It is redundant to wake the grantee task if it is already running, and
the call to wake_up_process is relatively expensive. If we can safely
skip it we can measurably improve the performance of the adaptive-locks.
Credit goes to Peter Morreale for the general idea.
Signed-off-by: Gregory Haskins <ghaskins@xxxxxxxxxx>
Signed-off-by: Peter Morreale <pmorreale@xxxxxxxxxx>
---
kernel/rtmutex.c | 45 ++++++++++++++++++++++++++++++++++++++++-----
1 files changed, 40 insertions(+), 5 deletions(-)
diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c
index 6c1debb..8ae9de3 100644
--- a/kernel/rtmutex.c
+++ b/kernel/rtmutex.c
@@ -522,6 +522,41 @@ static void wakeup_next_waiter(struct rt_mutex *lock, int savestate)
pendowner = waiter->task;
waiter->task = NULL;
+ /*
+ * Do the wakeup before the ownership change to give any spinning
+ * waiter grantees a headstart over the other threads that will
+ * trigger once owner changes.
+ */
+ if (!savestate)
+ wake_up_process(pendowner);
+ else {
+ /*
+ * We can skip the actual (expensive) wakeup if the
+ * waiter is already running, but we have to be careful
+ * of race conditions because they may be about to sleep.
+ *
+ * The waiter-side protocol has the following pattern:
+ * 1: Set state != RUNNING
+ * 2: Conditionally sleep if waiter->task != NULL;
+ *
+ * And the owner-side has the following:
+ * A: Set waiter->task = NULL
+ * B: Conditionally wake if the state != RUNNING
+ *
+ * As long as we ensure 1->2 order, and A->B order, we
+ * will never miss a wakeup.
+ *
+ * Therefore, this barrier ensures that waiter->task = NULL
+ * is visible before we test the pendowner->state. The
+ * corresponding barrier is in the sleep logic.
+ */
+ smp_mb();
+
+ /* If !RUNNING && !RUNNING_MUTEX */
+ if (pendowner->state & ~TASK_RUNNING_MUTEX)
+ wake_up_process_mutex(pendowner);
+ }
+
rt_mutex_set_owner(lock, pendowner, RT_MUTEX_OWNER_PENDING);
spin_unlock(¤t->pi_lock);
@@ -548,11 +583,6 @@ static void wakeup_next_waiter(struct rt_mutex *lock, int savestate)
plist_add(&next->pi_list_entry, &pendowner->pi_waiters);
}
spin_unlock(&pendowner->pi_lock);
-
- if (savestate)
- wake_up_process_mutex(pendowner);
- else
- wake_up_process(pendowner);
}
/*
@@ -803,6 +833,11 @@ rt_spin_lock_slowlock(struct rt_mutex *lock)
if (adaptive_wait(&waiter, orig_owner)) {
update_current(TASK_UNINTERRUPTIBLE, &saved_state);
+ /*
+ * The xchg() in update_current() is an implicit
+ * barrier which we rely upon to ensure current->state
+ * is visible before we test waiter.task.
+ */
if (waiter.task)
schedule_rt_mutex(lock);
}
--
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