We can safely check the wait_list to see if waiters are present without lock when there are spinners to fall back on in case we miss a waiter. The advantage is that we can save a pair of spin_lock/unlock calls when the wait_list is empty. This translates to a reduction in latency and hence slightly better performance.
On a 2-socket 36-core 72-thread x86-64 E5-2699 v3 system, a rwsem microbenchmark was run with 36 locking threads (one/core) doing 1 million writer lock/unlock operations each, the resulting locking rates (avg of 4 runs) on a 4.10 kernel were 7,755 Mop/s and 8,276 Mop/s without and with the patch respectively. That was an increase of about 7%. Signed-off-by: Waiman Long <[email protected]> --- kernel/locking/rwsem-xadd.c | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c index 34e727f..b5d7055 100644 --- a/kernel/locking/rwsem-xadd.c +++ b/kernel/locking/rwsem-xadd.c @@ -611,6 +611,17 @@ struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem) * state is consulted before reading the wait_lock. */ smp_rmb(); + + /* + * Normally checking wait_list without wait_lock isn't safe + * as we may miss an incoming waiter. With spinners present, + * however, we have someone to fall back on in case that + * happens. This can save a pair of spin_lock/unlock calls + * when there is no waiter. + */ + if (list_empty(&sem->wait_list)) + return sem; + if (!raw_spin_trylock_irqsave(&sem->wait_lock, flags)) return sem; goto locked; -- 1.8.3.1
