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Subject[PATCH -v6][RFC]: mutex: implement adaptive spinning
From
Date
On Wed, 2009-01-07 at 15:10 -0800, Linus Torvalds wrote:

> Please take all my patches to be pseudo-code. They've neither been
> compiled nor tested, and I'm just posting them in the hope that somebody
> else will then do things in the direction I think is the proper one ;)

Linux opteron 2.6.28-tip #585 SMP PREEMPT Thu Jan 8 10:38:09 CET 2009 x86_64 x86_64 x86_64 GNU/Linux

[root@opteron bench]# echo NO_OWNER_SPIN > /debug/sched_features; ./timec -e -5,-4,-3,-2 ./test-mutex V 16 10
2 CPUs, running 16 parallel test-tasks.
checking VFS performance.
| loops/sec: 69415
avg ops/sec: 74996
average cost per op: 0.00 usecs
average cost per lock: 0.00 usecs
average cost per unlock: 0.00 usecs
max cost per op: 0.00 usecs
max cost per lock: 0.00 usecs
max cost per unlock: 0.00 usecs
average deviance per op: 0.00 usecs

Performance counter stats for './test-mutex':

12098.324578 task clock ticks (msecs)

1081 CPU migrations (events)
7102 context switches (events)
2763 pagefaults (events)
12098.324578 task clock ticks (msecs)

Wall-clock time elapsed: 12026.804839 msecs

[root@opteron bench]# echo OWNER_SPIN > /debug/sched_features; ./timec -e -5,-4,-3,-2 ./test-mutex V 16 10
2 CPUs, running 16 parallel test-tasks.
checking VFS performance.
| loops/sec: 208147
avg ops/sec: 228126
average cost per op: 0.00 usecs
average cost per lock: 0.00 usecs
average cost per unlock: 0.00 usecs
max cost per op: 0.00 usecs
max cost per lock: 0.00 usecs
max cost per unlock: 0.00 usecs
average deviance per op: 0.00 usecs

Performance counter stats for './test-mutex':

22280.283224 task clock ticks (msecs)

117 CPU migrations (events)
5711 context switches (events)
2781 pagefaults (events)
22280.283224 task clock ticks (msecs)

Wall-clock time elapsed: 12307.053737 msecs

* WOW *

---
Subject: mutex: implement adaptive spin
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
Date: Thu Jan 08 09:41:22 CET 2009

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
include/linux/mutex.h | 4 +-
include/linux/sched.h | 1
kernel/mutex-debug.c | 7 ----
kernel/mutex-debug.h | 18 +++++-----
kernel/mutex.c | 81 ++++++++++++++++++++++++++++++++++++++++++------
kernel/mutex.h | 22 +++++++++++--
kernel/sched.c | 63 +++++++++++++++++++++++++++++++++++++
kernel/sched_features.h | 1
8 files changed, 170 insertions(+), 27 deletions(-)

Index: linux-2.6/include/linux/mutex.h
===================================================================
--- linux-2.6.orig/include/linux/mutex.h
+++ linux-2.6/include/linux/mutex.h
@@ -50,8 +50,10 @@ struct mutex {
atomic_t count;
spinlock_t wait_lock;
struct list_head wait_list;
-#ifdef CONFIG_DEBUG_MUTEXES
+#if defined(CONFIG_DEBUG_MUTEXES) || defined(CONFIG_SMP)
struct thread_info *owner;
+#endif
+#ifdef CONFIG_DEBUG_MUTEXES
const char *name;
void *magic;
#endif
Index: linux-2.6/kernel/mutex-debug.c
===================================================================
--- linux-2.6.orig/kernel/mutex-debug.c
+++ linux-2.6/kernel/mutex-debug.c
@@ -26,11 +26,6 @@
/*
* Must be called with lock->wait_lock held.
*/
-void debug_mutex_set_owner(struct mutex *lock, struct thread_info *new_owner)
-{
- lock->owner = new_owner;
-}
-
void debug_mutex_lock_common(struct mutex *lock, struct mutex_waiter *waiter)
{
memset(waiter, MUTEX_DEBUG_INIT, sizeof(*waiter));
@@ -82,7 +77,6 @@ void debug_mutex_unlock(struct mutex *lo
DEBUG_LOCKS_WARN_ON(lock->magic != lock);
DEBUG_LOCKS_WARN_ON(lock->owner != current_thread_info());
DEBUG_LOCKS_WARN_ON(!lock->wait_list.prev && !lock->wait_list.next);
- DEBUG_LOCKS_WARN_ON(lock->owner != current_thread_info());
}

