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 10ms waiting period. The waiting period is used to
avoid discouraging lock stealing too much to affect performance.
A rwsem microbenchmark was run for 5 seconds on a 2-socket 40-core
80-thread x86-64 system with a 4.14-rc2 based kernel and 60 write_lock
threads with 1us sleep critical section.
For the unpatched kernel, the locking rate was 15,519 kop/s. However
there were 28 threads with only 1 locking operation done (practically
starved). The thread with the highest locking rate had done more than
646k of them.
For the patched kernel, the locking rate dropped to 12,590 kop/s. The
number of locking operations done per thread had a range of 14,450 -
22,648. The rwsem became much more fair with the tradeoff of lower
overall throughput.
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
kernel/locking/rwsem-xadd.c | 98 +++++++++++++++++++++++++++++++++++++--------
kernel/locking/rwsem-xadd.h | 22 ++++++----
2 files changed, 96 insertions(+), 24 deletions(-)
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index e3ab430..bca412f 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -70,6 +70,7 @@ struct rwsem_waiter {
struct list_head list;
struct task_struct *task;
enum rwsem_waiter_type type;
+ unsigned long timeout;
};
enum rwsem_wake_type {
@@ -79,6 +80,16 @@ enum rwsem_wake_type {
};
/*
+ * The minimum waiting time (10ms) in the wait queue before initiating the
+ * handoff protocol.
+ *
+ * The queue head waiter that is aborted (killed) will pass the handoff
+ * bit, if set, to the next waiter, but the bit has to be cleared when
+ * the wait queue becomes empty.
+ */
+#define RWSEM_WAIT_TIMEOUT ((HZ - 1)/100 + 1)
+
+/*
* handle the lock release when processes blocked on it that can now run
* - if we come here from up_xxxx(), then:
* - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
@@ -128,6 +139,13 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
adjustment = RWSEM_READER_BIAS;
oldcount = atomic_fetch_add(adjustment, &sem->count);
if (unlikely(oldcount & RWSEM_WRITER_LOCKED)) {
+ /*
+ * Initiate handoff to reader, if applicable.
+ */
+ if (!(oldcount & RWSEM_FLAG_HANDOFF) &&
+ time_after(jiffies, waiter->timeout))
+ adjustment -= RWSEM_FLAG_HANDOFF;
+
atomic_sub(adjustment, &sem->count);
return;
}
@@ -170,6 +188,12 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
adjustment -= RWSEM_FLAG_WAITERS;
}
+ /*
+ * Clear the handoff flag
+ */
+ if (woken && RWSEM_COUNT_IS_HANDOFF(atomic_read(&sem->count)))
+ adjustment -= RWSEM_FLAG_HANDOFF;
+
if (adjustment)
atomic_add(adjustment, &sem->count);
}
@@ -179,15 +203,20 @@ 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(int count, struct rw_semaphore *sem)
+static inline bool
+rwsem_try_write_lock(int count, struct rw_semaphore *sem, bool first)
{
int new;
if (RWSEM_COUNT_IS_LOCKED(count))
return false;
+ if (!first && RWSEM_COUNT_IS_HANDOFF(count))
+ return false;
+
new = count + RWSEM_WRITER_LOCKED -
- (list_is_singular(&sem->wait_list) ? RWSEM_FLAG_WAITERS : 0);
+ (list_is_singular(&sem->wait_list) ? RWSEM_FLAG_WAITERS : 0) -
+ (count & RWSEM_FLAG_HANDOFF);
if (atomic_cmpxchg_acquire(&sem->count, count, new) == count) {
rwsem_set_owner(sem);
@@ -206,7 +235,7 @@ static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
int old, count = atomic_read(&sem->count);
while (true) {
- if (RWSEM_COUNT_IS_LOCKED(count))
+ if (RWSEM_COUNT_IS_LOCKED_OR_HANDOFF(count))
return false;
old = atomic_cmpxchg_acquire(&sem->count, count,
@@ -362,6 +391,16 @@ 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 *
@@ -373,6 +412,7 @@ 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))
@@ -413,8 +453,13 @@ 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_add(-RWSEM_FLAG_WAITERS, &sem->count);
+ if (list_empty(&sem->wait_list)) {
+ int adjustment = -RWSEM_FLAG_WAITERS;
+
+ if (RWSEM_COUNT_IS_HANDOFF(atomic_read(&sem->count)))
+ adjustment -= RWSEM_FLAG_HANDOFF;
+ atomic_add(adjustment, &sem->count);
+ }
raw_spin_unlock_irq(&sem->wait_lock);
