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locking/rtmutex: Implement lockless top-waiter wakeup 

Mark the task for later wakeup after the wait_lock has been released.
This way, once the next task is awoken, it will have a better chance
to of finding the wait_lock free when continuing executing in
__rt_mutex_slowlock() when trying to acquire the rtmutex, calling
try_to_take_rt_mutex(). Upon contended scenarios, other tasks attempting
take the lock may acquire it first, right after the wait_lock is released,
but (a) this can also occur with the current code, as it relies on the
spinlock fairness, and (b) we are dealing with the top-waiter anyway,
so it will always take the lock next.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1432056298-18738-2-git-send-email-dave@stgolabs.net
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
This commit is contained in:
Davidlohr Bueso 2015-05-19 10:24:55 -07:00 committed by Thomas Gleixner
parent 6f9aad0bc3
commit 45ab4effc3
1 changed files with 10 additions and 11 deletions
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21
kernel/locking/rtmutex.c
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@ -955,14 +955,13 @@ static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
}
/*
* Wake up the next waiter on the lock.
*
* Remove the top waiter from the current tasks pi waiter list and
* wake it up.
* queue it up.
*
* Called with lock->wait_lock held.
*/
static void wakeup_next_waiter(struct rt_mutex *lock)
static void mark_wakeup_next_waiter(struct wake_q_head *wake_q,
struct rt_mutex *lock)
{
struct rt_mutex_waiter *waiter;
unsigned long flags;
@ -991,12 +990,7 @@ static void wakeup_next_waiter(struct rt_mutex *lock)
raw_spin_unlock_irqrestore(&current->pi_lock, flags);
/*
* It's safe to dereference waiter as it cannot go away as
* long as we hold lock->wait_lock. The waiter task needs to
* acquire it in order to dequeue the waiter.
*/
wake_up_process(waiter->task);
wake_q_add(wake_q, waiter->task);
}
/*
@ -1258,6 +1252,8 @@ static inline int rt_mutex_slowtrylock(struct rt_mutex *lock)
static void __sched
rt_mutex_slowunlock(struct rt_mutex *lock)
{
WAKE_Q(wake_q);
raw_spin_lock(&lock->wait_lock);
debug_rt_mutex_unlock(lock);
@ -1306,10 +1302,13 @@ rt_mutex_slowunlock(struct rt_mutex *lock)
/*
* The wakeup next waiter path does not suffer from the above
* race. See the comments there.
*
* Queue the next waiter for wakeup once we release the wait_lock.
*/
wakeup_next_waiter(lock);
mark_wakeup_next_waiter(&wake_q, lock);
raw_spin_unlock(&lock->wait_lock);
wake_up_q(&wake_q);
/* Undo pi boosting if necessary: */
rt_mutex_adjust_prio(current);