[PATCH 2/2] locking/mutex,rwsem: Reduce vcpu_is_preempted() calling frequency

[Waiman Long]

As the vcpu_is_preempted() call is pretty costly compared with other
checks within mutex_spin_on_owner() and rwsem_spin_on_owner(), they
are done at a reduce frequency of once every 256 iterations.

Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 kernel/locking/mutex.c      | 5 ++++-
 kernel/locking/rwsem-xadd.c | 6 ++++--
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index ad2d9e2..2ece0c4 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -423,6 +423,7 @@ bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner,
 			 struct ww_acquire_ctx *ww_ctx, struct mutex_waiter *waiter)
 {
 	bool ret = true;
+	int loop = 0;
 
 	rcu_read_lock();
 	while (__mutex_owner(lock) == owner) {
@@ -436,9 +437,11 @@ bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner,
 
 		/*
 		 * Use vcpu_is_preempted to detect lock holder preemption issue.
+		 * As vcpu_is_preempted is more costly to use, it is called at
+		 * a reduced frequencey (once every 256 iterations).
 		 */
 		if (!owner->on_cpu || need_resched() ||
-				vcpu_is_preempted(task_cpu(owner))) {
+		   (!(++loop & 0xff) &&	vcpu_is_preempted(task_cpu(owner)))) {
 			ret = false;
 			break;
 		}
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 2ad8d8d..7a884a6 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -351,6 +351,7 @@ static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
 static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem)
 {
 	struct task_struct *owner = READ_ONCE(sem->owner);
+	int loop = 0;
 
 	if (!rwsem_owner_is_writer(owner))
 		goto out;
@@ -367,10 +368,11 @@ static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem)
 
 		/*
 		 * abort spinning when need_resched or owner is not running or
-		 * owner's cpu is preempted.
+		 * owner's cpu is preempted. The preemption check is done at
+		 * a lower frequencey because of its high cost.
 		 */
 		if (!owner->on_cpu || need_resched() ||
-				vcpu_is_preempted(task_cpu(owner))) {
+		   (!(++loop & 0xff) &&	vcpu_is_preempted(task_cpu(owner)))) {
 			rcu_read_unlock();
 			return false;
 		}
-- 
1.8.3.1