[lttng-dev] [PATCH 8/8] urcu-qsbr: batch concurrent synchronize_rcu()

[Mathieu Desnoyers]

CC: Paul E. McKenney <paulmck at linux.vnet.ibm.com>
CC: Lai Jiangshan <laijs at cn.fujitsu.com>
CC: Alan Stern <stern at rowland.harvard.edu>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
---
 urcu-qsbr.c |  151 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 151 insertions(+)

diff --git a/urcu-qsbr.c b/urcu-qsbr.c
index 5b341b5..7f747ed 100644
--- a/urcu-qsbr.c
+++ b/urcu-qsbr.c
@@ -36,6 +36,7 @@
 #include <poll.h>
 
 #include "urcu/wfcqueue.h"
+#include "urcu/wfstack.h"
 #include "urcu/map/urcu-qsbr.h"
 #define BUILD_QSBR_LIB
 #include "urcu/static/urcu-qsbr.h"
@@ -78,6 +79,35 @@ DEFINE_URCU_TLS(unsigned int, rcu_rand_yield);
 
 static CDS_LIST_HEAD(registry);
 
+/*
+ * Number of busy-loop attempts before waiting on futex for grace period
+ * batching.
+ */
+#define RCU_AWAKE_ATTEMPTS 1000
+
+enum adapt_wakeup_state {
+	/* AWAKE_WAITING is compared directly (futex compares it). */
+	AWAKE_WAITING =		0,
+	/* non-zero are used as masks. */
+	AWAKE_WAKEUP =		(1 << 0),
+	AWAKE_AWAKENED =	(1 << 1),
+	AWAKE_TEARDOWN =	(1 << 2),
+};
+
+struct gp_waiters_thread {
+	struct cds_wfs_node node;
+	int32_t wait_futex;
+};
+
+/*
+ * Stack keeping threads awaiting to wait for a grace period. Contains
+ * struct gp_waiters_thread objects.
+ */
+static struct cds_wfs_stack gp_waiters = {
+	.head = CDS_WFS_END,
+	.lock = PTHREAD_MUTEX_INITIALIZER,
+};
+
 static void mutex_lock(pthread_mutex_t *mutex)
 {
 	int ret;
@@ -116,6 +146,58 @@ static void wait_gp(void)
 		      NULL, NULL, 0);
 }
 
+/*
+ * Note: urcu_adaptative_wake_up needs "value" to stay allocated
+ * throughout its execution. In this scheme, the waiter owns the futex
+ * memory, and we only allow it to free this memory when it receives the
+ * AWAKE_TEARDOWN flag.
+ */
+static void urcu_adaptative_wake_up(int32_t *value)
+{
+	cmm_smp_mb();
+	assert(uatomic_read(value) == AWAKE_WAITING);
+	uatomic_set(value, AWAKE_WAKEUP);
+	if (!(uatomic_read(value) & AWAKE_AWAKENED))
+		futex_noasync(value, FUTEX_WAKE, 1, NULL, NULL, 0);
+	/* Allow teardown of "value" memory. */
+	uatomic_or(value, AWAKE_TEARDOWN);
+}
+
+/*
+ * Caller must initialize "value" to AWAKE_WAITING before passing its
+ * memory to waker thread.
+ */
+static void urcu_adaptative_busy_wait(int32_t *value)
+{
+	unsigned int i;
+
+	/* Load and test condition before read futex */
+	cmm_smp_rmb();
+	for (i = 0; i < RCU_AWAKE_ATTEMPTS; i++) {
+		if (uatomic_read(value) != AWAKE_WAITING)
+			goto skip_futex_wait;
+		caa_cpu_relax();
+	}
+	futex_noasync(value, FUTEX_WAIT, AWAKE_WAITING, NULL, NULL, 0);
+skip_futex_wait:
+
+	/* Tell waker thread than we are awakened. */
+	uatomic_or(value, AWAKE_AWAKENED);
+
+	/*
+	 * Wait until waker thread lets us know it's ok to tear down
+	 * memory allocated for value.
+	 */
+	for (i = 0; i < RCU_AWAKE_ATTEMPTS; i++) {
+		if (uatomic_read(value) & AWAKE_TEARDOWN)
+			break;
+		caa_cpu_relax();
+	}
+	while (!(uatomic_read(value) & AWAKE_TEARDOWN))
+		poll(NULL, 0, 10);
+	assert(uatomic_read(value) & AWAKE_TEARDOWN);
+}
+
 static void wait_for_readers(struct cds_list_head *input_readers,
 			struct cds_list_head *cur_snap_readers,
 			struct cds_list_head *qsreaders)
@@ -198,6 +280,9 @@ void synchronize_rcu(void)
 	CDS_LIST_HEAD(cur_snap_readers);
 	CDS_LIST_HEAD(qsreaders);
 	unsigned long was_online;
+	struct gp_waiters_thread gp_waiters_thread;
+	struct cds_wfs_head *gp_waiters_head;
+	struct cds_wfs_node *waiters_iter, *waiters_iter_n;
 
 	was_online = URCU_TLS(rcu_reader).ctr;
 
