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Subject[(RT RFC) PATCH v2 8/9] adjust pi_lock usage in wakeup
Date
From: Peter W.Morreale <pmorreale@novell.com>

In wakeup_next_waiter(), we take the pi_lock, and then find out whether
we have another waiter to add to the pending owner. We can reduce
contention on the pi_lock for the pending owner if we first obtain the
pointer to the next waiter outside of the pi_lock.

This patch adds a measureable increase in throughput.

Signed-off-by: Peter W. Morreale <pmorreale@novell.com>
---
kernel/rtmutex.c | 14 +++++++++-----
1 files changed, 9 insertions(+), 5 deletions(-)
diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c
index ea593e0..b81bbef 100644
--- a/kernel/rtmutex.c
+++ b/kernel/rtmutex.c
@@ -526,6 +526,7 @@ static void wakeup_next_waiter(struct rt_mutex *lock, int savestate)
{
struct rt_mutex_waiter *waiter;
struct task_struct *pendowner;
+ struct rt_mutex_waiter *next;

spin_lock(&current->pi_lock);

@@ -587,6 +588,12 @@ static void wakeup_next_waiter(struct rt_mutex *lock, int savestate)
* waiter with higher priority than pending-owner->normal_prio
* is blocked on the unboosted (pending) owner.
*/
+
+ if (rt_mutex_has_waiters(lock))
+ next = rt_mutex_top_waiter(lock);
+ else
+ next = NULL;
+
spin_lock(&pendowner->pi_lock);

WARN_ON(!pendowner->pi_blocked_on);
@@ -595,12 +602,9 @@ static void wakeup_next_waiter(struct rt_mutex *lock, int savestate)

pendowner->pi_blocked_on = NULL;

- if (rt_mutex_has_waiters(lock)) {
- struct rt_mutex_waiter *next;
-
- next = rt_mutex_top_waiter(lock);
+ if (next)
plist_add(&next->pi_list_entry, &pendowner->pi_waiters);
- }
+
spin_unlock(&pendowner->pi_lock);
}



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