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Commit a8654596 authored by Waiman Long's avatar Waiman Long Committed by Ingo Molnar
Browse files

locking/rwsem: Enable lock event counting 



Add lock event counting calls so that we can track the number of lock
events happening in the rwsem code.

With CONFIG_LOCK_EVENT_COUNTS on and booting a 4-socket 112-thread x86-64
system, the rwsem counts after system bootup were as follows:

  rwsem_opt_fail=261
  rwsem_opt_wlock=50636
  rwsem_rlock=445
  rwsem_rlock_fail=0
  rwsem_rlock_fast=22
  rwsem_rtrylock=810144
  rwsem_sleep_reader=441
  rwsem_sleep_writer=310
  rwsem_wake_reader=355
  rwsem_wake_writer=2335
  rwsem_wlock=261
  rwsem_wlock_fail=0
  rwsem_wtrylock=20583

It can be seen that most of the lock acquisitions in the slowpath were
write-locks in the optimistic spinning code path with no sleeping at
all. For this system, over 97% of the locks are acquired via optimistic
spinning. It illustrates the importance of optimistic spinning in
improving the performance of rwsem.

Signed-off-by: default avatarWaiman Long <longman@redhat.com>
Acked-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: default avatarDavidlohr Bueso <dbueso@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Will Deacon <will.deacon@arm.com>
Link: http://lkml.kernel.org/r/20190404174320.22416-11-longman@redhat.com


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent bf20616f

arch/Kconfig

+0 −1
Original line number Diff line number Diff line
@@ -904,7 +904,6 @@ config ARCH_USE_MEMREMAP_PROT 
config LOCK_EVENT_COUNTS

	bool "Locking event counts collection"

	depends on DEBUG_FS

	depends on QUEUED_SPINLOCKS

	---help---

	  Enable light-weight counting of various locking related events

	  in the system with minimal performance impact. This reduces

kernel/locking/lock_events_list.h

+17 −0
Original line number Diff line number Diff line
@@ -48,3 +48,20 @@ LOCK_EVENT(lock_use_node3) /* # of locking ops that use 3rd percpu node */ 
LOCK_EVENT(lock_use_node4)	/* # of locking ops that use 4th percpu node */

LOCK_EVENT(lock_no_node)	/* # of locking ops w/o using percpu node    */

#endif /* CONFIG_QUEUED_SPINLOCKS */



/*

 * Locking events for rwsem

 */

LOCK_EVENT(rwsem_sleep_reader)	/* # of reader sleeps			*/

LOCK_EVENT(rwsem_sleep_writer)	/* # of writer sleeps			*/

LOCK_EVENT(rwsem_wake_reader)	/* # of reader wakeups			*/

LOCK_EVENT(rwsem_wake_writer)	/* # of writer wakeups			*/

LOCK_EVENT(rwsem_opt_wlock)	/* # of write locks opt-spin acquired	*/

LOCK_EVENT(rwsem_opt_fail)	/* # of failed opt-spinnings		*/

LOCK_EVENT(rwsem_rlock)		/* # of read locks acquired		*/

LOCK_EVENT(rwsem_rlock_fast)	/* # of fast read locks acquired	*/

LOCK_EVENT(rwsem_rlock_fail)	/* # of failed read lock acquisitions	*/

LOCK_EVENT(rwsem_rtrylock)	/* # of read trylock calls		*/

LOCK_EVENT(rwsem_wlock)		/* # of write locks acquired		*/

LOCK_EVENT(rwsem_wlock_fail)	/* # of failed write lock acquisitions	*/

LOCK_EVENT(rwsem_wtrylock)	/* # of write trylock calls		*/

kernel/locking/rwsem-xadd.c

+11 −0
Original line number Diff line number Diff line
@@ -147,6 +147,7 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem, 
			 * will notice the queued writer.

