Re: [PATCH][RFC]: mutex: adaptive spin

Nick Piggin
2009-01-06T13:29:40Z

On Tue, Jan 06, 2009 at 12:40:31PM +0100, Peter Zijlstra wrote:
> Subject: mutex: adaptive spin
> From: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Date: Tue Jan 06 12:32:12 CET 2009
>
> Based on the code in -rt, provide adaptive spins on generic mutexes.

I guess it would be nice to add another type so you can test/convert
callsites individually. I've got no objections to improving
synchronisation primitives, but I would be interested to see good
results from some mutex that can't be achieved by improving the
locking (by improving I don't mean inventing some crazy lockless
algorithm, but simply making it reasonably sane and scalable).

Good area to investigate though, I think.

>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> ---
> include/linux/mutex.h | 4 ++--
> include/linux/sched.h | 1 +
> kernel/mutex-debug.c | 11 ++---------
> kernel/mutex-debug.h | 8 --------
> kernel/mutex.c | 46 +++++++++++++++++++++++++++++++++++++++-------
> kernel/mutex.h | 2 --
> kernel/sched.c | 5 +++++
> 7 files changed, 49 insertions(+), 28 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 +50,8 @@ struct mutex {
> atomic_t count;
> spinlock_t wait_lock;
> struct list_head wait_list;
> + struct task_struct *owner;
> #ifdef CONFIG_DEBUG_MUTEXES
> - struct thread_info *owner;
> const char *name;
> void *magic;
> #endif
> @@ -67,8 +67,8 @@ struct mutex {
> struct mutex_waiter {
> struct list_head list;
> struct task_struct *task;
> -#ifdef CONFIG_DEBUG_MUTEXES
> struct mutex *lock;
> +#ifdef CONFIG_DEBUG_MUTEXES
> void *magic;
> #endif
> };
> Index: linux-2.6/include/linux/sched.h
> ===================================================================
> --- linux-2.6.orig/include/linux/sched.h
> +++ linux-2.6/include/linux/sched.h
> @@ -249,6 +249,7 @@ extern void init_idle(struct task_struct
> extern void init_idle_bootup_task(struct task_struct *idle);
>
> extern int runqueue_is_locked(void);
> +extern int task_is_current(struct task_struct *p);
> extern void task_rq_unlock_wait(struct task_struct *p);
>
> extern cpumask_var_t nohz_cpu_mask;
> 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));
> @@ -59,7 +54,6 @@ void debug_mutex_add_waiter(struct mutex
>
> /* Mark the current thread as blocked on the lock: */
> ti->task->blocked_on = waiter;
> - waiter->lock = lock;
> }
>
> void mutex_remove_waiter(struct mutex *lock, struct mutex_waiter *waiter,
> @@ -80,9 +74,9 @@ void debug_mutex_unlock(struct mutex *lo
> return;
>
> DEBUG_LOCKS_WARN_ON(lock->magic != lock);
> - DEBUG_LOCKS_WARN_ON(lock->owner != current_thread_info());
> + DEBUG_LOCKS_WARN_ON(lock->owner != current);
> DEBUG_LOCKS_WARN_ON(!lock->wait_list.prev && !lock->wait_list.next);
> - DEBUG_LOCKS_WARN_ON(lock->owner != current_thread_info());
> + DEBUG_LOCKS_WARN_ON(lock->owner != current);
> }
>
> 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,
> Index: linux-2.6/kernel/mutex.c
> ===================================================================
> --- linux-2.6.orig/kernel/mutex.c
> +++ linux-2.6/kernel/mutex.c
> @@ -46,6 +46,7 @@ __mutex_init(struct mutex *lock, const c
> atomic_set(&lock->count, 1);
> spin_lock_init(&lock->wait_lock);
> INIT_LIST_HEAD(&lock->wait_list);
> + lock->owner = NULL;
>
> debug_mutex_init(lock, name, key);
> }
> @@ -120,6 +121,28 @@ void __sched mutex_unlock(struct mutex *
>
> EXPORT_SYMBOL(mutex_unlock);
>
> +#ifdef CONFIG_SMP
> +static int adaptive_wait(struct mutex_waiter *waiter,
> + struct task_struct *owner, long state)
> +{
> + for (;;) {
> + if (signal_pending_state(state, waiter->task))
> + return 0;
> + if (waiter->lock->owner != owner)
> + return 0;
> + if (!task_is_current(owner))
> + return 1;
> + cpu_relax();
> + }
> +}
> +#else
> +static int adaptive_wait(struct mutex_waiter *waiter,
> + struct task_struct *owner, long state)
> +{
> + return 1;
> +}
> +#endif
> +
> /*
> * Lock a mutex (possibly interruptible), slowpath:
> */
> @@ -127,7 +150,7 @@ static inline int __sched
> __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> unsigned long ip)
> {
> - struct task_struct *task = current;
> + struct task_struct *owner, *task = current;
> struct mutex_waiter waiter;
> unsigned int old_val;
> unsigned long flags;
> @@ -141,6 +164,7 @@ __mutex_lock_common(struct mutex *lock,
> /* add waiting tasks to the end of the waitqueue (FIFO): */
> list_add_tail(&waiter.list, &lock->wait_list);
> waiter.task = task;
> + waiter.lock = lock;
>
> old_val = atomic_xchg(&lock->count, -1);
> if (old_val == 1)
> @@ -175,11 +199,19 @@ __mutex_lock_common(struct mutex *lock,
> debug_mutex_free_waiter(&waiter);
> return -EINTR;
> }
> - __set_task_state(task, state);
>
> - /* didnt get the lock, go to sleep: */
> + owner = lock->owner;
> + get_task_struct(owner);
> spin_unlock_mutex(&lock->wait_lock, flags);
> - schedule();
> +
> + if (adaptive_wait(&waiter, owner, state)) {
> + put_task_struct(owner);
> + __set_task_state(task, state);
> + /* didnt get the lock, go to sleep: */
> + schedule();
> + } else
> + put_task_struct(owner);
> +
> spin_lock_mutex(&lock->wait_lock, flags);
> }
>
> @@ -187,7 +219,7 @@ 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));
> + lock->owner = task;
>
> /* set it to 0 if there are no waiters left: */
> if (likely(list_empty(&lock->wait_list)))
> @@ -260,7 +292,7 @@ __mutex_unlock_common_slowpath(atomic_t
> wake_up_process(waiter->task);
> }
>
> - debug_mutex_clear_owner(lock);
> + lock->owner = NULL;
>
> spin_unlock_mutex(&lock->wait_lock, flags);
> }
> @@ -352,7 +384,7 @@ static inline int __mutex_trylock_slowpa
>
> prev = atomic_xchg(&lock->count, -1);
> if (likely(prev == 1)) {
> - debug_mutex_set_owner(lock, current_thread_info());
> + lock->owner = current;
> mutex_acquire(&lock->dep_map, 0, 1, _RET_IP_);
> }
> /* Set it back to 0 if there are no waiters: */
> Index: linux-2.6/kernel/sched.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched.c
> +++ linux-2.6/kernel/sched.c
> @@ -697,6 +697,11 @@ int runqueue_is_locked(void)
> return ret;
> }
>
> +int task_is_current(struct task_struct *p)
> +{
> + return task_rq(p)->curr == p;
> +}
> +
> /*
> * Debugging: various feature bits
> */
> Index: linux-2.6/kernel/mutex.h
> ===================================================================
> --- linux-2.6.orig/kernel/mutex.h
> +++ linux-2.6/kernel/mutex.h
> @@ -16,8 +16,6 @@
> #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)
> #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)