Commit 992caf7f authored by Steven Rostedt's avatar Steven Rostedt Committed by Ingo Molnar
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locking/rtmutex: Add adaptive spinwait mechanism 



Going to sleep when locks are contended can be quite inefficient when the
contention time is short and the lock owner is running on a different CPU.

The MCS mechanism cannot be used because MCS is strictly FIFO ordered while
for rtmutex based locks the waiter ordering is priority based.

Provide a simple adaptive spinwait mechanism which currently restricts the
spinning to the top priority waiter.

[ tglx: Provide a contemporary changelog, extended it to all rtmutex based
  	locks and updated it to match the other spin on owner implementations ]

Originally-by: default avatarGregory Haskins <ghaskins@novell.com>
Signed-off-by: default avatarSteven Rostedt <rostedt@goodmis.org>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20210815211305.912050691@linutronix.de
parent 48eb3f4f

kernel/locking/rtmutex.c

+65 −2
Original line number Diff line number Diff line
@@ -8,6 +8,11 @@ 
 *  Copyright (C) 2005-2006 Timesys Corp., Thomas Gleixner <tglx@timesys.com>

 *  Copyright (C) 2005 Kihon Technologies Inc., Steven Rostedt

 *  Copyright (C) 2006 Esben Nielsen

 * Adaptive Spinlocks:

 *  Copyright (C) 2008 Novell, Inc., Gregory Haskins, Sven Dietrich,

 *				     and Peter Morreale,

 * Adaptive Spinlocks simplification:

 *  Copyright (C) 2008 Red Hat, Inc., Steven Rostedt <srostedt@redhat.com>

 *

 *  See Documentation/locking/rt-mutex-design.rst for details.

 */
@@ -1297,6 +1302,52 @@ static __always_inline void __rt_mutex_unlock(struct rt_mutex_base *lock) 
	rt_mutex_slowunlock(lock);

}



#ifdef CONFIG_SMP

static bool rtmutex_spin_on_owner(struct rt_mutex_base *lock,

				  struct rt_mutex_waiter *waiter,

				  struct task_struct *owner)

{

	bool res = true;



	rcu_read_lock();

	for (;;) {

		/* If owner changed, trylock again. */

		if (owner != rt_mutex_owner(lock))

			break;

		/*

		 * Ensure that @owner is dereferenced after checking that

		 * the lock owner still matches @owner. If that fails,

		 * @owner might point to freed memory. If it still matches,

		 * the rcu_read_lock() ensures the memory stays valid.

		 */

		barrier();

		/*

		 * Stop spinning when:

		 *  - the lock owner has been scheduled out

		 *  - current is not longer the top waiter

		 *  - current is requested to reschedule (redundant

		 *    for CONFIG_PREEMPT_RCU=y)

		 *  - the VCPU on which owner runs is preempted

		 */

		if (!owner->on_cpu || waiter != rt_mutex_top_waiter(lock) ||

		    need_resched() || vcpu_is_preempted(task_cpu(owner))) {

			res = false;

			break;

		}

		cpu_relax();

	}

	rcu_read_unlock();

	return res;

}

#else

static bool rtmutex_spin_on_owner(struct rt_mutex_base *lock,

				  struct rt_mutex_waiter *waiter,

				  struct task_struct *owner)

{

	return false;

}

#endif



#ifdef RT_MUTEX_BUILD_MUTEX

/*

 * Functions required for:
@@ -1381,6 +1432,7 @@ static int __sched rt_mutex_slowlock_block(struct rt_mutex_base *lock, 
					   struct rt_mutex_waiter *waiter)

{

	struct rt_mutex *rtm = container_of(lock, struct rt_mutex, rtmutex);

	struct task_struct *owner;

	int ret = 0;



	for (;;) {
@@ -1403,8 +1455,13 @@ static int __sched rt_mutex_slowlock_block(struct rt_mutex_base *lock, 
				break;

		}



		if (waiter == rt_mutex_top_waiter(lock))

			owner = rt_mutex_owner(lock);

		else

			owner = NULL;

		raw_spin_unlock_irq(&lock->wait_lock);



		if (!owner || !rtmutex_spin_on_owner(lock, waiter, owner))

			schedule();



		raw_spin_lock_irq(&lock->wait_lock);
@@ -1561,6 +1618,7 @@ static __always_inline int __rt_mutex_lock(struct rt_mutex_base *lock, 
static void __sched rtlock_slowlock_locked(struct rt_mutex_base *lock)

{

	struct rt_mutex_waiter waiter;

	struct task_struct *owner;



	lockdep_assert_held(&lock->wait_lock);
@@ -1579,8 +1637,13 @@ static void __sched rtlock_slowlock_locked(struct rt_mutex_base *lock) 
		if (try_to_take_rt_mutex(lock, current, &waiter))

			break;



		if (&waiter == rt_mutex_top_waiter(lock))

			owner = rt_mutex_owner(lock);

		else

			owner = NULL;

		raw_spin_unlock_irq(&lock->wait_lock);



		if (!owner || !rtmutex_spin_on_owner(lock, &waiter, owner))

			schedule_rtlock();



		raw_spin_lock_irq(&lock->wait_lock);