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[tip:sched/core] sched/deadline: Fix migration of SCHED_DEADLINE tasks

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  [tip:sched/core] sched/deadline:   Fix migration of SCHED_DEADLINE tasks tip-bot for Luca Abeni <tipbot@zytor.com> - 2015-10-20 11:40 +0200

#1251526 — [tip:sched/core] sched/deadline: Fix migration of SCHED_DEADLINE tasks

Commit-ID:  5aa5050787f449e7eaef2c5ec93c7b357aa7dcdc
Gitweb:     http://git.kernel.org/tip/5aa5050787f449e7eaef2c5ec93c7b357aa7dcdc
Author:     Luca Abeni <luca.abeni@unitn.it>
AuthorDate: Fri, 16 Oct 2015 10:06:21 +0200
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Tue, 20 Oct 2015 10:13:36 +0200

sched/deadline: Fix migration of SCHED_DEADLINE tasks

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 patch fixes the issue by checking if dl.dl_nr_running == 0.

Signed-off-by: Luca Abeni <luca.abeni@unitn.it>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@arm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Wanpeng Li <wanpeng.li@linux.intel.com>
Fixes: 9d5142624256 ("sched/deadline: Reduce rq lock contention by eliminating locking of non-feasible target")
Link: http://lkml.kernel.org/r/1444982781-15608-1-git-send-email-luca.abeni@unitn.it
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 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..142df26 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.dl_nr_running == 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.dl_nr_running &&
+		    !dl_time_before(task->dl.deadline,
 					later_rq->dl.earliest_dl.curr)) {
 			/*
 			 * Target rq has tasks of equal or earlier deadline,
--
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