			 */

			wake_q_add(wake_q, waiter->task);

			lockevent_inc(rwsem_wake_writer);

		}



		return;
@@ -214,6 +215,7 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem, 
	}



	adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;

	lockevent_cond_inc(rwsem_wake_reader, woken);

	if (list_empty(&sem->wait_list)) {

		/* hit end of list above */

		adjustment -= RWSEM_WAITING_BIAS;
@@ -265,6 +267,7 @@ static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem) 
		if (atomic_long_try_cmpxchg_acquire(&sem->count, &count,

					count + RWSEM_ACTIVE_WRITE_BIAS)) {

			rwsem_set_owner(sem);

			lockevent_inc(rwsem_opt_wlock);

			return true;

		}

	}
@@ -389,6 +392,7 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem) 
	osq_unlock(&sem->osq);

done:

	preempt_enable();

	lockevent_cond_inc(rwsem_opt_fail, !taken);

	return taken;

}
@@ -436,6 +440,7 @@ __rwsem_down_read_failed_common(struct rw_semaphore *sem, int state) 
		if (atomic_long_read(&sem->count) >= 0) {

			raw_spin_unlock_irq(&sem->wait_lock);

			rwsem_set_reader_owned(sem);

			lockevent_inc(rwsem_rlock_fast);

			return sem;

		}

		adjustment += RWSEM_WAITING_BIAS;
@@ -472,9 +477,11 @@ __rwsem_down_read_failed_common(struct rw_semaphore *sem, int state) 
			break;

		}

		schedule();

		lockevent_inc(rwsem_sleep_reader);

	}



	__set_current_state(TASK_RUNNING);

	lockevent_inc(rwsem_rlock);

	return sem;

out_nolock:

	list_del(&waiter.list);
@@ -482,6 +489,7 @@ __rwsem_down_read_failed_common(struct rw_semaphore *sem, int state) 
		atomic_long_add(-RWSEM_WAITING_BIAS, &sem->count);

	raw_spin_unlock_irq(&sem->wait_lock);

	__set_current_state(TASK_RUNNING);

	lockevent_inc(rwsem_rlock_fail);

	return ERR_PTR(-EINTR);

}
@@ -575,6 +583,7 @@ __rwsem_down_write_failed_common(struct rw_semaphore *sem, int state) 
				goto out_nolock;



			schedule();

			lockevent_inc(rwsem_sleep_writer);

			set_current_state(state);

		} while ((count = atomic_long_read(&sem->count)) & RWSEM_ACTIVE_MASK);
@@ -583,6 +592,7 @@ __rwsem_down_write_failed_common(struct rw_semaphore *sem, int state) 
	__set_current_state(TASK_RUNNING);

	list_del(&waiter.list);

	raw_spin_unlock_irq(&sem->wait_lock);

	lockevent_inc(rwsem_wlock);



	return ret;
@@ -596,6 +606,7 @@ __rwsem_down_write_failed_common(struct rw_semaphore *sem, int state) 
		__rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);

	raw_spin_unlock_irq(&sem->wait_lock);

	wake_up_q(&wake_q);

	lockevent_inc(rwsem_wlock_fail);



	return ERR_PTR(-EINTR);

}

kernel/locking/rwsem.h

+4 −0
Original line number Diff line number Diff line
@@ -23,6 +23,8 @@ 
 * is involved. Ideally we would like to track all the readers that own

 * a rwsem, but the overhead is simply too big.

 */

#include "lock_events.h"



#define RWSEM_READER_OWNED	(1UL << 0)

#define RWSEM_ANONYMOUSLY_OWNED	(1UL << 1)
@@ -200,6 +202,7 @@ static inline int __down_read_trylock(struct rw_semaphore *sem) 
	 */

	long tmp = RWSEM_UNLOCKED_VALUE;



	lockevent_inc(rwsem_rtrylock);

	do {

		if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,

					tmp + RWSEM_ACTIVE_READ_BIAS)) {
@@ -241,6 +244,7 @@ static inline int __down_write_trylock(struct rw_semaphore *sem) 
{

	long tmp;



	lockevent_inc(rwsem_wtrylock);

	tmp = atomic_long_cmpxchg_acquire(&sem->count, RWSEM_UNLOCKED_VALUE,

		      RWSEM_ACTIVE_WRITE_BIAS);

	if (tmp == RWSEM_UNLOCKED_VALUE) {