Because of writer lock stealing, it is possible that a constant
stream of incoming writers will cause a waiting writer or reader to
wait indefinitely leading to lock starvation.
The mutex code has a lock handoff mechanism to prevent lock starvation.
This patch implements a similar lock handoff mechanism to disable
lock stealing and force lock handoff to the first waiter in the queue
after at least a 5ms waiting period. The waiting period is used to
avoid discouraging lock stealing too much to affect performance.
A rwsem microbenchmark was run for 8 seconds on a 4-socket 56-core
128-thread x86-64 system with a v5.0 based kernel and 112 write_lock
threads with 5us sleep critical section.
Before the patch, the min/mean/max numbers of locking operations for
the locking threads were 1/7,708/542,988. After the patch, the figures
became 6,031/7,267/9,476. It can be seen that the rwsem became much more
fair, though there was a slight drop of about 6% in the mean locking
operations done which was a tradeoff of having better fairness.
Signed-off-by: Waiman Long <long...@redhat.com>
---
kernel/locking/lock_events_list.h | 2 +
kernel/locking/rwsem-xadd.c | 110 +++++++++++++++++++++++++++++++-------
kernel/locking/rwsem-xadd.h | 23 +++++---
3 files changed, 109 insertions(+), 26 deletions(-)
diff --git a/kernel/locking/lock_events_list.h
b/kernel/locking/lock_events_list.h
index c33c5df..4cde507 100644
--- a/kernel/locking/lock_events_list.h
+++ b/kernel/locking/lock_events_list.h
@@ -62,6 +62,8 @@
LOCK_EVENT(rwsem_rlock) /* # of read locks acquired
*/
LOCK_EVENT(rwsem_rlock_fast) /* # of fast read locks acquired */
LOCK_EVENT(rwsem_rlock_fail) /* # of failed read lock acquisitions */
+LOCK_EVENT(rwsem_rlock_handoff) /* # of read lock handoffs
*/
LOCK_EVENT(rwsem_wlock) /* # of write locks acquired
*/
LOCK_EVENT(rwsem_wlock_fail) /* # of failed write lock acquisitions */
+LOCK_EVENT(rwsem_wlock_handoff) /* # of write lock handoffs
*/
#endif /* CONFIG_RWSEM_XCHGADD_ALGORITHM */
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 18a414e..12b1d61 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -74,6 +74,7 @@ struct rwsem_waiter {
struct list_head list;
struct task_struct *task;
enum rwsem_waiter_type type;
+ unsigned long timeout;
};
enum rwsem_wake_type {
@@ -83,6 +84,12 @@ enum rwsem_wake_type {
};
/*
+ * The minimum waiting time (5ms) in the wait queue before initiating the
+ * handoff protocol.
+ */
+#define RWSEM_WAIT_TIMEOUT ((HZ - 1)/200 + 1)
+
+/*
* handle the lock release when processes blocked on it that can now run
* - if we come here from up_xxxx(), then the RWSEM_FLAG_WAITERS bit must
* have been set.
@@ -132,6 +139,15 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
adjustment = RWSEM_READER_BIAS;
oldcount = atomic_long_fetch_add(adjustment, &sem->count);
if (unlikely(oldcount & RWSEM_WRITER_MASK)) {
+ /*
+ * Initiate handoff to reader, if applicable.
+ */
+ if (!(oldcount & RWSEM_FLAG_HANDOFF) &&
+ time_after(jiffies, waiter->timeout)) {
+ adjustment -= RWSEM_FLAG_HANDOFF;
+ lockevent_inc(rwsem_rlock_handoff);
+ }
+
atomic_long_sub(adjustment, &sem->count);
return;
}
@@ -180,6 +196,12 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
adjustment -= RWSEM_FLAG_WAITERS;
}
+ /*
+ * Clear the handoff flag
+ */
+ if (woken && RWSEM_COUNT_HANDOFF(atomic_long_read(&sem->count)))
+ adjustment -= RWSEM_FLAG_HANDOFF;
+
if (adjustment)
atomic_long_add(adjustment, &sem->count);
}
@@ -189,15 +211,19 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
* race conditions between checking the rwsem wait list and setting the
* sem->count accordingly.
