[patch V3 64/64] locking/rtmutex: Add adaptive spinwait mechanism

Thomas Gleixner
2021-08-05T15:46:24Z

From: Steven Rostedt <rostedt@goodmis.org>

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 ]

Originally-by: Gregory Haskins <ghaskins@novell.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
V3: Fold the extension for regular sleeping locks and add the missing spin
wait checks (PeterZ)
---
kernel/locking/rtmutex.c | 59 +++++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 57 insertions(+), 2 deletions(-)
---
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -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.
*/
@@ -1278,6 +1283,44 @@ static __always_inline void __rt_mutex_u
rt_mutex_slowunlock(lock);
}

+#ifdef CONFIG_SMP
+/*
+ * Note that owner is a speculative pointer and dereferencing relies
+ * on rcu_read_lock() and the check against the lock owner.
+ */
+static bool rtmutex_adaptive_spinwait(struct rt_mutex_base *lock,
+ struct task_struct *owner)
+{
+ bool res = true;
+
+ rcu_read_lock();
+ for (;;) {
+ /* Owner changed. Trylock again */
+ if (owner != rt_mutex_owner(lock))
+ break;
+ /*
+ * Ensure that owner->on_cpu is dereferenced _after_
+ * checking the above to be valid.
+ */
+ barrier();
+ if (!owner->on_cpu || need_resched() ||
+ vcpu_is_preempted(task_cpu(owner))) {
+ res = false;
+ break;
+ }
+ cpu_relax();
+ }
+ rcu_read_unlock();
+ return res;
+}
+#else
+static bool rtmutex_adaptive_spinwait(struct rt_mutex_base *lock,
+ struct task_struct *owner)
+{
+ return false;
+}
+#endif
+
#ifdef RT_MUTEX_BUILD_MUTEX
/*
* Functions required for:
@@ -1362,6 +1405,7 @@ static int __sched rt_mutex_slowlock_blo
struct rt_mutex_waiter *waiter)
{
struct rt_mutex *rtm = container_of(lock, struct rt_mutex, rtmutex);
+ struct task_struct *owner;
int ret = 0;

for (;;) {
@@ -1384,9 +1428,14 @@ static int __sched rt_mutex_slowlock_blo
break;
}

+ if (waiter == rt_mutex_top_waiter(lock))
+ owner = rt_mutex_owner(lock);
+ else
+ owner = NULL;
raw_spin_unlock_irq(&lock->wait_lock);

- schedule();
+ if (!owner || !rtmutex_adaptive_spinwait(lock, owner))
+ schedule();

raw_spin_lock_irq(&lock->wait_lock);
set_current_state(state);
@@ -1542,6 +1591,7 @@ static __always_inline int __rt_mutex_lo
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);

@@ -1560,9 +1610,14 @@ static void __sched rtlock_slowlock_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);

- schedule_rtlock();
+ if (!owner || !rtmutex_adaptive_spinwait(lock, owner))
+ schedule_rtlock();

raw_spin_lock_irq(&lock->wait_lock);
set_current_state(TASK_RTLOCK_WAIT);