[PATCH -v7][RFC]: mutex: implement adaptive spinning

Peter Zijlstra
2009-01-08T14:46:54Z

On Thu, 2009-01-08 at 15:18 +0100, Ingo Molnar wrote:

> [ A detail, -tip testing found that the patch breaks mutex debugging:
>
> =====================================
> [ BUG: bad unlock balance detected! ]
> -------------------------------------
> swapper/0 is trying to release lock (cpu_add_remove_lock) at:
> [<ffffffff8089f540>] mutex_unlock+0xe/0x10
> but there are no more locks to release!
>
> but that's a detail for -v7 ;-) ]

Here it is..

I changed the optimistic spin cmpxchg trickery to not require the -1000
state, please double and tripple check the code.

This was done because the interaction between trylock_slowpath and that
-1000 state hurt my head.

---
Subject:
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 | 24 ++++++++++-----
kernel/mutex-debug.h | 18 ++++++-----
kernel/mutex.c | 76 ++++++++++++++++++++++++++++++++++++++++++------
kernel/mutex.h | 22 ++++++++++++-
kernel/sched.c | 66 +++++++++++++++++++++++++++++++++++++++++
kernel/sched_features.h | 1
8 files changed, 185 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));
@@ -80,9 +75,25 @@ void debug_mutex_unlock(struct mutex *lo
return;

DEBUG_LOCKS_WARN_ON(lock->magic != lock);
+#if 0
+ /*
+ * XXX something is iffy with owner tracking, but lockdep - which has
+ * similar checks - finds everything dandy, so disable for now.
+ */
+ if (lock->owner != current_thread_info()) {
+ printk(KERN_ERR "%p %p\n", lock->owner, current_thread_info());
+ if (lock->owner) {
+ printk(KERN_ERR "%d %s\n",
+ lock->owner->task->pid,
+ lock->owner->task->comm);
+ }
+ printk(KERN_ERR "%d %s\n",
+ current_thread_info()->task->pid,
+ current_thread_info()->task->comm);
+ }
DEBUG_LOCKS_WARN_ON(lock->owner != current_thread_info());
+#endif
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 +106,6 @@ 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 inline 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);
@@ -115,6 +122,14 @@ 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);
}

@@ -129,13 +144,38 @@ __mutex_lock_common(struct mutex *lock,
{
struct task_struct *task = current;
struct mutex_waiter waiter;
- unsigned int old_val;
unsigned long flags;
+ int old_val;
+
+ mutex_acquire(&lock->dep_map, subclass, 0, ip);
+
+#ifdef CONFIG_SMP
+ /* Optimistic spinning.. */
+ for (;;) {
+ struct thread_info *owner;
+
+ old_val = atomic_cmpxchg(&lock->count, 1, 0);
+ if (old_val == 1) {
+ lock_acquired(&lock->dep_map, ip);
+ mutex_set_owner(lock);
+ return 0;
+ }
+
+ if (old_val < 0 && !list_empty(&lock->wait_list))
+ break;
+
+ /* See who owns it, and spin on him if anybody */
+ owner = ACCESS_ONCE(lock->owner);
+ if (owner && !spin_on_owner(lock, owner))
+ break;
+
+ cpu_relax();
+ }
+#endif

spin_lock_mutex(&lock->wait_lock, flags);

debug_mutex_lock_common(lock, &waiter);
- mutex_acquire(&lock->dep_map, subclass, 0, ip);
debug_mutex_add_waiter(lock, &waiter, task_thread_info(task));

/* add waiting tasks to the end of the waitqueue (FIFO): */
@@ -187,7 +227,7 @@ 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 +300,7 @@ __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 +338,30 @@ __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 +404,10 @@ 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 +433,13 @@ 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 +4672,72 @@ 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;
+ int ret = 1;
+
+ 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()) {
+ ret = 0;
+ break;
+ }
+
+ cpu_relax();
+ }
+out:
+ preempt_enable_no_resched();
+ return ret;
+}
+#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)