[PATCH v2 4/4] sched: use highest_prio.next to optimize pull operations

Gregory Haskins
2008-12-03T22:07:31Z

We currently take the rq->lock for every cpu in an overload state during
pull_rt_tasks(). However, we now have enough information via the
highest_prio.[curr|next] fields to determine if there is any tasks of
interest to warrant the overhead of the rq->lock, before we actually take
it. So we use this information to reduce lock contention during the
pull for the case where the source-rq doesnt have tasks that preempt
the current task.

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

kernel/sched_rt.c | 12 ++++++++++++
1 files changed, 12 insertions(+), 0 deletions(-)

diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index 7431d19..6072f24 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -1220,6 +1220,18 @@ static int pull_rt_task(struct rq *this_rq)
continue;

src_rq = cpu_rq(cpu);
+
+ /*
+ * Don't bother taking the src_rq->lock if the next highest
+ * task is known to be lower-priority than our current task.
+ * This may look racy, but if this value is about to go
+ * logically higher, the src_rq will push this task away.
+ * And if its going logically lower, we do not care
+ */
+ if (src_rq->rt.highest_prio.next >=
+ this_rq->rt.highest_prio.curr)
+ continue;
+
/*
* We can potentially drop this_rq's lock in
* double_lock_balance, and another CPU could