[PATCH 4/5] locking/rwsem: Enable timeout check when staying in the OSQ

Waiman Long
2019-09-11T15:06:25Z

Use the break function allowed by the new osq_lock() to enable early
break from the OSQ when a timeout value is specified and expiration
time has been reached.

Signed-off-by: Waiman Long <longman@redhat.com>
---
kernel/locking/rwsem.c | 35 +++++++++++++++++++++++++++++++----
1 file changed, 31 insertions(+), 4 deletions(-)

diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index c15926ecb21e..78708097162a 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -794,23 +794,50 @@ static inline u64 rwsem_rspin_threshold(struct rw_semaphore *sem)
return sched_clock() + delta;
}

+struct rwsem_break_arg {
+ u64 timeout;
+ int loopcnt;
+};
+
+static bool rwsem_osq_break(void *brk_arg)
+{
+ struct rwsem_break_arg *arg = brk_arg;
+
+ arg->loopcnt++;
+ /*
+ * Check sched_clock() only once every 256 iterations.
+ */
+ if (!(arg->loopcnt++ & 0xff) && (sched_clock() >= arg->timeout))
+ return true;
+ return false;
+}
+
static bool rwsem_optimistic_spin(struct rw_semaphore *sem, bool wlock,
ktime_t timeout)
{
- bool taken = false;
+ bool taken = false, locked;
int prev_owner_state = OWNER_NULL;
int loop = 0;
u64 rspin_threshold = 0, curtime;
+ struct rwsem_break_arg break_arg;
unsigned long nonspinnable = wlock ? RWSEM_WR_NONSPINNABLE
: RWSEM_RD_NONSPINNABLE;

preempt_disable();

/* sem->wait_lock should not be held when doing optimistic spinning */
- if (!osq_lock(&sem->osq, NULL, NULL))
- goto done;
+ if (timeout) {
+ break_arg.timeout = ktime_to_ns(timeout);
+ break_arg.loopcnt = 0;
+ locked = osq_lock(&sem->osq, rwsem_osq_break, &break_arg);
+ curtime = sched_clock();
+ } else {
+ locked = osq_lock(&sem->osq, NULL, NULL);
+ curtime = 0;
+ }

- curtime = timeout ? sched_clock() : 0;
+ if (!locked)
+ goto done;

/*
* Optimistically spin on the owner field and attempt to acquire the
--
2.18.1