Commit 59aabfc7 authored by Waiman Long's avatar Waiman Long Committed by Ingo Molnar
Browse files

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: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: default avatarWaiman Long <Waiman.Long@hp.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarDavidlohr Bueso <dave@stgolabs.net>
Acked-by: default avatarJason 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: default avatarIngo Molnar <mingo@kernel.org>
parent 3e0283a5

include/linux/osq_lock.h

+5 −0
Original line number Diff line number Diff line
@@ -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

kernel/locking/rwsem-xadd.c

+44 −0
Original line number Diff line number Diff line
@@ -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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