linux-kernel - [PATCH 1/3] sched: fix RT task-wakeup logic

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Message-ID: <20080528005934.4146.77828.stgit@lsg.lsg.lab.novell.com>
Date:	Tue, 27 May 2008 18:59:34 -0600
From:	Gregory Haskins <ghaskins@...ell.com>
To:	Steven Rostedt <rostedt@...dmis.org>,
	linux-rt-users@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Gregory Haskins <ghaskins@...ell.com>
Subject: [PATCH 1/3] sched: fix RT task-wakeup logic

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@...ell.com>
CC: Dmitry Adamushko <dmitry.adamushko@...il.com>
CC: Steven Rostedt <rostedt@...dmis.org>
Signed-off-by: Ingo Molnar <mingo@...e.hu>
---

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

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 823819c..6fff051 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2118,6 +2118,11 @@ 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));
@@ -2132,7 +2137,7 @@ 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));
 }
 
 static inline int need_resched(void)
diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index 5a5da00..80ef2ee 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -987,7 +987,7 @@ static void post_schedule_rt(struct rq *rq)
 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);
  		schedstat_inc(rq, rto_wakeup);

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