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Message-ID: <162923123763.25758.11891289655344170456.tip-bot2@tip-bot2>
Date: Tue, 17 Aug 2021 20:13:57 -0000
From: "tip-bot2 for Steven Rostedt" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Gregory Haskins <ghaskins@...ell.com>,
Steven Rostedt <rostedt@...dmis.org>,
Thomas Gleixner <tglx@...utronix.de>,
"Peter Zijlstra (Intel)" <peterz@...radead.org>,
Ingo Molnar <mingo@...nel.org>, x86@...nel.org,
linux-kernel@...r.kernel.org
Subject: [tip: locking/core] locking/rtmutex: Add adaptive spinwait mechanism
The following commit has been merged into the locking/core branch of tip:
Commit-ID: 992caf7f17243d736fc996770bac6566103778f6
Gitweb: https://git.kernel.org/tip/992caf7f17243d736fc996770bac6566103778f6
Author: Steven Rostedt <rostedt@...dmis.org>
AuthorDate: Sun, 15 Aug 2021 23:29:25 +02:00
Committer: Ingo Molnar <mingo@...nel.org>
CommitterDate: Tue, 17 Aug 2021 19:06:11 +02:00
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: Gregory Haskins <ghaskins@...ell.com>
Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Link: https://lore.kernel.org/r/20210815211305.912050691@linutronix.de
---
kernel/locking/rtmutex.c | 67 +++++++++++++++++++++++++++++++++++++--
1 file changed, 65 insertions(+), 2 deletions(-)
diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 3eaf636..8aaa352 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -8,6 +8,11 @@
* Copyright (C) 2005-2006 Timesys Corp., Thomas Gleixner <tglx@...esys.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@...hat.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);
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