[PATCH 1/2] sched: fix RT task-wakeup logic

[Gregory Haskins]

Dmitry Adamushko pointed out a logic error in task_wake_up_rt() where we
will always evaluate to "true".  You can find the thread here:

http://lkml.org/lkml/2008/4/22/296

In reality, we only want to try to push tasks away when a wake up request is
not going to preempt the current task.  So lets fix it.

Note: We introduce test_tsk_need_resched() instead of open-coding the flag
check so that the merge-conflict with -rt should help remind us that we
may need to support NEEDS_RESCHED_DELAYED in the future, too.

Signed-off-by: Gregory Haskins <ghaskins@xxxxxxxxxx>
CC: Dmitry Adamushko <dmitry.adamushko@xxxxxxxxx>
CC: Ingo Molnar <mingo@xxxxxxx>
CC: Steven Rostedt <rostedt@xxxxxxxxxxx>
---

 include/linux/sched.h |    7 ++++++-
 kernel/sched_rt.c     |    7 +++++--
 2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 949bd54..55d4bd4 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1966,6 +1966,11 @@ static inline void clear_tsk_need_resched(struct task_struct *tsk)
 	clear_tsk_thread_flag(tsk,TIF_NEED_RESCHED);
 }
 
+static inline int test_tsk_need_resched(struct task_struct *tsk)
+{
+	return unlikely(test_tsk_thread_flag(tsk,TIF_NEED_RESCHED));
+}
+
 static inline int signal_pending(struct task_struct *p)
 {
 	return unlikely(test_tsk_thread_flag(p,TIF_SIGPENDING));
@@ -1980,7 +1985,7 @@ static inline int fatal_signal_pending(struct task_struct *p)
 
 static inline int need_resched(void)
 {
-	return unlikely(test_thread_flag(TIF_NEED_RESCHED));
+	return unlikely(test_tsk_need_resched(current));
 }
 
 /*
diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index 79e5ad5..ae6b0e6 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -1058,11 +1058,14 @@ static void post_schedule_rt(struct rq *rq)
 	}
 }
 
-
+/*
+ * If we are not running and we are not going to reschedule soon, we should
+ * try to push tasks away now
+ */
 static void task_wake_up_rt(struct rq *rq, struct task_struct *p)
 {
 	if (!task_running(rq, p) &&
-	    (p->prio >= rq->rt.highest_prio) &&
+	    !test_tsk_need_resched(rq->curr) &&
 	    rq->rt.overloaded)
 		push_rt_tasks(rq);
 }

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