*/
-static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem)
+static inline bool
+rwsem_try_write_lock(long count, struct rw_semaphore *sem, bool first)
{
long new;
if (RWSEM_COUNT_LOCKED(count))
return false;
- new = count + RWSEM_WRITER_LOCKED -
- (list_is_singular(&sem->wait_list) ? RWSEM_FLAG_WAITERS : 0);
+ if (!first && RWSEM_COUNT_HANDOFF(count))
+ return false;
+
+ new = (count & ~RWSEM_FLAG_HANDOFF) + RWSEM_WRITER_LOCKED -
+ (list_is_singular(&sem->wait_list) ? RWSEM_FLAG_WAITERS : 0);
if (atomic_long_cmpxchg_acquire(&sem->count, count, new) == count) {
rwsem_set_owner(sem);
@@ -216,7 +242,7 @@ static inline bool rwsem_try_write_lock_unqueued(struct
rw_semaphore *sem)
long old, count = atomic_long_read(&sem->count);
while (true) {
- if (RWSEM_COUNT_LOCKED(count))
+ if (RWSEM_COUNT_LOCKED_OR_HANDOFF(count))
return false;
old = atomic_long_cmpxchg_acquire(&sem->count, count,
@@ -374,6 +400,16 @@ static inline bool rwsem_has_spinner(struct rw_semaphore
*sem)
#endif
/*
+ * This is safe to be called without holding the wait_lock.
+ */
+static inline bool
+rwsem_waiter_is_first(struct rw_semaphore *sem, struct rwsem_waiter *waiter)
+{
+ return list_first_entry(&sem->wait_list, struct rwsem_waiter, list)
+ == waiter;
+}
+
+/*
* Wait for the read lock to be granted
*/
static inline struct rw_semaphore __sched *
@@ -385,6 +421,7 @@ static inline bool rwsem_has_spinner(struct rw_semaphore
*sem)
waiter.task = current;
waiter.type = RWSEM_WAITING_FOR_READ;
+ waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
raw_spin_lock_irq(&sem->wait_lock);
if (list_empty(&sem->wait_list)) {
@@ -441,8 +478,12 @@ static inline bool rwsem_has_spinner(struct rw_semaphore
*sem)
return sem;
out_nolock:
list_del(&waiter.list);
- if (list_empty(&sem->wait_list))
- atomic_long_add(-RWSEM_FLAG_WAITERS, &sem->count);
+ if (list_empty(&sem->wait_list)) {
+ int adjustment = -RWSEM_FLAG_WAITERS -
+ (atomic_long_read(&sem->count) & RWSEM_FLAG_HANDOFF);
+
+ atomic_long_add(adjustment, &sem->count);
+ }
raw_spin_unlock_irq(&sem->wait_lock);
__set_current_state(TASK_RUNNING);
lockevent_inc(rwsem_rlock_fail);
@@ -469,8 +510,8 @@ static inline bool rwsem_has_spinner(struct rw_semaphore
*sem)
static inline struct rw_semaphore *
__rwsem_down_write_failed_common(struct rw_semaphore *sem, int state)
{
- long count;
- bool waiting = true; /* any queued threads before us */
+ long count, adjustment;
+ bool first; /* First one in queue */
struct rwsem_waiter waiter;
struct rw_semaphore *ret = sem;
DEFINE_WAKE_Q(wake_q);
@@ -485,17 +526,17 @@ static inline bool rwsem_has_spinner(struct rw_semaphore
*sem)
*/
waiter.task = current;
waiter.type = RWSEM_WAITING_FOR_WRITE;
+ waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
raw_spin_lock_irq(&sem->wait_lock);
/* account for this before adding a new element to the list */
- if (list_empty(&sem->wait_list))
- waiting = false;
+ first = list_empty(&sem->wait_list);
