Messages in this thread Patch in this message | | | From | Waiman Long <> | Subject | [PATCH v4 2/2] locking/rwsem: check for active writer before wakeup | Date | Thu, 30 Apr 2015 17:12:17 -0400 |
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On a highly contended rwsem, spinlock contention due to the slow rwsem_wake() call can be a significant portion of the total CPU cycles used. With writer lock stealing and writer optimistic spinning, there is also a chance that the lock may have been stolen by the time that the wait_lock is acquired.
This patch adds a low cost checking code after acquiring the wait_lock to look for active writer. The presence of an active writer will abort the wakeup operation.
Signed-off-by: Waiman Long <Waiman.Long@hp.com> --- kernel/locking/rwsem-xadd.c | 21 +++++++++++++++++++-- 1 files changed, 19 insertions(+), 2 deletions(-)
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c index 2bb25e2..815f0cc 100644 --- a/kernel/locking/rwsem-xadd.c +++ b/kernel/locking/rwsem-xadd.c @@ -399,6 +399,15 @@ static inline bool rwsem_has_spinner(struct rw_semaphore *sem) return osq_is_locked(&sem->osq); } +/* + * Return true if there is an active writer by checking the owner field which + * should be set if there is one. + */ +static inline bool rwsem_has_active_writer(struct rw_semaphore *sem) +{ + return READ_ONCE(sem->owner) != NULL; +} + #else static bool rwsem_optimistic_spin(struct rw_semaphore *sem) { @@ -409,6 +418,11 @@ static inline bool rwsem_has_spinner(struct rw_semaphore *sem) { return false; } + +static inline bool rwsem_has_active_writer(struct rw_semaphore *sem) +{ + return false; /* Assume it has no active writer */ +} #endif /* @@ -524,8 +538,11 @@ struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem) raw_spin_lock_irqsave(&sem->wait_lock, flags); locked: - /* do nothing if list empty */ - if (!list_empty(&sem->wait_list)) + /* + * Do nothing if list empty or the lock has just been stolen by a + * writer after a possibly long wait in getting the wait_lock. + */ + if (!list_empty(&sem->wait_list) && !rwsem_has_active_writer(sem)) sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY); raw_spin_unlock_irqrestore(&sem->wait_lock, flags); -- 1.7.1
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