void debug_mutex_init(struct mutex *lock, const char *name,
@@ -95,7 +89,6 @@ void debug_mutex_init(struct mutex *lock
debug_check_no_locks_freed((void *)lock, sizeof(*lock));
lockdep_init_map(&lock->dep_map, name, key, 0);
#endif
- lock->owner = NULL;
lock->magic = lock;
}

Index: linux-2.6/kernel/mutex-debug.h
===================================================================
--- linux-2.6.orig/kernel/mutex-debug.h
+++ linux-2.6/kernel/mutex-debug.h
@@ -13,14 +13,6 @@
/*
* This must be called with lock->wait_lock held.
*/
-extern void
-debug_mutex_set_owner(struct mutex *lock, struct thread_info *new_owner);
-
-static inline void debug_mutex_clear_owner(struct mutex *lock)
-{
- lock->owner = NULL;
-}
-
extern void debug_mutex_lock_common(struct mutex *lock,
struct mutex_waiter *waiter);
extern void debug_mutex_wake_waiter(struct mutex *lock,
@@ -35,6 +27,16 @@ extern void debug_mutex_unlock(struct mu
extern void debug_mutex_init(struct mutex *lock, const char *name,
struct lock_class_key *key);

+static inline void mutex_set_owner(struct mutex *lock)
+{
+ lock->owner = current_thread_info();
+}
+
+static incline void mutex_clear_owner(struct mutex *lock)
+{
+ lock->owner = NULL;
+}
+
#define spin_lock_mutex(lock, flags) \
do { \
struct mutex *l = container_of(lock, struct mutex, wait_lock); \
Index: linux-2.6/kernel/mutex.c
===================================================================
--- linux-2.6.orig/kernel/mutex.c
+++ linux-2.6/kernel/mutex.c
@@ -10,6 +10,11 @@
* Many thanks to Arjan van de Ven, Thomas Gleixner, Steven Rostedt and
* David Howells for suggestions and improvements.
*
+ * - Adaptive spinning for mutexes by Peter Zijlstra. (Ported to mainline
+ * from the -rt tree, where it was originally implemented for rtmutexes
+ * by Steven Rostedt, based on work by Gregory Haskins, Peter Morreale
+ * and Sven Dietrich.
+ *
* Also see Documentation/mutex-design.txt.
*/
#include <linux/mutex.h>
@@ -46,6 +51,7 @@ __mutex_init(struct mutex *lock, const c
atomic_set(&lock->count, 1);
spin_lock_init(&lock->wait_lock);
INIT_LIST_HEAD(&lock->wait_list);
+ mutex_clear_owner(lock);

debug_mutex_init(lock, name, key);
}
@@ -91,6 +97,7 @@ void inline __sched mutex_lock(struct mu
* 'unlocked' into 'locked' state.
*/
__mutex_fastpath_lock(&lock->count, __mutex_lock_slowpath);
+ mutex_set_owner(lock);
}

EXPORT_SYMBOL(mutex_lock);
@@ -115,11 +122,21 @@ void __sched mutex_unlock(struct mutex *
* The unlocking fastpath is the 0->1 transition from 'locked'
* into 'unlocked' state:
*/
+#ifndef CONFIG_DEBUG_MUTEXES
+ /*
+ * When debugging is enabled we must not clear the owner before time,
+ * the slow path will always be taken, and that clears the owner field
+ * after verifying that it was indeed current.
+ */
+ mutex_clear_owner(lock);
+#endif
__mutex_fastpath_unlock(&lock->count, __mutex_unlock_slowpath);
}

EXPORT_SYMBOL(mutex_unlock);

+#define MUTEX_SLEEPERS (-1000)
+
/*
* Lock a mutex (possibly interruptible), slowpath:
*/
@@ -132,6 +149,34 @@ __mutex_lock_common(struct mutex *lock,
unsigned int old_val;
unsigned long flags;