list_add_tail(&waiter.list, &sem->wait_list);
/* we're now waiting on the lock, but no longer actively locking */
- if (waiting) {
+ if (!first) {
count = atomic_long_read(&sem->count);
/*
@@ -529,12 +570,13 @@ static inline bool rwsem_has_spinner(struct rw_semaphore
*sem)
/* wait until we successfully acquire the lock */
set_current_state(state);
while (true) {
- if (rwsem_try_write_lock(count, sem))
+ if (rwsem_try_write_lock(count, sem, first))
break;
+
raw_spin_unlock_irq(&sem->wait_lock);
/* Block until there are no active lockers. */
- do {
+ for (;;) {
if (signal_pending_state(state, current))
goto out_nolock;
@@ -542,7 +584,29 @@ static inline bool rwsem_has_spinner(struct rw_semaphore
*sem)
lockevent_inc(rwsem_sleep_writer);
set_current_state(state);
count = atomic_long_read(&sem->count);
- } while (RWSEM_COUNT_LOCKED(count));
+
+ if (!first)
+ first = rwsem_waiter_is_first(sem, &waiter);
+
+ if (!RWSEM_COUNT_LOCKED(count))
+ break;
+
+ if (first && !RWSEM_COUNT_HANDOFF(count) &&
+ time_after(jiffies, waiter.timeout)) {
+ atomic_long_or(RWSEM_FLAG_HANDOFF, &sem->count);
+ /*
+ * Make sure the handoff bit is seen by
+ * others before proceeding.
+ */
+ smp_mb__after_atomic();
+ lockevent_inc(rwsem_wlock_handoff);
+ /*
+ * Do a trylock after setting the handoff
+ * flag to avoid missed wakeup.
+ */
+ break;
+ }
+ }
raw_spin_lock_irq(&sem->wait_lock);
}
@@ -557,9 +621,15 @@ static inline bool rwsem_has_spinner(struct rw_semaphore
*sem)
__set_current_state(TASK_RUNNING);
raw_spin_lock_irq(&sem->wait_lock);
list_del(&waiter.list);
+ adjustment = 0;
if (list_empty(&sem->wait_list))
- atomic_long_add(-RWSEM_FLAG_WAITERS, &sem->count);
- else
+ adjustment -= RWSEM_FLAG_WAITERS;
+ if (first)
+ adjustment -= (atomic_long_read(&sem->count) &
+ RWSEM_FLAG_HANDOFF);
+ if (adjustment)
+ atomic_long_add(adjustment, &sem->count);
+ if (!list_empty(&sem->wait_list))
__rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
raw_spin_unlock_irq(&sem->wait_lock);
wake_up_q(&wake_q);
@@ -587,7 +657,7 @@ static inline bool rwsem_has_spinner(struct rw_semaphore
*sem)
* - up_read/up_write has decremented the active part of count if we come here
*/
__visible
-struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
+struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem, long count)
{
unsigned long flags;
DEFINE_WAKE_Q(wake_q);
@@ -620,7 +690,9 @@ struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
smp_rmb();
/*
- * If a spinner is present, it is not necessary to do the wakeup.
+ * If a spinner is present and the handoff flag isn't set, it is
+ * not necessary to do the wakeup.
+ *
* Try to do wakeup only if the trylock succeeds to minimize
* spinlock contention which may introduce too much delay in the
* unlock operation.
@@ -639,7 +711,7 @@ struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
* rwsem_has_spinner() is true, it will guarantee at least one
* trylock attempt on the rwsem later on.
