[dpdk-dev] [PATCH v1] ticketlock: ticket based to improve fairness

[Gavin Hu <gavin.hu@arm.com>]

From: Joyce Kong <joyce.kong@arm.com>

The spinlock implementation is unfair, some threads may take locks
aggressively while leaving the other threads starving for long time. As
shown in the following test, within same period of time, there are
threads taking locks much more times than the others.

The ticketlock gives each waiting thread a ticket and they can take the
lock one by one, first come, first serviced, this avoids starvation for
too long time and is more predictable.

*** spinlock_autotest without this patch ***
Core [0] count = 89
Core [1] count = 84
Core [2] count = 94
...
Core [208] count = 171
Core [209] count = 152
Core [210] count = 161
Core [211] count = 187

*** spinlock_autotest with this patch ***
Core [0] count = 534
Core [1] count = 533
Core [2] count = 542
...
Core [208] count = 554
Core [209] count = 556
Core [210] count = 555
Core [211] count = 551

Signed-off-by: Joyce Kong <joyce.kong@arm.com>
Signed-off-by: Gavin Hu <gavin.hu@arm.com>
---
 .../common/include/generic/rte_ticketlock.h        | 203 +++++++++++++++++++++
 lib/librte_eal/rte_eal_version.map                 |   8 +
 2 files changed, 211 insertions(+)
 create mode 100644 lib/librte_eal/common/include/generic/rte_ticketlock.h