+#ifdef CONFIG_SMP
+ /* Optimistic spinning.. */
+ for (;;) {
+ struct thread_info *owner;
+ int oldval = atomic_read(&lock->count);
+
+ if (oldval <= MUTEX_SLEEPERS)
+ break;
+ if (oldval == 1) {
+ oldval = atomic_cmpxchg(&lock->count, oldval, 0);
+ if (oldval == 1) {
+ mutex_set_owner(lock);
+ return 0;
+ }
+ } else {
+ /* See who owns it, and spin on him if anybody */
+ owner = ACCESS_ONCE(lock->owner);
+ if (owner && !spin_on_owner(lock, owner))
+ break;
+ }
+
+ if (need_resched())
+ break;
+
+ cpu_relax();
+ }
+#endif
+
spin_lock_mutex(&lock->wait_lock, flags);

debug_mutex_lock_common(lock, &waiter);
@@ -142,7 +187,7 @@ __mutex_lock_common(struct mutex *lock,
list_add_tail(&waiter.list, &lock->wait_list);
waiter.task = task;

- old_val = atomic_xchg(&lock->count, -1);
+ old_val = atomic_xchg(&lock->count, MUTEX_SLEEPERS);
if (old_val == 1)
goto done;

@@ -158,7 +203,7 @@ __mutex_lock_common(struct mutex *lock,
* that when we release the lock, we properly wake up the
* other waiters:
*/
- old_val = atomic_xchg(&lock->count, -1);
+ old_val = atomic_xchg(&lock->count, MUTEX_SLEEPERS);
if (old_val == 1)
break;

@@ -187,7 +232,7 @@ done:
lock_acquired(&lock->dep_map, ip);
/* got the lock - rejoice! */
mutex_remove_waiter(lock, &waiter, task_thread_info(task));
- debug_mutex_set_owner(lock, task_thread_info(task));
+ mutex_set_owner(lock);

/* set it to 0 if there are no waiters left: */
if (likely(list_empty(&lock->wait_list)))
@@ -260,7 +305,7 @@ __mutex_unlock_common_slowpath(atomic_t
wake_up_process(waiter->task);
}

- debug_mutex_clear_owner(lock);
+ mutex_clear_owner(lock);

spin_unlock_mutex(&lock->wait_lock, flags);
}
@@ -298,18 +343,30 @@ __mutex_lock_interruptible_slowpath(atom
*/
int __sched mutex_lock_interruptible(struct mutex *lock)
{
+ int ret;
+
might_sleep();
- return __mutex_fastpath_lock_retval
+ ret = __mutex_fastpath_lock_retval
(&lock->count, __mutex_lock_interruptible_slowpath);
+ if (!ret)
+ mutex_set_owner(lock);
+
+ return ret;
}

EXPORT_SYMBOL(mutex_lock_interruptible);

int __sched mutex_lock_killable(struct mutex *lock)
{
+ int ret;
+
might_sleep();
- return __mutex_fastpath_lock_retval
+ ret = __mutex_fastpath_lock_retval
(&lock->count, __mutex_lock_killable_slowpath);
+ if (!ret)
+ mutex_set_owner(lock);
+
+ return ret;
}
EXPORT_SYMBOL(mutex_lock_killable);

@@ -352,9 +409,10 @@ static inline int __mutex_trylock_slowpa

prev = atomic_xchg(&lock->count, -1);
if (likely(prev == 1)) {
- debug_mutex_set_owner(lock, current_thread_info());
+ mutex_set_owner(lock);
mutex_acquire(&lock->dep_map, 0, 1, _RET_IP_);
}
+
/* Set it back to 0 if there are no waiters: */
if (likely(list_empty(&lock->wait_list)))
atomic_set(&lock->count, 0);
@@ -380,8 +438,13 @@ static inline int __mutex_trylock_slowpa
*/
int __sched mutex_trylock(struct mutex *lock)
{
- return __mutex_fastpath_trylock(&lock->count,
- __mutex_trylock_slowpath);
+ int ret;
+
+ ret = __mutex_fastpath_trylock(&lock->count, __mutex_trylock_slowpath);
+ if (ret)
+ mutex_set_owner(lock);
+
+ return ret;
}

