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Subject[PATCH v2 2/2] kernel/locking/rtmutex.c: some code optimization
Date
Adding the following code optimization,

- Reducing the number of cmpxchgs. Only call mark_rt_mutex_waiters() when
needed, waiters bit is not set.
- Reducing the hold time of wait_lock lock.
- Calling fixup_rt_mutex_waiters() only when needed.
- When unlocking rt_spin_lock in IRQ, alternate between attempting fast
unlocking and attempting to lock mutex's wait_lock.

Signed-off-by: T. Makphaibulchoke <tmac@hp.com>
---
kernel/locking/rtmutex.c | 30 +++++++++++++++++-------------
1 file changed, 17 insertions(+), 13 deletions(-)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index ae5c13f..cadba20 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -608,7 +608,8 @@ __try_to_take_rt_mutex(struct rt_mutex *lock, struct task_struct *task,
* any more. This is fixed up when we take the ownership.
* This is the transitional state explained at the top of this file.
*/
- mark_rt_mutex_waiters(lock);
+ if (!((unsigned long)lock->owner & RT_MUTEX_HAS_WAITERS))
+ mark_rt_mutex_waiters(lock);

if (rt_mutex_owner(lock))
return 0;
@@ -832,8 +833,8 @@ static void remove_waiter(struct rt_mutex *lock,
struct rt_mutex *next_lock = NULL;
unsigned long flags;

- raw_spin_lock_irqsave(&current->pi_lock, flags);
rt_mutex_dequeue(lock, waiter);
+ raw_spin_lock_irqsave(&current->pi_lock, flags);
current->pi_blocked_on = NULL;
raw_spin_unlock_irqrestore(&current->pi_lock, flags);

@@ -1019,11 +1020,11 @@ static void noinline __sched rt_spin_lock_slowlock(struct rt_mutex *lock)
if (top_waiter != &waiter || adaptive_wait(lock, lock_owner))
schedule_rt_mutex(lock);

- raw_spin_lock(&lock->wait_lock);
-
pi_lock(&self->pi_lock);
__set_current_state(TASK_UNINTERRUPTIBLE);
pi_unlock(&self->pi_lock);
+
+ raw_spin_lock(&lock->wait_lock);
}

/*
@@ -1038,12 +1039,6 @@ static void noinline __sched rt_spin_lock_slowlock(struct rt_mutex *lock)
self->saved_state = TASK_RUNNING;
pi_unlock(&self->pi_lock);

- /*
- * try_to_take_rt_mutex() sets the waiter bit
- * unconditionally. We might have to fix that up:
- */
- fixup_rt_mutex_waiters(lock);
-
BUG_ON(rt_mutex_has_waiters(lock) && &waiter == rt_mutex_top_waiter(lock));
BUG_ON(!RB_EMPTY_NODE(&waiter.tree_entry));

@@ -1096,7 +1091,14 @@ static inline void rt_spin_lock_fastunlock_in_irq(struct rt_mutex *lock,
return;
}
do {
+ /*
+ * Alternate between fast acquire and try lock and proceed
+ * to slow lock whichever succeeds first.
+ */
ret = raw_spin_trylock(&lock->wait_lock);
+ if (!ret && unlikely(rt_mutex_cmpxchg(lock, intr_owner,
+ NULL)))
+ return;
} while (!ret);

slowfn(lock, intr_owner);
@@ -1499,10 +1501,12 @@ rt_mutex_slowtrylock(struct rt_mutex *lock, struct task_struct *task)

ret = try_to_take_rt_mutex(lock, task, NULL);
/*
- * try_to_take_rt_mutex() sets the lock waiters
- * bit unconditionally. Clean this up.
+ * try_to_take_rt_mutex() keeps the lock waiters
+ * bit set when failed to grab lock. Clean this
+ * in case of a failure.
*/
- fixup_rt_mutex_waiters(lock);
+ if (!ret)
+ fixup_rt_mutex_waiters(lock);
}

raw_spin_unlock(&lock->wait_lock);
--
1.9.1


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