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Subject[PATCH 10/10] locking/rwsem: Enable lock event counting
Date
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=634
rwsem_opt_wlock=53735
rwsem_rlock=824
rwsem_rlock_fail=0
rwsem_rlock_fast=20
rwsem_sleep_reader=814
rwsem_sleep_writer=852
rwsem_wake_reader=462
rwsem_wake_writer=3747
rwsem_wlock=634
rwsem_wlock_fail=0

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: Waiman Long <longman@redhat.com>
---
arch/Kconfig | 2 +-
kernel/locking/lock_events_list.h | 17 +++++++++++++++++
kernel/locking/rwsem-xadd.c | 12 ++++++++++++
3 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 5fccff4..39d9ab8 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -891,7 +891,7 @@ config ARCH_USE_MEMREMAP_PROT
config LOCK_EVENT_COUNTS
bool "Locking event counts collection"
depends on DEBUG_FS
- depends on QUEUED_SPINLOCKS
+ depends on (QUEUED_SPINLOCKS || RWSEM_XCHGADD_ALGORITHM)
---help---
Enable light-weight counting of various locking related events
in the system with minimal performance impact. This reduces
diff --git a/kernel/locking/lock_events_list.h b/kernel/locking/lock_events_list.h
index 8b4d2e1..c33c5df 100644
--- a/kernel/locking/lock_events_list.h
+++ b/kernel/locking/lock_events_list.h
@@ -48,3 +48,20 @@
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 */
+
+#ifdef CONFIG_RWSEM_XCHGADD_ALGORITHM
+/*
+ * 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_wlock) /* # of write locks acquired */
+LOCK_EVENT(rwsem_wlock_fail) /* # of failed write lock acquisitions */
+#endif /* CONFIG_RWSEM_XCHGADD_ALGORITHM */
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 62422a6..fff231a 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -20,6 +20,7 @@
#include <linux/osq_lock.h>

#include "rwsem-xadd.h"
+#include "lock_events.h"

/*
* Guide to the rw_semaphore's count field for common values.
@@ -147,6 +148,7 @@ 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 +216,7 @@ 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;
@@ -269,6 +272,7 @@ static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
count + RWSEM_ACTIVE_WRITE_BIAS);
if (old == count) {
rwsem_set_owner(sem);
+ lockevent_inc(rwsem_opt_wlock);
return true;
}

@@ -394,6 +398,7 @@ 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;
}

@@ -441,6 +446,7 @@ static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
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;
@@ -477,9 +483,11 @@ static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
break;
}
schedule();
+ lockevent_inc(rwsem_sleep_reader);
}

__set_current_state(TASK_RUNNING);
+ lockevent_inc(rwsem_rlock);
return sem;
out_nolock:
list_del(&waiter.list);
@@ -487,6 +495,7 @@ static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
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);
}

@@ -580,6 +589,7 @@ static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
goto out_nolock;

schedule();
+ lockevent_inc(rwsem_sleep_writer);
set_current_state(state);
} while ((count = atomic_long_read(&sem->count)) & RWSEM_ACTIVE_MASK);

@@ -588,6 +598,7 @@ static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
__set_current_state(TASK_RUNNING);
list_del(&waiter.list);
raw_spin_unlock_irq(&sem->wait_lock);
+ lockevent_inc(rwsem_wlock);

return ret;

@@ -601,6 +612,7 @@ static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
__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);
}
--
1.8.3.1
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