__set_current_state(TASK_RUNNING);
return ERR_PTR(-EINTR);
@@ -441,7 +486,7 @@ static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
__rwsem_down_write_failed_common(struct rw_semaphore *sem, int state)
{
int count;
- bool waiting = true; /* any queued threads before us */
+ bool first = false; /* First one in queue */
struct rwsem_waiter waiter;
struct rw_semaphore *ret = sem;
DEFINE_WAKE_Q(wake_q);
@@ -456,17 +501,18 @@ 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 = true;
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_read(&sem->count);
/*
@@ -498,19 +544,30 @@ 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;
schedule();
set_current_state(state);
count = atomic_read(&sem->count);
- } while (RWSEM_COUNT_IS_LOCKED(count));
+
+ if (!first)
+ first = rwsem_waiter_is_first(sem, &waiter);
+
+ if (!RWSEM_COUNT_IS_LOCKED(count))
+ break;
+
+ if (first && !RWSEM_COUNT_IS_HANDOFF(count) &&
+ time_after(jiffies, waiter.timeout))
+ atomic_or(RWSEM_FLAG_HANDOFF, &sem->count);
+ }
raw_spin_lock_irq(&sem->wait_lock);
}
@@ -524,10 +581,15 @@ 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);
- if (list_empty(&sem->wait_list))
- atomic_add(-RWSEM_FLAG_WAITERS, &sem->count);
- else
+ if (list_empty(&sem->wait_list)) {
+ int adjustment = -RWSEM_FLAG_WAITERS;
+
+ if (RWSEM_COUNT_IS_HANDOFF(atomic_read(&sem->count)))
+ adjustment -= RWSEM_FLAG_HANDOFF;
+ atomic_add(adjustment, &sem->count);
+ } else {
__rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
+ }
raw_spin_unlock_irq(&sem->wait_lock);
wake_up_q(&wake_q);
@@ -553,7 +615,7 @@ 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, int count)
{
unsigned long flags;
DEFINE_WAKE_Q(wake_q);
@@ -586,7 +648,9 @@ 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.
@@ -605,7 +669,7 @@ 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) && !(count & RWSEM_FLAG_HANDOFF)) {
/*
* 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 9b30f0c..8cb12ed 100644
--- a/kernel/locking/rwsem-xadd.h
+++ b/kernel/locking/rwsem-xadd.h
@@ -74,7 +74,8 @@ static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
*
* Bit 0 - writer locked bit
* Bit 1 - waiters present bit
- * Bits 2-7 - reserved
+ * Bit 2 - lock handoff bit
+ * Bits 3-7 - reserved
* Bits 8-31 - 24-bit reader count
*
* atomic_fetch_add() is used to obtain reader lock, whereas atomic_cmpxchg()
@@ -82,19 +83,24 @@ static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
*/
#define RWSEM_WRITER_LOCKED 0X00000001
#define RWSEM_FLAG_WAITERS 0X00000002
+#define RWSEM_FLAG_HANDOFF 0X00000004
#define RWSEM_READER_BIAS 0x00000100
#define RWSEM_READER_SHIFT 8
#define RWSEM_READER_MASK (~((1U << RWSEM_READER_SHIFT) - 1))
#define RWSEM_LOCK_MASK (RWSEM_WRITER_LOCKED|RWSEM_READER_MASK)
-#define RWSEM_READ_FAILED_MASK (RWSEM_WRITER_LOCKED|RWSEM_FLAG_WAITERS)
+#define RWSEM_READ_FAILED_MASK (RWSEM_WRITER_LOCKED|RWSEM_FLAG_WAITERS|\
+ RWSEM_FLAG_HANDOFF)
#define RWSEM_COUNT_IS_LOCKED(c) ((c) & RWSEM_LOCK_MASK)
+#define RWSEM_COUNT_IS_HANDOFF(c) ((c) & RWSEM_FLAG_HANDOFF)
+#define RWSEM_COUNT_IS_LOCKED_OR_HANDOFF(c) \
+ ((c) & (RWSEM_LOCK_MASK|RWSEM_FLAG_HANDOFF))
extern struct rw_semaphore *rwsem_down_read_failed(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 *);
+extern struct rw_semaphore *rwsem_wake(struct rw_semaphore *, int count);
extern struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem);
/*
@@ -165,7 +171,7 @@ static inline void __up_read(struct rw_semaphore *sem)
tmp = atomic_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);
}
/*
@@ -173,10 +179,12 @@ static inline void __up_read(struct rw_semaphore *sem)
*/
static inline void __up_write(struct rw_semaphore *sem)
{
+ int tmp;
+
rwsem_clear_owner(sem);
- if (unlikely(atomic_fetch_add_release(-RWSEM_WRITER_LOCKED,
- &sem->count) & RWSEM_FLAG_WAITERS))
- rwsem_wake(sem);
+ tmp = atomic_fetch_add_release(-RWSEM_WRITER_LOCKED, &sem->count);
+ if (unlikely(tmp & RWSEM_FLAG_WAITERS))
+ rwsem_wake(sem, tmp);
}
/*
--
1.8.3.1
--
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