[PATCH] s390/rwlock: Add contention tracepoints to rwlock slowpath

[Jan Polensky <japo@linux.ibm.com>]

Instrument arch_read_lock_wait() and arch_write_lock_wait() with
trace_contention_begin() and trace_contention_end().

These tracepoints are used by lock contention analysis tools such as
perf lock contention to identify contended rwlocks and measure wait
times. The generic implementation in kernel/locking/qrwlock.c already
emits the same events from its read and write slowpaths.

For arch_read_lock_wait(), the in_interrupt() fast-path is intentionally
left outside the tracepoint scope. That path spins without entering the
wait queue, so it does not represent queue-based contention and its
duration is not comparable to the normal slowpath. This matches the
behaviour of queued_read_lock_slowpath() in qrwlock.c.

The include for <trace/events/lock.h> is already present from
commit ffa796cc1f45 ("s390/spinlock: Add contention tracepoints to
lock slowpath").

Signed-off-by: Jan Polensky <japo@linux.ibm.com>
---
 arch/s390/lib/spinlock.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/s390/lib/spinlock.c b/arch/s390/lib/spinlock.c
index dbabca35c008..9094617a5e00 100644
--- a/arch/s390/lib/spinlock.c
+++ b/arch/s390/lib/spinlock.c
@@ -316,6 +316,7 @@ void arch_read_lock_wait(arch_rwlock_t *rw)
 		return;
 	}
 
+	trace_contention_begin(rw, LCB_F_SPIN | LCB_F_READ);
 	/* Remove this reader again to allow recursive read locking */
 	__atomic_add_const(-1, &rw->cnts);
 	/* Put the reader into the wait queue */
@@ -326,6 +327,7 @@ void arch_read_lock_wait(arch_rwlock_t *rw)
 	while (READ_ONCE(rw->cnts) & 0x10000)
 		barrier();
 	arch_spin_unlock(&rw->wait);
+	trace_contention_end(rw, 0);
 }
 EXPORT_SYMBOL(arch_read_lock_wait);
 
@@ -333,6 +335,7 @@ void arch_write_lock_wait(arch_rwlock_t *rw)
 {
 	int old;
 
+	trace_contention_begin(rw, LCB_F_SPIN | LCB_F_WRITE);
 	/* Add this CPU to the write waiters */
 	__atomic_add(0x20000, &rw->cnts);
 
@@ -349,6 +352,7 @@ void arch_write_lock_wait(arch_rwlock_t *rw)
 	}
 
 	arch_spin_unlock(&rw->wait);
+	trace_contention_end(rw, 0);
 }
 EXPORT_SYMBOL(arch_write_lock_wait);
 
-- 
2.53.0