@@ -214,8 +299,26 @@ void synchronize_rcu(void)
 	else
 		cmm_smp_mb();
 
+	/*
+	 * Add ourself to gp_waiters stack of threads awaiting to wait
+	 * for a grace period. Proceed to perform the grace period only
+	 * if we are the first thread added into the stack.
+	 */
+	cds_wfs_node_init(&gp_waiters_thread.node);
+	gp_waiters_thread.wait_futex = AWAKE_WAITING;
+	if (cds_wfs_push(&gp_waiters, &gp_waiters_node) != 0) {
+		/* Not first in stack: will be awakened by another thread. */
+		urcu_adaptative_busy_wait(&gp_waiters_thread.wait_futex);
+		goto gp_end;
+	}
+
 	mutex_lock(&rcu_gp_lock);
 
+	/*
+	 * Pop all waiters into our local stack head.
+	 */
+	gp_waiters_head = __cds_wfs_pop_all(&gp_waiters);
+
 	if (cds_list_empty(&registry))
 		goto out;
 
@@ -272,6 +375,19 @@ void synchronize_rcu(void)
 out:
 	mutex_unlock(&rcu_gp_lock);
 
+	/* Wake all waiters in our stack head, excluding ourself. */
+	cds_wfs_for_each_blocking_safe(gp_waiters_head, waiters_iter,
+				waiters_iter_n) {
+		struct gp_waiters_thread *wt;
+
+		wt = caa_container_of(waiters_iter,
+				struct gp_waiters_thread, node);
+		if (wt == &gp_waiters_thread)
+			continue;
+		urcu_adaptative_wake_up(&wt->wait_futex);
+	}
+
+gp_end:
 	/*
 	 * Finish waiting for reader threads before letting the old ptr being
 	 * freed.
@@ -286,6 +402,9 @@ void synchronize_rcu(void)
 {
 	CDS_LIST_HEAD(qsreaders);
 	unsigned long was_online;
+	struct gp_waiters_thread gp_waiters_thread;
+	struct cds_wfs_head *gp_waiters_head;
+	struct cds_wfs_node *waiters_iter, *waiters_iter_n;
 
 	was_online = URCU_TLS(rcu_reader).ctr;
 
@@ -299,7 +418,26 @@ void synchronize_rcu(void)
 	else
 		cmm_smp_mb();
 
+	/*
+	 * Add ourself to gp_waiters stack of threads awaiting to wait
+	 * for a grace period. Proceed to perform the grace period only
+	 * if we are the first thread added into the stack.
+	 */
+	cds_wfs_node_init(&gp_waiters_thread.node);
+	gp_waiters_thread.wait_futex = AWAKE_WAITING;
+	if (cds_wfs_push(&gp_waiters, &gp_waiters_thread.node) != 0) {
+		/* Not first in stack: will be awakened by another thread. */
+		urcu_adaptative_busy_wait(&gp_waiters_thread.wait_futex);
+		goto gp_end;
+	}
+
 	mutex_lock(&rcu_gp_lock);
+
+	/*
+	 * Pop all waiters into our local stack head.
+	 */
+	gp_waiters_head = __cds_wfs_pop_all(&gp_waiters);
+
 	if (cds_list_empty(&registry))
 		goto out;
 
@@ -334,6 +472,19 @@ void synchronize_rcu(void)
 out:
 	mutex_unlock(&rcu_gp_lock);
 
+	/* Wake all waiters in our stack head, excluding ourself. */
+	cds_wfs_for_each_blocking_safe(gp_waiters_head, waiters_iter,
+				waiters_iter_n) {
+		struct gp_waiters_thread *wt;
+
+		wt = caa_container_of(waiters_iter,
+				struct gp_waiters_thread, node);
+		if (wt == &gp_waiters_thread)
+			continue;
+		urcu_adaptative_wake_up(&wt->wait_futex);
+	}
+
+gp_end:
 	if (was_online)
 		rcu_thread_online();
 	else
-- 
1.7.10.4