[PATCH v6 6/6] locking/rwsem: Update handoff lock events tracking

From: Waiman Long
Date: Thu Nov 17 2022 - 21:21:28 EST


With the new direct rwsem lock handoff, the corresponding handoff lock
events are updated to also track the number of secondary lock handoffs
in rwsem_down_read_slowpath() to see how prevalent those handoff
events are. The number of primary lock handoffs in the unlock paths is
(rwsem_handoff_read + rwsem_handoff_write - rwsem_handoff_rslow).

After running a 96-thread rwsem microbenchmark with equal number
of readers and writers on a 2-socket 96-thread system for 40s, the
following handoff stats were obtained:

rwsem_handoff_read=189
rwsem_handoff_rslow=1
rwsem_handoff_write=6678
rwsem_handoff_wspin=6681

The number of primary handoffs was 6866, whereas there was only one
secondary handoff for this test run.

Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
kernel/locking/lock_events_list.h | 6 ++++--
kernel/locking/rwsem.c | 9 +++++----
2 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/kernel/locking/lock_events_list.h b/kernel/locking/lock_events_list.h
index 97fb6f3f840a..04d101767c2c 100644
--- a/kernel/locking/lock_events_list.h
+++ b/kernel/locking/lock_events_list.h
@@ -63,7 +63,9 @@ LOCK_EVENT(rwsem_rlock) /* # of read locks acquired */
LOCK_EVENT(rwsem_rlock_steal) /* # of read locks by lock stealing */
LOCK_EVENT(rwsem_rlock_fast) /* # of fast read locks acquired */
LOCK_EVENT(rwsem_rlock_fail) /* # of failed read lock acquisitions */
-LOCK_EVENT(rwsem_rlock_handoff) /* # of read lock handoffs */
LOCK_EVENT(rwsem_wlock) /* # of write locks acquired */
LOCK_EVENT(rwsem_wlock_fail) /* # of failed write lock acquisitions */
-LOCK_EVENT(rwsem_wlock_handoff) /* # of write lock handoffs */
+LOCK_EVENT(rwsem_handoff_read) /* # of read lock handoffs */
+LOCK_EVENT(rwsem_handoff_write) /* # of write lock handoffs */
+LOCK_EVENT(rwsem_handoff_rslow) /* # of handoffs in read slowpath */
+LOCK_EVENT(rwsem_handoff_wspin) /* # of handoff spins in write slowpath */
diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index 047e5fcb2457..b3135e12ca22 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -469,10 +469,8 @@ static void rwsem_mark_wake(struct rw_semaphore *sem,
* force the issue.
*/
if (time_after(jiffies, waiter->timeout)) {
- if (!(oldcount & RWSEM_FLAG_HANDOFF)) {
+ if (!(oldcount & RWSEM_FLAG_HANDOFF))
adjustment -= RWSEM_FLAG_HANDOFF;
- lockevent_inc(rwsem_rlock_handoff);
- }
WRITE_ONCE(waiter->handoff_state, HANDOFF_REQUESTED);
}

@@ -677,7 +675,6 @@ static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
*/
if (new & RWSEM_FLAG_HANDOFF) {
WRITE_ONCE(first->handoff_state, HANDOFF_REQUESTED);
- lockevent_inc(rwsem_wlock_handoff);
return false;
}

@@ -1011,10 +1008,12 @@ static void rwsem_handoff(struct rw_semaphore *sem, long adj,
wake_type = RWSEM_WAKE_ANY;
adj += RWSEM_WRITER_LOCKED;
atomic_long_set(&sem->owner, (long)waiter->task);
+ lockevent_inc(rwsem_handoff_write);
} else {
wake_type = RWSEM_WAKE_READ_OWNED;
adj += RWSEM_READER_BIAS;
__rwsem_set_reader_owned(sem, waiter->task);
+ lockevent_inc(rwsem_handoff_read);
}
atomic_long_add(adj, &sem->count);
rwsem_mark_wake(sem, wake_type, wake_q);
@@ -1123,6 +1122,7 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, unsigned int stat
if (rwsem_first_waiter(sem)->type == RWSEM_WAITING_FOR_READ)
adjustment = 0;
rwsem_handoff(sem, adjustment, &wake_q);
+ lockevent_inc(rwsem_handoff_rslow);

if (!adjustment) {
raw_spin_unlock_irq(&sem->wait_lock);
@@ -1253,6 +1253,7 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
if (handoff == HANDOFF_REQUESTED) {
rwsem_spin_on_owner(sem);
handoff = READ_ONCE(waiter.handoff_state);
+ lockevent_inc(rwsem_handoff_wspin);
}

if (handoff == HANDOFF_GRANTED)
--
2.31.1