EXPORT_SYMBOL(mutex_trylock);
Index: linux-2.6/kernel/mutex.h
===================================================================
--- linux-2.6.orig/kernel/mutex.h
+++ linux-2.6/kernel/mutex.h
@@ -16,8 +16,26 @@
#define mutex_remove_waiter(lock, waiter, ti) \
__list_del((waiter)->list.prev, (waiter)->list.next)

-#define debug_mutex_set_owner(lock, new_owner) do { } while (0)
-#define debug_mutex_clear_owner(lock) do { } while (0)
+#ifdef CONFIG_SMP
+static inline void mutex_set_owner(struct mutex *lock)
+{
+ lock->owner = current_thread_info();
+}
+
+static inline void mutex_clear_owner(struct mutex *lock)
+{
+ lock->owner = NULL;
+}
+#else
+static inline void mutex_set_owner(struct mutex *lock)
+{
+}
+
+static inline void mutex_clear_owner(struct mutex *lock)
+{
+}
+#endif
+
#define debug_mutex_wake_waiter(lock, waiter) do { } while (0)
#define debug_mutex_free_waiter(waiter) do { } while (0)
#define debug_mutex_add_waiter(lock, waiter, ti) do { } while (0)
Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c
+++ linux-2.6/kernel/sched.c
@@ -4672,6 +4672,69 @@ need_resched_nonpreemptible:
}
EXPORT_SYMBOL(schedule);

+#ifdef CONFIG_SMP
+/*
+ * Look out! "owner" is an entirely speculative pointer
+ * access and not reliable.
+ */
+int spin_on_owner(struct mutex *lock, struct thread_info *owner)
+{
+ unsigned int cpu;
+ struct rq *rq;
+
+ if (unlikely(!sched_feat(OWNER_SPIN)))
+ return 0;
+
+ preempt_disable();
+#ifdef CONFIG_DEBUG_PAGEALLOC
+ /*
+ * Need to access the cpu field knowing that
+ * DEBUG_PAGEALLOC could have unmapped it if
+ * the mutex owner just released it and exited.
+ */
+ if (probe_kernel_address(&owner->cpu, cpu))
+ goto out;
+#else
+ cpu = owner->cpu;
+#endif
+
+ /*
+ * Even if the access succeeded (likely case),
+ * the cpu field may no longer be valid.
+ */
+ if (cpu >= nr_cpumask_bits)
+ goto out;
+
+ /*
+ * We need to validate that we can do a
+ * get_cpu() and that we have the percpu area.
+ */
+ if (!cpu_online(cpu))
+ goto out;
+
+ rq = cpu_rq(cpu);
+
+ for (;;) {
+ if (lock->owner != owner)
+ break;
+
+ /*
+ * Is that owner really running on that cpu?
+ */
+ if (task_thread_info(rq->curr) != owner)
+ break;
+
+ if (need_resched())
+ break;
+
+ cpu_relax();
+ }
+out:
+ preempt_enable_no_resched();
+ return 1;
+}
+#endif
+
#ifdef CONFIG_PREEMPT
/*
* this is the entry point to schedule() from in-kernel preemption
Index: linux-2.6/include/linux/sched.h
===================================================================
--- linux-2.6.orig/include/linux/sched.h
+++ linux-2.6/include/linux/sched.h
@@ -330,6 +330,7 @@ extern signed long schedule_timeout_inte
extern signed long schedule_timeout_killable(signed long timeout);
extern signed long schedule_timeout_uninterruptible(signed long timeout);
asmlinkage void schedule(void);
+extern int spin_on_owner(struct mutex *lock, struct thread_info *owner);

struct nsproxy;
struct user_namespace;
Index: linux-2.6/kernel/sched_features.h
===================================================================
--- linux-2.6.orig/kernel/sched_features.h
+++ linux-2.6/kernel/sched_features.h
@@ -13,3 +13,4 @@ SCHED_FEAT(LB_WAKEUP_UPDATE, 1)
SCHED_FEAT(ASYM_EFF_LOAD, 1)
SCHED_FEAT(WAKEUP_OVERLAP, 0)
SCHED_FEAT(LAST_BUDDY, 1)
+SCHED_FEAT(OWNER_SPIN, 1)


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