result, if an application creates multiple SCHED_DEADLINE threads,
they will never be pushed to runqueues that do not already contain
SCHED_DEADLINE tasks.
This patches fixes the issue by checking if dl.earliest_dl.curr == 0.

Signed-off-by: Luca Abeni <luca.abeni@unitn.it>
---
kernel/sched/deadline.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index fc8f010..0d86d60 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1066,8 +1066,9 @@ select_task_rq_dl(struct task_struct *p, int cpu, int sd_flag, int flags)
int target = find_later_rq(p);

if (target != -1 &&
- dl_time_before(p->dl.deadline,
- cpu_rq(target)->dl.earliest_dl.curr))
+ (dl_time_before(p->dl.deadline,
+ cpu_rq(target)->dl.earliest_dl.curr) ||
+ (cpu_rq(target)->dl.earliest_dl.curr == 0)))
cpu = target;
}
rcu_read_unlock();
@@ -1417,7 +1418,8 @@ static struct rq *find_lock_later_rq(struct task_struct *task, struct rq *rq)

later_rq = cpu_rq(cpu);

- if (!dl_time_before(task->dl.deadline,
+ if (later_rq->dl.earliest_dl.curr &&
+ !dl_time_before(task->dl.deadline,
later_rq->dl.earliest_dl.curr)) {
/*
* Target rq has tasks of equal or earlier deadline,
--
1.9.1