[PATCH v3 4/4] sched: use highest_prio.next to optimize pulloperations

From: Gregory Haskins
Date: Thu Dec 04 2008 - 10:41:58 EST

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@xxxxxxxxxx>

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 ffdd00e..afcc67a 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -1220,6 +1220,18 @@ static int pull_rt_task(struct rq *this_rq)

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

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