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locking/rtmutex: Add adaptive spinwait mechanism

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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: Gregory Haskins <ghaskins@novell.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20210815211305.912050691@linutronix.de
This commit is contained in:
Steven Rostedt 2021-08-15 23:29:25 +02:00 committed by Ingo Molnar
parent 48eb3f4fcf
commit 992caf7f17
1 changed files with 65 additions and 2 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

67
kernel/locking/rtmutex.c
View file

@ -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 (;;) {
@ -1403,9 +1455,14 @@ 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);
schedule();
if (!owner || !rtmutex_spin_on_owner(lock, waiter, owner))
schedule();
raw_spin_lock_irq(&lock->wait_lock);
set_current_state(state);
@ -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);
@ -1579,9 +1637,14 @@ 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);
schedule_rtlock();
if (!owner || !rtmutex_spin_on_owner(lock, &waiter, owner))
schedule_rtlock();
raw_spin_lock_irq(&lock->wait_lock);
set_current_state(TASK_RTLOCK_WAIT);