From 07d2413a61db6500f58e614e873eed79d7f2ed72 Mon Sep 17 00:00:00 2001 From: Jason Low Date: Mon, 2 Feb 2015 13:59:26 -0800 Subject: [PATCH] locking/mutex: In mutex_spin_on_owner(), return true when owner changes In the mutex_spin_on_owner(), we return true only if lock->owner == NULL. This was beneficial in situations where there were multiple threads simultaneously spinning for the mutex. If another thread got the lock while other spinner(s) were also doing mutex_spin_on_owner(), then the other spinners would stop spinning. This workaround helped reduce the chance that many spinners were simultaneously spinning for the mutex which can help reduce contention in highly contended cases. However, recent changes were made to the optimistic spinning code such that instead of having all spinners simultaneously spin for the mutex, we queue the spinners with an MCS lock such that only one thread spins for the mutex at a time. Furthermore, the OSQ optimizations ensure that spinners in the queue will stop waiting if it needs to reschedule. Now, we don't have to worry about multiple threads spinning on owner at the same time, and if lock->owner is not NULL at this point, it likely means another thread happens to obtain the lock in the fastpath. In this case, it would make sense for the spinner to continue spinning as long as the spinner doesn't need to schedule and the mutex owner is running. This patch changes this so that mutex_spin_on_owner() returns true when the lock owner changes, which means a thread will only stop spinning if it either needs to reschedule or if the lock owner is not running. We saw up to a 5% performance improvement in the fserver workload with this patch. Signed-off-by: Jason Low Signed-off-by: Peter Zijlstra (Intel) Acked-by: Davidlohr Bueso Cc: Aswin Chandramouleeswaran Cc: Linus Torvalds Cc: Paul E. McKenney Cc: Tim Chen Cc: chegu_vinod@hp.com Cc: tglx@linutronix.de Link: http://lkml.kernel.org/r/1422914367-5574-2-git-send-email-jason.low2@hp.com Signed-off-by: Ingo Molnar --- kernel/locking/mutex.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c index 94674e5919cb..49cce442f3ff 100644 --- a/kernel/locking/mutex.c +++ b/kernel/locking/mutex.c @@ -250,11 +250,11 @@ int mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner) rcu_read_unlock(); /* - * We break out the loop above on need_resched() and when the - * owner changed, which is a sign for heavy contention. Return - * success only when lock->owner is NULL. + * We break out of the loop above on either need_resched(), when + * the owner is not running, or when the lock owner changed. + * Return success only when the lock owner changed. */ - return lock->owner == NULL; + return lock->owner != owner; } /* -- 2.30.2