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Subject[PATCH 4/5] sched: add sched_class->needs_post_schedule() member
Date
We currently run class->post_schedule() outside of the rq->lock, which
means that we need to test for the need to post_schedule outside of
the lock to avoid a forced reacquistion. This is currently not a problem
as we only look at rq->rt.overloaded. However, we want to enhance this
going forward to look at more state to reduce the need to post_schedule to
a bare minimum set. Therefore, we introduce a new member-func called
needs_post_schedule() which tests for the post_schedule condtion without
actually performing the work. Therefore it is safe to call this
function before the rq->lock is released, because we are guaranteed not
to drop the lock at an intermediate point (such as what post_schedule()
may do).

We will use this later in the series

Signed-off-by: Gregory Haskins <ghaskins@novell.com>
---

include/linux/sched.h | 1 +
kernel/sched.c | 10 +++++++++-
kernel/sched_rt.c | 24 ++++++++++++++----------
3 files changed, 24 insertions(+), 11 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 08a87b5..cf8cd8c 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -912,6 +912,7 @@ struct sched_class {
struct rq *busiest, struct sched_domain *sd,
enum cpu_idle_type idle);
void (*pre_schedule) (struct rq *this_rq, struct task_struct *task);
+ int (*needs_post_schedule) (struct rq *this_rq);
void (*post_schedule) (struct rq *this_rq);
void (*task_wake_up) (struct rq *this_rq, struct task_struct *task);
#endif
diff --git a/kernel/sched.c b/kernel/sched.c
index b7326cd..78fae75 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2525,6 +2525,14 @@ static void finish_task_switch(struct rq *rq, struct task_struct *prev)
{
struct mm_struct *mm = rq->prev_mm;
long prev_state;
+#ifdef CONFIG_SMP
+ int post_schedule = 0;
+
+ if (current->sched_class->needs_post_schedule) {
+ BUG_ON(!current->sched_class->post_schedule);
+ post_schedule = current->sched_class->needs_post_schedule(rq);
+ }
+#endif

rq->prev_mm = NULL;

@@ -2543,7 +2551,7 @@ static void finish_task_switch(struct rq *rq, struct task_struct *prev)
finish_arch_switch(prev);
finish_lock_switch(rq, prev);
#ifdef CONFIG_SMP
- if (current->sched_class->post_schedule)
+ if (post_schedule)
current->sched_class->post_schedule(rq);
#endif

diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index 196c024..a3d695b 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -1212,20 +1212,23 @@ static void pre_schedule_rt(struct rq *rq, struct task_struct *prev)
pull_rt_task(rq);
}

+/*
+ * assumes rq->lock is held
+ */
+static int needs_post_schedule_rt(struct rq *rq)
+{
+ return rq->rt.overloaded ? 1 : 0;
+}
+
static void post_schedule_rt(struct rq *rq)
{
/*
- * If we have more than one rt_task queued, then
- * see if we can push the other rt_tasks off to other CPUS.
- * Note we may release the rq lock, and since
- * the lock was owned by prev, we need to release it
- * first via finish_lock_switch and then reaquire it here.
+ * This is only called if needs_post_schedule_rt() indicates that
+ * we need to push tasks away
*/
- if (unlikely(rq->rt.overloaded)) {
- spin_lock_irq(&rq->lock);
- push_rt_tasks(rq);
- spin_unlock_irq(&rq->lock);
- }
+ spin_lock_irq(&rq->lock);
+ push_rt_tasks(rq);
+ spin_unlock_irq(&rq->lock);
}

/*
@@ -1473,6 +1476,7 @@ static const struct sched_class rt_sched_class = {
.rq_online = rq_online_rt,
.rq_offline = rq_offline_rt,
.pre_schedule = pre_schedule_rt,
+ .needs_post_schedule = needs_post_schedule_rt,
.post_schedule = post_schedule_rt,
.task_wake_up = task_wake_up_rt,
.switched_from = switched_from_rt,


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