*/
- if (rwsem_has_spinner(sem)) {
+ if (rwsem_has_spinner(sem) && !RWSEM_COUNT_HANDOFF(count)) {
/*
* The smp_rmb() here is to make sure that the spinner
* state is consulted before reading the wait_lock.
diff --git a/kernel/locking/rwsem-xadd.h b/kernel/locking/rwsem-xadd.h
index 1febd17..6d4890d 100644
--- a/kernel/locking/rwsem-xadd.h
+++ b/kernel/locking/rwsem-xadd.h
@@ -41,7 +41,8 @@
*
* Bit 0 - writer locked bit
* Bit 1 - waiters present bit
- * Bits 2-7 - reserved
+ * Bit 2 - lock handoff bit
+ * Bits 3-7 - reserved
* Bits 8-X - 24-bit (32-bit) or 56-bit reader count
*
* atomic_long_fetch_add() is used to obtain reader lock, whereas
@@ -49,14 +50,20 @@
*/
#define RWSEM_WRITER_LOCKED (1UL << 0)
#define RWSEM_FLAG_WAITERS (1UL << 1)
+#define RWSEM_FLAG_HANDOFF (1UL << 2)
+
#define RWSEM_READER_SHIFT 8
#define RWSEM_READER_BIAS (1UL << RWSEM_READER_SHIFT)
#define RWSEM_READER_MASK (~(RWSEM_READER_BIAS - 1))
#define RWSEM_WRITER_MASK RWSEM_WRITER_LOCKED
#define RWSEM_LOCK_MASK (RWSEM_WRITER_MASK|RWSEM_READER_MASK)
-#define RWSEM_READ_FAILED_MASK (RWSEM_WRITER_MASK|RWSEM_FLAG_WAITERS)
+#define RWSEM_READ_FAILED_MASK (RWSEM_WRITER_MASK|RWSEM_FLAG_WAITERS|\
+ RWSEM_FLAG_HANDOFF)
#define RWSEM_COUNT_LOCKED(c) ((c) & RWSEM_LOCK_MASK)
+#define RWSEM_COUNT_HANDOFF(c) ((c) & RWSEM_FLAG_HANDOFF)
+#define RWSEM_COUNT_LOCKED_OR_HANDOFF(c) \
+ ((c) & (RWSEM_LOCK_MASK|RWSEM_FLAG_HANDOFF))
#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
/*
@@ -163,7 +170,7 @@ static inline void rwsem_clear_reader_owned(struct
rw_semaphore *sem)
extern struct rw_semaphore *rwsem_down_read_failed_killable(struct
rw_semaphore *sem);
extern struct rw_semaphore *rwsem_down_write_failed(struct rw_semaphore *sem);
extern struct rw_semaphore *rwsem_down_write_failed_killable(struct
rw_semaphore *sem);
-extern struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem);
+extern struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem, long count);
extern struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem);
/*
@@ -253,7 +260,7 @@ static inline void __up_read(struct rw_semaphore *sem)
tmp = atomic_long_add_return_release(-RWSEM_READER_BIAS, &sem->count);
if (unlikely((tmp & (RWSEM_LOCK_MASK|RWSEM_FLAG_WAITERS))
== RWSEM_FLAG_WAITERS))
- rwsem_wake(sem);
+ rwsem_wake(sem, tmp);
}
/*
@@ -261,11 +268,13 @@ static inline void __up_read(struct rw_semaphore *sem)
*/
static inline void __up_write(struct rw_semaphore *sem)
{
+ long tmp;
+
DEBUG_RWSEMS_WARN_ON(sem->owner != current, sem);
rwsem_clear_owner(sem);
- if (unlikely(atomic_long_fetch_add_release(-RWSEM_WRITER_LOCKED,
- &sem->count) & RWSEM_FLAG_WAITERS))
- rwsem_wake(sem);
+ tmp = atomic_long_fetch_add_release(-RWSEM_WRITER_LOCKED, &sem->count);
+ if (unlikely(tmp & RWSEM_FLAG_WAITERS))
+ rwsem_wake(sem, tmp);
}
/*
--
1.8.3.1
