[RFC] Avoid mutex starvation when optimistic spinning is disabled

[Jason Low]

Hi Imre,

Here is a patch which prevents a thread from spending too much "time"
waiting for a mutex in the !CONFIG_MUTEX_SPIN_ON_OWNER case.

Would you like to try this out and see if this addresses the mutex
starvation issue you are seeing in your workload when optimistic
spinning is disabled?

Thanks.

---
Signed-off-by: Jason Low <jason.low2@xxxxxxx>
---
 include/linux/mutex.h  |  2 ++
 kernel/locking/mutex.c | 61 +++++++++++++++++++++++++++++++++++++++++++++-----
 2 files changed, 58 insertions(+), 5 deletions(-)

diff --git a/include/linux/mutex.h b/include/linux/mutex.h
index 2cb7531..c1ca68d 100644
--- a/include/linux/mutex.h
+++ b/include/linux/mutex.h
@@ -57,6 +57,8 @@ struct mutex {
 #endif
 #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
 	struct optimistic_spin_queue osq; /* Spinner MCS lock */
+#else
+	bool yield_to_waiter; /* Prevent starvation when spinning disabled */
 #endif
 #ifdef CONFIG_DEBUG_MUTEXES
 	void			*magic;
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index a70b90d..fec78a7 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -55,6 +55,8 @@ __mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key)
 	mutex_clear_owner(lock);
 #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
 	osq_lock_init(&lock->osq);
+#else
+	lock->yield_to_waiter = false;
 #endif
 
 	debug_mutex_init(lock, name, key);
@@ -71,6 +73,9 @@ EXPORT_SYMBOL(__mutex_init);
  */
 __visible void __sched __mutex_lock_slowpath(atomic_t *lock_count);
 
+
+static inline bool need_yield_to_waiter(struct mutex *lock);
+
 /**
  * mutex_lock - acquire the mutex
  * @lock: the mutex to be acquired
@@ -95,11 +100,15 @@ __visible void __sched __mutex_lock_slowpath(atomic_t *lock_count);
 void __sched mutex_lock(struct mutex *lock)
 {
 	might_sleep();
+
 	/*
 	 * The locking fastpath is the 1->0 transition from
 	 * 'unlocked' into 'locked' state.
 	 */
-	__mutex_fastpath_lock(&lock->count, __mutex_lock_slowpath);
+	if (!need_yield_to_waiter(lock))
+		__mutex_fastpath_lock(&lock->count, __mutex_lock_slowpath);
+	else
+		__mutex_lock_slowpath(&lock->count);
 	mutex_set_owner(lock);
 }
 
@@ -398,12 +407,39 @@ done:
 
 	return false;
 }
+
+static inline void do_yield_to_waiter(struct mutex *lock, int loops)
+{
+	return;
+}
+
+static inline bool need_yield_to_waiter(struct mutex *lock)
+{
+	return false;
+}
+
 #else
 static bool mutex_optimistic_spin(struct mutex *lock,
 				  struct ww_acquire_ctx *ww_ctx, const bool use_ww_ctx)
 {
 	return false;
 }
+
+#define MUTEX_MAX_WAIT 16
+
+static inline void do_yield_to_waiter(struct mutex *lock, int loops)
+{
+	if (loops < MUTEX_MAX_WAIT)
+		return;
+
+	if (lock->yield_to_waiter != true)
+		lock->yield_to_waiter =true;
+}
+
+static inline bool need_yield_to_waiter(struct mutex *lock)
+{
+	return lock->yield_to_waiter;
+}
 #endif
 
 __visible __used noinline
@@ -510,6 +546,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 	struct mutex_waiter waiter;
 	unsigned long flags;
 	int ret;
+	int loop = 0;
 
 	if (use_ww_ctx) {
 		struct ww_mutex *ww = container_of(lock, struct ww_mutex, base);
@@ -546,6 +583,8 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 	lock_contended(&lock->dep_map, ip);
 
 	for (;;) {
+		loop++;
+
 		/*
 		 * Lets try to take the lock again - this is needed even if
 		 * we get here for the first time (shortly after failing to
@@ -580,6 +619,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 		/* didn't get the lock, go to sleep: */
 		spin_unlock_mutex(&lock->wait_lock, flags);
 		schedule_preempt_disabled();
+		do_yield_to_waiter(lock, loop);
 		spin_lock_mutex(&lock->wait_lock, flags);
 	}
 	__set_task_state(task, TASK_RUNNING);
@@ -590,6 +630,11 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 		atomic_set(&lock->count, 0);
 	debug_mutex_free_waiter(&waiter);
 
+#ifndef CONFIG_MUTEX_SPIN_ON_OWNER
+	if (lock->yield_to_waiter)
+		lock->yield_to_waiter = false;
+#endif
+
 skip_wait:
 	/* got the lock - cleanup and rejoice! */
 	lock_acquired(&lock->dep_map, ip);
@@ -789,10 +834,13 @@ __mutex_lock_interruptible_slowpath(struct mutex *lock);
  */
 int __sched mutex_lock_interruptible(struct mutex *lock)
 {
-	int ret;
+	int ret = 1;
 
 	might_sleep();
-	ret =  __mutex_fastpath_lock_retval(&lock->count);
+
+	if (!need_yield_to_waiter(lock))
+		ret =  __mutex_fastpath_lock_retval(&lock->count);
+
 	if (likely(!ret)) {
 		mutex_set_owner(lock);
 		return 0;
@@ -804,10 +852,13 @@ EXPORT_SYMBOL(mutex_lock_interruptible);
 
 int __sched mutex_lock_killable(struct mutex *lock)
 {
-	int ret;
+	int ret = 1;
 
 	might_sleep();
-	ret = __mutex_fastpath_lock_retval(&lock->count);
+
+	if (!need_yield_to_waiter(lock))
+		ret = __mutex_fastpath_lock_retval(&lock->count);
+
 	if (likely(!ret)) {
 		mutex_set_owner(lock);
 		return 0;
-- 
1.9.1