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Subject[tip:locking/core] locking/rwsem: Reduce spinlock contention in wakeup after up_read()/up_write()
Commit-ID:  59aabfc7e959f5f213e4e5cc7567ab4934da2adf
Gitweb: http://git.kernel.org/tip/59aabfc7e959f5f213e4e5cc7567ab4934da2adf
Author: Waiman Long <Waiman.Long@hp.com>
AuthorDate: Thu, 30 Apr 2015 17:12:16 -0400
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Fri, 8 May 2015 12:27:59 +0200

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>
---
include/linux/osq_lock.h | 5 +++++
kernel/locking/rwsem-xadd.c | 44 ++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 49 insertions(+)

diff --git a/include/linux/osq_lock.h b/include/linux/osq_lock.h
index 3a6490e..703ea5c 100644
--- a/include/linux/osq_lock.h
+++ b/include/linux/osq_lock.h
@@ -32,4 +32,9 @@ static inline void osq_lock_init(struct optimistic_spin_queue *lock)
extern bool osq_lock(struct optimistic_spin_queue *lock);
extern void osq_unlock(struct optimistic_spin_queue *lock);

+static inline bool osq_is_locked(struct optimistic_spin_queue *lock)
+{
+ return atomic_read(&lock->tail) != OSQ_UNLOCKED_VAL;
+}
+
#endif
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 3417d01..0f18971 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))

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