From 1a99367023f6ac664365a37fa508b059e31d0e88 Mon Sep 17 00:00:00 2001 From: Davidlohr Bueso Date: Fri, 30 Jan 2015 01:14:27 -0800 Subject: [PATCH] locking/rwsem: Check for active lock before bailing on spinning 37e9562453b ("locking/rwsem: Allow conservative optimistic spinning when readers have lock") forced the default for optimistic spinning to be disabled if the lock owner was nil, which makes much sense for readers. However, while it is not our priority, we can make some optimizations for write-mostly workloads. We can bail the spinning step and still be conservative if there are any active tasks, otherwise there's really no reason not to spin, as the semaphore is most likely unlocked. This patch recovers most of a Unixbench 'execl' benchmark throughput by sleeping less and making better average system usage: before: CPU %user %nice %system %iowait %steal %idle all 0.60 0.00 8.02 0.00 0.00 91.38 after: CPU %user %nice %system %iowait %steal %idle all 1.22 0.00 70.18 0.00 0.00 28.60 Signed-off-by: Davidlohr Bueso Signed-off-by: Peter Zijlstra (Intel) Acked-by: Jason Low Cc: Linus Torvalds Cc: Michel Lespinasse Cc: Paul E. McKenney Cc: Tim Chen Link: http://lkml.kernel.org/r/1422609267-15102-6-git-send-email-dave@stgolabs.net Signed-off-by: Ingo Molnar --- kernel/locking/rwsem-xadd.c | 27 +++++++++++++++++---------- 1 file changed, 17 insertions(+), 10 deletions(-) diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c index 1c0d11e8ce34..e4ad019e23f5 100644 --- a/kernel/locking/rwsem-xadd.c +++ b/kernel/locking/rwsem-xadd.c @@ -298,23 +298,30 @@ static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem) static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem) { struct task_struct *owner; - bool on_cpu = false; + bool ret = true; if (need_resched()) return false; rcu_read_lock(); owner = ACCESS_ONCE(sem->owner); - if (owner) - on_cpu = owner->on_cpu; - rcu_read_unlock(); + if (!owner) { + long count = ACCESS_ONCE(sem->count); + /* + * If sem->owner is not set, yet we have just recently entered the + * slowpath with the lock being active, then there is a possibility + * reader(s) may have the lock. To be safe, bail spinning in these + * situations. + */ + if (count & RWSEM_ACTIVE_MASK) + ret = false; + goto done; + } - /* - * If sem->owner is not set, yet we have just recently entered the - * slowpath, then there is a possibility reader(s) may have the lock. - * To be safe, avoid spinning in these situations. - */ - return on_cpu; + ret = owner->on_cpu; +done: + rcu_read_unlock(); + return ret; } static inline bool owner_running(struct rw_semaphore *sem, -- 2.20.1