diff --git a/lib/librte_eal/common/include/generic/rte_ticketlock.h b/lib/librte_eal/common/include/generic/rte_ticketlock.h
new file mode 100644
index 000000000..9d044bb31
--- /dev/null
+++ b/lib/librte_eal/common/include/generic/rte_ticketlock.h
@@ -0,0 +1,203 @@
+/* SPDX-License-Identifier: BSD-3-Clause
+ * Copyright(c) 2018-2019 Arm Corporation
+ */
+
+#ifndef _RTE_TICKETLOCK_H_
+#define _RTE_TICKETLOCK_H_
+
+/**
+ * @file
+ *
+ * RTE ticketlocks
+ *
+ * This file defines an API for read-write locks, which are implemented
+ * in an architecture-specific way. This kind of lock simply waits in
+ * a loop repeatedly checking until the lock becomes available.
+ *
+ * All locks must be initialised before use, and only initialised once.
+ *
+ */
+
+#include <rte_lcore.h>
+#include <rte_common.h>
+#include <rte_pause.h>
+
+/**
+ * The rte_ticketlock_t type.
+ */
+typedef struct {
+	uint16_t current;
+	uint16_t next;
+} rte_ticketlock_t;
+
+/**
+ * A static ticketlock initializer.
+ */
+#define RTE_TICKETLOCK_INITIALIZER { 0 }
+
+/**
+ * Initialize the ticketlock to an unlocked state.
+ *
+ * @param tl
+ *   A pointer to the ticketlock.
+ */
+static inline __rte_experimental void
+rte_ticketlock_init(rte_ticketlock_t *tl)
+{
+	__atomic_store_n(&tl->current, 0, __ATOMIC_RELAXED);
+	__atomic_store_n(&tl->next, 0, __ATOMIC_RELAXED);
+}
+
+/**
+ * Take the ticketlock.
+ *
+ * @param tl
+ *   A pointer to the ticketlock.
+ */
+static inline __rte_experimental void
+rte_ticketlock_lock(rte_ticketlock_t *tl);
+
+static inline __rte_experimental void
+rte_ticketlock_lock(rte_ticketlock_t *tl)
+{
+	uint16_t me = __atomic_fetch_add(&tl->next, 1, __ATOMIC_RELAXED);
+	while (__atomic_load_n(&tl->current, __ATOMIC_ACQUIRE) != me)
+		rte_pause();
+}
+
+/**
+ * Release the ticketlock.
+ *
+ * @param tl
+ *   A pointer to the ticketlock.
+ */
+static inline __rte_experimental void
+rte_ticketlock_unlock(rte_ticketlock_t *tl);
+
+static inline __rte_experimental void
+rte_ticketlock_unlock(rte_ticketlock_t *tl)
+{
+	uint16_t i = __atomic_load_n(&tl->current, __ATOMIC_RELAXED);
+	i++;
+	__atomic_store_n(&tl->current, i, __ATOMIC_RELEASE);
+}
+
+/**
+ * Try to take the lock.
+ *
+ * @param tl
+ *   A pointer to the ticketlock.
+ * @return
+ *   1 if the lock is successfully taken; 0 otherwise.
+ */
+static inline __rte_experimental int
+rte_ticketlock_trylock(rte_ticketlock_t *tl)
+{
+	uint16_t me = __atomic_fetch_add(&tl->next, 1, __ATOMIC_RELAXED);
+	while (__atomic_load_n(&tl->current, __ATOMIC_RELAXED) != me) {
+		__atomic_sub_fetch(&tl->next, 1, __ATOMIC_RELAXED);
+		return 0;
+	}
+
+	return 1;
+}
+
+/**
+ * Test if the lock is taken.
+ *
+ * @param tl
+ *   A pointer to the ticketlock.
+ * @return
+ *   1 if the lock icurrently taken; 0 otherwise.
+ */
+static inline __rte_experimental int
+rte_ticketlock_is_locked(rte_ticketlock_t *tl)
+{
+	return (__atomic_load_n(&tl->current, __ATOMIC_RELAXED) !=
+			__atomic_load_n(&tl->next, __ATOMIC_RELAXED));
+}
+
+/**
+ * The rte_ticketlock_recursive_t type.
+ */
+typedef struct {
+	rte_ticketlock_t tl; /**< the actual ticketlock */
+	volatile int user; /**< core id using lock, -1 for unused */
+	volatile int count; /**< count of time this lock has been called */
+} rte_ticketlock_recursive_t;
+
+/**
+ * A static recursive ticketlock initializer.
+ */
+#define RTE_TICKETLOCK_RECURSIVE_INITIALIZER {RTE_TICKETLOCK_INITIALIZER, -1, 0}
+
+/**
+ * Initialize the recursive ticketlock to an unlocked state.
+ *
+ * @param tlr
+ *   A pointer to the recursive ticketlock.
+ */
+static inline __rte_experimental void rte_ticketlock_recursive_init(
+					rte_ticketlock_recursive_t *tlr)
+{
+	rte_ticketlock_init(&tlr->tl);
+	tlr->user = -1;
+	tlr->count = 0;
+}
+
+/**
+ * Take the recursive ticketlock.
+ *
+ * @param tlr
+ *   A pointer to the recursive ticketlock.
+ */
+static inline __rte_experimental void rte_ticketlock_recursive_lock(
+					rte_ticketlock_recursive_t *tlr)
+{
+	int id = rte_gettid();
+
+	if (tlr->user != id) {
+		rte_ticketlock_lock(&tlr->tl);
+		tlr->user = id;
+	}
+	tlr->count++;
+}
+/**
+ * Release the recursive ticketlock.
+ *
+ * @param tlr
+ *   A pointer to the recursive ticketlock.
+ */
+static inline __rte_experimental void rte_ticketlock_recursive_unlock(
+					rte_ticketlock_recursive_t *tlr)
+{
+	if (--(tlr->count) == 0) {
+		tlr->user = -1;
+		rte_ticketlock_unlock(&tlr->tl);
+	}
+
+}
+
+/**
+ * Try to take the recursive lock.
+ *
+ * @param tlr
+ *   A pointer to the recursive ticketlock.
+ * @return
+ *   1 if the lock is successfully taken; 0 otherwise.
+ */
+static inline __rte_experimental int rte_ticketlock_recursive_trylock(
+					rte_ticketlock_recursive_t *tlr)
+{
+	int id = rte_gettid();
+
+	if (tlr->user != id) {
+		if (rte_ticketlock_trylock(&tlr->tl) == 0)
+			return 0;
+		tlr->user = id;
+	}
+	tlr->count++;
+	return 1;
+}
+
+#endif /* _RTE_TICKETLOCK_H_ */
diff --git a/lib/librte_eal/rte_eal_version.map b/lib/librte_eal/rte_eal_version.map
index eb5f7b9cb..f1841effa 100644
--- a/lib/librte_eal/rte_eal_version.map
+++ b/lib/librte_eal/rte_eal_version.map
@@ -364,4 +364,12 @@ EXPERIMENTAL {
 	rte_service_may_be_active;
 	rte_socket_count;
 	rte_socket_id_by_idx;
+	rte_ticketlock_lock;
+	rte_ticketlock_unlock;
+	rte_ticketlock_islocked;
+	rte_ticketlock_trylock;
+	rte_ticketlock_recurrsive_lock;
+	rte_ticketlock_recurrsive_unlock;
+	rte_ticketlock_recurrsive_islocked;
+	rte_ticketlock_recurrsive_trylock;
 };
-- 
2.11.0