Messages in this thread Patch in this message | | | Date | Mon, 10 Feb 2014 20:58:24 +0100 | From | Peter Zijlstra <> | Subject | [PATCH 4/8] mutex: Unlock the mutex without the wait_lock |
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When running workloads that have high contention in mutexes on an 8 socket machine, mutex spinners would often spin for a long time with no lock owner.
The main reason why this is occuring is in __mutex_unlock_common_slowpath(), if __mutex_slowpath_needs_to_unlock(), then the owner needs to acquire the mutex->wait_lock before releasing the mutex (setting lock->count to 1). When the wait_lock is contended, this delays the mutex from being released. We should be able to release the mutex without holding the wait_lock.
Cc: chegu_vinod@hp.com Cc: paulmck@linux.vnet.ibm.com Cc: Waiman.Long@hp.com Cc: torvalds@linux-foundation.org Cc: tglx@linutronix.de Cc: riel@redhat.com Cc: akpm@linux-foundation.org Cc: davidlohr@hp.com Cc: hpa@zytor.com Cc: andi@firstfloor.org Cc: aswin@hp.com Cc: mingo@kernel.org Cc: scott.norton@hp.com Signed-off-by: Jason Low <jason.low2@hp.com> Signed-off-by: Peter Zijlstra <peterz@infradead.org> Link: http://lkml.kernel.org/r/1390936396-3962-4-git-send-email-jason.low2@hp.com --- kernel/locking/mutex.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-)
--- a/kernel/locking/mutex.c +++ b/kernel/locking/mutex.c @@ -673,10 +673,6 @@ __mutex_unlock_common_slowpath(atomic_t struct mutex *lock = container_of(lock_count, struct mutex, count); unsigned long flags; - spin_lock_mutex(&lock->wait_lock, flags); - mutex_release(&lock->dep_map, nested, _RET_IP_); - debug_mutex_unlock(lock); - /* * some architectures leave the lock unlocked in the fastpath failure * case, others need to leave it locked. In the later case we have to @@ -685,6 +681,10 @@ __mutex_unlock_common_slowpath(atomic_t if (__mutex_slowpath_needs_to_unlock()) atomic_set(&lock->count, 1); + spin_lock_mutex(&lock->wait_lock, flags); + mutex_release(&lock->dep_map, nested, _RET_IP_); + debug_mutex_unlock(lock); + if (!list_empty(&lock->wait_list)) { /* get the first entry from the wait-list: */ struct mutex_waiter *waiter =
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