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Subject[tip:locking/core] locking/rwsem: Enable lock event counting
Commit-ID:  a8654596f0371c2604c4d475422c48f4fc6a56c9
Gitweb: https://git.kernel.org/tip/a8654596f0371c2604c4d475422c48f4fc6a56c9
Author: Waiman Long <longman@redhat.com>
AuthorDate: Thu, 4 Apr 2019 13:43:19 -0400
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Wed, 10 Apr 2019 10:56:06 +0200

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: Waiman Long <longman@redhat.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Davidlohr 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: Ingo Molnar <mingo@kernel.org>
---
arch/Kconfig | 1 -
kernel/locking/lock_events_list.h | 17 +++++++++++++++++
kernel/locking/rwsem-xadd.c | 11 +++++++++++
kernel/locking/rwsem.h | 4 ++++
4 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 28c0f1ad80d7..02cb4e6a3e38 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -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
diff --git a/kernel/locking/lock_events_list.h b/kernel/locking/lock_events_list.h
index 8b4d2e180475..ad7668cfc9da 100644
--- a/kernel/locking/lock_events_list.h
+++ b/kernel/locking/lock_events_list.h
@@ -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 */
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index f6198e1a58f6..6b3ee9948bf1 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -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 @@ out_nolock:
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 @@ out_nolock:
__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);
}
diff --git a/kernel/locking/rwsem.h b/kernel/locking/rwsem.h
index 3059a2dc39f8..37db17890e36 100644
--- a/kernel/locking/rwsem.h
+++ b/kernel/locking/rwsem.h
@@ -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) {
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