locking/rwsem: Reduce spinlock contention in wakeup after up_read()/up_write()

In up_write()/up_read(), rwsem_wake() will be called whenever it
detects that some writers/readers are waiting. The rwsem_wake()
function will take the wait_lock and call __rwsem_do_wake() to do the
real wakeup.  For a heavily contended rwsem, doing a spin_lock() on
wait_lock will cause further contention on the heavily contended rwsem
cacheline resulting in delay in the completion of the up_read/up_write
operations.

This patch makes the wait_lock taking and the call to __rwsem_do_wake()
optional if at least one spinning writer is present. The spinning
writer will be able to take the rwsem and call rwsem_wake() later
when it calls up_write(). With the presence of a spinning writer,
rwsem_wake() will now try to acquire the lock using trylock. If that
fails, it will just quit.

Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Waiman Long <Waiman.Long@hp.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Acked-by: Jason Low <jason.low2@hp.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Douglas Hatch <doug.hatch@hp.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Scott J Norton <scott.norton@hp.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1430428337-16802-2-git-send-email-Waiman.Long@hp.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 44 insertions, 0 deletions

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 3417d0172a5d..0f189714e457 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -409,11 +409,24 @@ done:
         return taken;
 }
 
+/*
+ * Return true if the rwsem has active spinner
+ */
+static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
+{
+        return osq_is_locked(&sem->osq);
+}
+
 #else
 static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
 {
         return false;
 }
+
+static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
+{
+        return false;
+}
 #endif
 
 /*
@@ -496,7 +509,38 @@ struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
 {
         unsigned long flags;
 
+        /*
+         * If a spinner is present, it is not necessary to do the wakeup.
+         * Try to do wakeup only if the trylock succeeds to minimize
+         * spinlock contention which may introduce too much delay in the
+         * unlock operation.
+         *
+         *    spinning writer           up_write/up_read caller
+         *    ---------------           -----------------------
+         * [S]   osq_unlock()           [L]   osq
+         *       MB                           RMB
+         * [RmW] rwsem_try_write_lock() [RmW] spin_trylock(wait_lock)
+         *
+         * Here, it is important to make sure that there won't be a missed
+         * wakeup while the rwsem is free and the only spinning writer goes
+         * to sleep without taking the rwsem. Even when the spinning writer
+         * is just going to break out of the waiting loop, it will still do
+         * a trylock in rwsem_down_write_failed() before sleeping. IOW, if
+         * rwsem_has_spinner() is true, it will guarantee at least one
+         * trylock attempt on the rwsem later on.
+         */
+        if (rwsem_has_spinner(sem)) {
+                /*
+                 * The smp_rmb() here is to make sure that the spinner
+                 * state is consulted before reading the wait_lock.
+                 */
+                smp_rmb();
+                if (!raw_spin_trylock_irqsave(&sem->wait_lock, flags))
+                        return sem;
+                goto locked;
+        }
         raw_spin_lock_irqsave(&sem->wait_lock, flags);
+locked:
 
         /* do nothing if list empty */
         if (!list_empty(&sem->wait_list))