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locking/rwsem: Optimize write lock by reducing operations in slowpath

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When acquiring the rwsem write lock in the slowpath, we first try
to set count to RWSEM_WAITING_BIAS. When that is successful,
we then atomically add the RWSEM_WAITING_BIAS in cases where
there are other tasks on the wait list. This causes write lock
operations to often issue multiple atomic operations.

We can instead make the list_is_singular() check first, and then
set the count accordingly, so that we issue at most 1 atomic
operation when acquiring the write lock and reduce unnecessary
cacheline contention.

Signed-off-by: Jason Low <jason.low2@hpe.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Waiman Long<Waiman.Long@hpe.com>
Acked-by: Davidlohr Bueso <dave@stgolabs.net>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Christoph Lameter <cl@linux.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Jason Low <jason.low2@hp.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Terry Rudd <terry.rudd@hpe.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Tony Luck <tony.luck@intel.com>
Link: http://lkml.kernel.org/r/1463445486-16078-2-git-send-email-jason.low2@hpe.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Jason Low 2016-05-16 17:38:00 -07:00 committed by Ingo Molnar
parent e38513905e
commit c0fcb6c2d3
1 changed files with 18 additions and 7 deletions
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25
kernel/locking/rwsem-xadd.c
View file

@ -261,17 +261,28 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
}
EXPORT_SYMBOL(rwsem_down_read_failed);
/*
* This function must be called with the sem->wait_lock held to prevent
* race conditions between checking the rwsem wait list and setting the
* sem->count accordingly.
*/
static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem)
{
/*
* Try acquiring the write lock. Check count first in order
* to reduce unnecessary expensive cmpxchg() operations.
* Avoid trying to acquire write lock if count isn't RWSEM_WAITING_BIAS.
*/
if (count == RWSEM_WAITING_BIAS &&
cmpxchg_acquire(&sem->count, RWSEM_WAITING_BIAS,
RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_WAITING_BIAS) {
if (!list_is_singular(&sem->wait_list))
rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
if (count != RWSEM_WAITING_BIAS)
return false;
/*
* Acquire the lock by trying to set it to ACTIVE_WRITE_BIAS. If there
* are other tasks on the wait list, we need to add on WAITING_BIAS.
*/
count = list_is_singular(&sem->wait_list) ?
RWSEM_ACTIVE_WRITE_BIAS :
RWSEM_ACTIVE_WRITE_BIAS + RWSEM_WAITING_BIAS;
if (cmpxchg_acquire(&sem->count, RWSEM_WAITING_BIAS, count) == RWSEM_WAITING_BIAS) {
rwsem_set_owner(sem);
return true;
}