Messages in this thread | | | Subject | Re: [PATCH] mutex: do not unnecessarily deal with waiters | From | Davidlohr Bueso <> | Date | Thu, 30 May 2013 18:12:00 -0700 |
| |
ping?
On Thu, 2013-05-23 at 16:59 -0700, Davidlohr Bueso wrote: > From: Davidlohr Bueso <davidlohr.bueso@hp.com> > > Upon entering the slowpath, we immediately attempt to acquire the lock > by checking if it is already unlocked. If we are lucky enough that this > is the case, then we don't need to deal with any waiter related logic. > > Furthermore any checks for an empty wait_list are unnecessary as we > already know that count is non-negative and hence no one is waiting for > the lock. > > Move the count check and xchg calls to be done before any waiters are > setup - including waiter debugging. Upon failure to acquire the lock, > the xchg sets the counter to 0, instead of -1 as it was originally. > This can be done here since we set it back to -1 right at the beginning > of the loop so other waiters are woken up when the lock is released. > > When tested on a 8-socket (80 core) system against a vanilla 3.10-rc1 > kernel, this patch provides some small performance benefits (+2-6%). > While it could be considered in the noise level, the average percentages > were stable across multiple runs and no performance regressions were seen. > Two big winners, for small amounts of users (10-100), were the short and > compute workloads had a +19.36% and +%15.76% in jobs per minute. > > Signed-off-by: Davidlohr Bueso <davidlohr.bueso@hp.com> > --- > kernel/mutex.c | 26 ++++++++++++++------------ > 1 file changed, 14 insertions(+), 12 deletions(-) > > diff --git a/kernel/mutex.c b/kernel/mutex.c > index ad53a66..a8cd741 100644 > --- a/kernel/mutex.c > +++ b/kernel/mutex.c > @@ -306,7 +306,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, > owner = ACCESS_ONCE(lock->owner); > if (owner && !mutex_spin_on_owner(lock, owner)) { > mspin_unlock(MLOCK(lock), &node); > - break; > + goto slowpath; > } > > if ((atomic_read(&lock->count) == 1) && > @@ -314,8 +314,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, > lock_acquired(&lock->dep_map, ip); > mutex_set_owner(lock); > mspin_unlock(MLOCK(lock), &node); > - preempt_enable(); > - return 0; > + goto done; > } > mspin_unlock(MLOCK(lock), &node); > > @@ -326,7 +325,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, > * the owner complete. > */ > if (!owner && (need_resched() || rt_task(task))) > - break; > + goto slowpath; > > /* > * The cpu_relax() call is a compiler barrier which forces > @@ -340,6 +339,14 @@ slowpath: > #endif > spin_lock_mutex(&lock->wait_lock, flags); > > + /* once more, can we acquire the lock? */ > + if (MUTEX_SHOW_NO_WAITER(lock) && (atomic_xchg(&lock->count, 0) == 1)) { > + lock_acquired(&lock->dep_map, ip); > + mutex_set_owner(lock); > + spin_unlock_mutex(&lock->wait_lock, flags); > + goto done; > + } > + > debug_mutex_lock_common(lock, &waiter); > debug_mutex_add_waiter(lock, &waiter, task_thread_info(task)); > > @@ -347,9 +354,6 @@ slowpath: > list_add_tail(&waiter.list, &lock->wait_list); > waiter.task = task; > > - if (MUTEX_SHOW_NO_WAITER(lock) && (atomic_xchg(&lock->count, -1) == 1)) > - goto done; > - > lock_contended(&lock->dep_map, ip); > > for (;;) { > @@ -363,7 +367,7 @@ slowpath: > * other waiters: > */ > if (MUTEX_SHOW_NO_WAITER(lock) && > - (atomic_xchg(&lock->count, -1) == 1)) > + (atomic_xchg(&lock->count, -1) == 1)) > break; > > /* > @@ -388,9 +392,8 @@ slowpath: > spin_lock_mutex(&lock->wait_lock, flags); > } > > -done: > + /* got the lock - cleanup and rejoice! */ > lock_acquired(&lock->dep_map, ip); > - /* got the lock - rejoice! */ > mutex_remove_waiter(lock, &waiter, current_thread_info()); > mutex_set_owner(lock); > > @@ -399,10 +402,9 @@ done: > atomic_set(&lock->count, 0); > > spin_unlock_mutex(&lock->wait_lock, flags); > - > debug_mutex_free_waiter(&waiter); > +done: > preempt_enable(); > - > return 0; > } >
| |