[RFC PATCH-tip v4 09/10] locking/rwsem: Enable reactivation of reader spinning

Waiman Long
2016-08-18T21:11:11Z

Reader optimistic spinning will be disabled once the rspin_enabled
count reaches 0. After that, it cannot be re-enabled. This may cause
an eligible rwsem locked out of reader spinning because of a series
of unfortunate events.

This patch looks at the regular writer-on-writer spinning history. If
there are sufficient more successful spin attempts than failed ones,
it will try to reactivate reader spinning.

Signed-off-by: Waiman Long <Waiman.Long@hpe.com>
---
include/linux/rwsem.h | 12 ++++++++----
kernel/locking/rwsem-xadd.c | 27 +++++++++++++++++++++++++--
2 files changed, 33 insertions(+), 6 deletions(-)

diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
index 8978f87..98284b4 100644
--- a/include/linux/rwsem.h
+++ b/include/linux/rwsem.h
@@ -32,7 +32,11 @@ struct rw_semaphore {
raw_spinlock_t wait_lock;
#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
struct optimistic_spin_queue osq; /* spinner MCS lock */
- int rspin_enabled; /* protected by osq lock */
+ /*
+ * Reader optimistic spinning fields protected by osq lock
+ */
+ uint16_t rspin_enabled;
+ int16_t wspin_cnt;

/*
* Write owner. Used as a speculative check to see
@@ -74,10 +78,10 @@ static inline int rwsem_is_locked(struct rw_semaphore *sem)
/*
* Each successful reader spin will increment the rspin_enabled by 1.
* Each unsuccessful spin, on the other hand, will decrement it by 2.
- * Reader spinning will be permanently disabled when it reaches 0.
+ * Reader spinning will be disabled when it reaches 0.
*/
#ifndef RWSEM_RSPIN_ENABLED_DEFAULT
-# define RWSEM_RSPIN_ENABLED_DEFAULT 40
+# define RWSEM_RSPIN_ENABLED_DEFAULT 30
#endif
#define RWSEM_RSPIN_ENABLED_MAX 1024

@@ -87,7 +91,7 @@ static inline int rwsem_is_locked(struct rw_semaphore *sem)

#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
#define __RWSEM_OPT_INIT(lockname) , .osq = OSQ_LOCK_UNLOCKED, .owner = NULL, \
- .rspin_enabled = RWSEM_RSPIN_ENABLED_DEFAULT
+ .rspin_enabled = RWSEM_RSPIN_ENABLED_DEFAULT, .wspin_cnt = 0
#else
#define __RWSEM_OPT_INIT(lockname)
#endif
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index d850214..9978159 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -108,6 +108,7 @@ void __init_rwsem(struct rw_semaphore *sem, const char *name,
#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
sem->owner = NULL;
sem->rspin_enabled = RWSEM_RSPIN_ENABLED_DEFAULT;
+ sem->wspin_cnt = 0;
osq_lock_init(&sem->osq);
#endif
}
@@ -458,10 +459,32 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem,
if (taken && (sem->rspin_enabled < RWSEM_RSPIN_ENABLED_MAX)) {
sem->rspin_enabled++;
} else if (!taken) {
- if (sem->rspin_enabled > 2)
+ if (sem->rspin_enabled > 2) {
sem->rspin_enabled -= 2;
- else
+ } else if (sem->rspin_enabled) {
sem->rspin_enabled = 0;
+ /*
+ * Reset wspin_cnt so that it won't get
+ * re-enabled too soon.
+ */
+ if (sem->wspin_cnt > -30)
+ sem->wspin_cnt = -30;
+ }
+ }
+ } else if (type = RWSEM_WAITING_FOR_WRITE) {
+ /*
+ * Every 10 successful writer-on-writer spins more than failed
+ * spins will increment rspin_enabled to encourage more
+ * writer-on-reader spinning attempts.
+ */
+ if (taken) {
+ if ((++sem->wspin_cnt >= 10) &&
+ (sem->rspin_enabled < RWSEM_RSPIN_ENABLED_MAX)) {
+ sem->wspin_cnt = 0;
+ sem->rspin_enabled++;
+ }
+ } else if (sem->wspin_cnt > -100) {
+ sem->wspin_cnt--;
}
}
osq_unlock(&sem->osq);
--
1.7.1