Messages in this thread Patch in this message | | | From | Waiman Long <> | Subject | [PATCH 3/5] locking/rwsem: Enable reader optimistic lock stealing | Date | Tue, 17 Nov 2020 22:04:27 -0500 |
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If the optimistic spinning queue is empty and the rwsem does not have the handoff or write-lock bits set, it is actually not necessary to call rwsem_optimistic_spin() to spin on it. Instead, it can steal the lock directly as its reader bias is in the count already. If it is the first reader in this state, it will try to wake up other readers in the wait queue.
Signed-off-by: Waiman Long <longman@redhat.com> --- kernel/locking/lock_events_list.h | 1 + kernel/locking/rwsem.c | 27 +++++++++++++++++++++++++++ 2 files changed, 28 insertions(+)
diff --git a/kernel/locking/lock_events_list.h b/kernel/locking/lock_events_list.h index 239039d0ce21..270a0d351932 100644 --- a/kernel/locking/lock_events_list.h +++ b/kernel/locking/lock_events_list.h @@ -63,6 +63,7 @@ LOCK_EVENT(rwsem_opt_nospin) /* # of disabled optspins */ LOCK_EVENT(rwsem_opt_norspin) /* # of disabled reader-only optspins */ LOCK_EVENT(rwsem_opt_rlock2) /* # of opt-acquired 2ndary read locks */ LOCK_EVENT(rwsem_rlock) /* # of read locks acquired */ +LOCK_EVENT(rwsem_rlock_steal) /* # of read locks by lock stealing */ LOCK_EVENT(rwsem_rlock_fast) /* # of fast read locks acquired */ LOCK_EVENT(rwsem_rlock_fail) /* # of failed read lock acquisitions */ LOCK_EVENT(rwsem_rlock_handoff) /* # of read lock handoffs */ diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c index ee374ae061c3..930dd4af3639 100644 --- a/kernel/locking/rwsem.c +++ b/kernel/locking/rwsem.c @@ -957,6 +957,12 @@ static inline bool rwsem_reader_phase_trylock(struct rw_semaphore *sem, } return false; } + +static inline bool osq_is_empty(struct rw_semaphore *sem) +{ + return !osq_is_locked(&sem->osq); +} + #else static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable) @@ -977,6 +983,10 @@ static inline bool rwsem_reader_phase_trylock(struct rw_semaphore *sem, return false; } +static inline bool osq_is_empty(sem) +{ + return false; +} static inline int rwsem_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable) { @@ -1007,6 +1017,22 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, int state, long count) !(count & RWSEM_WRITER_LOCKED)) goto queue; + /* + * Reader optimistic lock stealing + * + * We can take the read lock directly without doing + * rwsem_optimistic_spin() if the conditions are right. + * Also wake up other readers if it is the first reader. + */ + if (!(count & (RWSEM_WRITER_LOCKED | RWSEM_FLAG_HANDOFF)) && + osq_is_empty(sem)) { + rwsem_set_reader_owned(sem); + lockevent_inc(rwsem_rlock_steal); + if (rcnt == 1) + goto wake_readers; + return sem; + } + /* * Save the current read-owner of rwsem, if available, and the * reader nonspinnable bit. @@ -1029,6 +1055,7 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, int state, long count) * Wake up other readers in the wait list if the front * waiter is a reader. */ +wake_readers: if ((atomic_long_read(&sem->count) & RWSEM_FLAG_WAITERS)) { raw_spin_lock_irq(&sem->wait_lock); if (!list_empty(&sem->wait_list)) -- 2.18.1
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