call_rcu: use wfcqueue, eliminate false-sharing

Eliminate false-sharing between call_rcu (enqueuer) and worker threads
on the queue head and tail.

Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/tests/Makefile.am b/tests/Makefile.am
index 81718bbaa19f07cff120312f3d614f085d2def49..c92bbe6cb5e0d0f17a6067312e0c5bac029954e1 100644 (file)
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -30,14 +30,14 @@ if COMPAT_FUTEX
 COMPAT+=$(top_srcdir)/compat_futex.c
 endif
 
-URCU=$(top_srcdir)/urcu.c $(top_srcdir)/urcu-pointer.c $(top_srcdir)/wfqueue.c $(COMPAT)
-URCU_QSBR=$(top_srcdir)/urcu-qsbr.c $(top_srcdir)/urcu-pointer.c $(top_srcdir)/wfqueue.c $(COMPAT)
+URCU=$(top_srcdir)/urcu.c $(top_srcdir)/urcu-pointer.c $(top_srcdir)/wfcqueue.c $(COMPAT)
+URCU_QSBR=$(top_srcdir)/urcu-qsbr.c $(top_srcdir)/urcu-pointer.c $(top_srcdir)/wfcqueue.c $(COMPAT)
 # URCU_MB uses urcu.c but -DRCU_MB must be defined
-URCU_MB=$(top_srcdir)/urcu.c $(top_srcdir)/urcu-pointer.c $(top_srcdir)/wfqueue.c $(COMPAT)
+URCU_MB=$(top_srcdir)/urcu.c $(top_srcdir)/urcu-pointer.c $(top_srcdir)/wfcqueue.c $(COMPAT)
 # URCU_SIGNAL uses urcu.c but -DRCU_SIGNAL must be defined
-URCU_SIGNAL=$(top_srcdir)/urcu.c $(top_srcdir)/urcu-pointer.c $(top_srcdir)/wfqueue.c $(COMPAT)
-URCU_BP=$(top_srcdir)/urcu-bp.c $(top_srcdir)/urcu-pointer.c $(top_srcdir)/wfqueue.c $(COMPAT)
-URCU_DEFER=$(top_srcdir)/urcu.c $(top_srcdir)/urcu-pointer.c $(top_srcdir)/wfqueue.c $(COMPAT)
+URCU_SIGNAL=$(top_srcdir)/urcu.c $(top_srcdir)/urcu-pointer.c $(top_srcdir)/wfcqueue.c $(COMPAT)
+URCU_BP=$(top_srcdir)/urcu-bp.c $(top_srcdir)/urcu-pointer.c $(top_srcdir)/wfcqueue.c $(COMPAT)
+URCU_DEFER=$(top_srcdir)/urcu.c $(top_srcdir)/urcu-pointer.c $(top_srcdir)/wfcqueue.c $(COMPAT)
 
 URCU_COMMON_LIB=$(top_builddir)/liburcu-common.la
 URCU_LIB=$(top_builddir)/liburcu.la

diff --git a/urcu-call-rcu-impl.h b/urcu-call-rcu-impl.h
index 13b24ff20b7c71c265c1fc5b0b3d17f1c28ce55c..cf65992f4e57a8bc86187744271c5ee4c58b6d26 100644 (file)
--- a/urcu-call-rcu-impl.h
+++ b/urcu-call-rcu-impl.h
@@ -21,6 +21,7 @@
  */
 
 #define _GNU_SOURCE
+#define _LGPL_SOURCE
 #include <stdio.h>
 #include <pthread.h>
 #include <signal.h>
@@ -35,7 +36,7 @@
 #include <sched.h>
 
 #include "config.h"
-#include "urcu/wfqueue.h"
+#include "urcu/wfcqueue.h"
 #include "urcu-call-rcu.h"
 #include "urcu-pointer.h"
 #include "urcu/list.h"
@@ -46,7 +47,14 @@
 /* Data structure that identifies a call_rcu thread. */
 
 struct call_rcu_data {
-       struct cds_wfq_queue cbs;
+       /*
+        * Align the tail on cache line size to eliminate false-sharing
+        * with head.
+        */
+       struct cds_wfcq_tail __attribute__((aligned(CAA_CACHE_LINE_SIZE))) cbs_tail;
+       /* Alignment on cache line size will add padding here */
+
+       struct cds_wfcq_head cbs_head;
        unsigned long flags;
        int32_t futex;
        unsigned long qlen; /* maintained for debugging. */
@@ -220,10 +228,7 @@ static void call_rcu_wake_up(struct call_rcu_data *crdp)
 static void *call_rcu_thread(void *arg)
 {
        unsigned long cbcount;
-       struct cds_wfq_node *cbs;
-       struct cds_wfq_node **cbs_tail;
-       struct call_rcu_data *crdp = (struct call_rcu_data *)arg;
-       struct rcu_head *rhp;
+       struct call_rcu_data *crdp = (struct call_rcu_data *) arg;
        int rt = !!(uatomic_read(&crdp->flags) & URCU_CALL_RCU_RT);
        int ret;
 
@@ -243,35 +248,33 @@ static void *call_rcu_thread(void *arg)
                cmm_smp_mb();
        }
        for (;;) {
-               if (&crdp->cbs.head != _CMM_LOAD_SHARED(crdp->cbs.tail)) {
-                       while ((cbs = _CMM_LOAD_SHARED(crdp->cbs.head)) == NULL)
-                               poll(NULL, 0, 1);
-                       _CMM_STORE_SHARED(crdp->cbs.head, NULL);
-                       cbs_tail = (struct cds_wfq_node **)
-                               uatomic_xchg(&crdp->cbs.tail, &crdp->cbs.head);
+               struct cds_wfcq_head cbs_tmp_head;
+               struct cds_wfcq_tail cbs_tmp_tail;
+               struct cds_wfcq_node *cbs, *cbs_tmp_n;
+
+               cds_wfcq_init(&cbs_tmp_head, &cbs_tmp_tail);
+               __cds_wfcq_splice_blocking(&cbs_tmp_head, &cbs_tmp_tail,
+                       &crdp->cbs_head, &crdp->cbs_tail);
+               if (!cds_wfcq_empty(&cbs_tmp_head, &cbs_tmp_tail)) {
                        synchronize_rcu();
                        cbcount = 0;
-                       do {
-                               while (cbs->next == NULL &&
-                                      &cbs->next != cbs_tail)
-                                       poll(NULL, 0, 1);
-                               if (cbs == &crdp->cbs.dummy) {
-                                       cbs = cbs->next;
-                                       continue;
-                               }
-                               rhp = (struct rcu_head *)cbs;
-                               cbs = cbs->next;
+                       __cds_wfcq_for_each_blocking_safe(&cbs_tmp_head,
+                                       &cbs_tmp_tail, cbs, cbs_tmp_n) {
+                               struct rcu_head *rhp;
+
+                               rhp = caa_container_of(cbs,
+                                       struct rcu_head, next);
                                rhp->func(rhp);
                                cbcount++;
-                       } while (cbs != NULL);
+                       }
                        uatomic_sub(&crdp->qlen, cbcount);
                }
                if (uatomic_read(&crdp->flags) & URCU_CALL_RCU_STOP)
                        break;
                rcu_thread_offline();
                if (!rt) {
-                       if (&crdp->cbs.head
-                           == _CMM_LOAD_SHARED(crdp->cbs.tail)) {
+                       if (cds_wfcq_empty(&crdp->cbs_head,
+                                       &crdp->cbs_tail)) {
                                call_rcu_wait(crdp);
                                poll(NULL, 0, 10);
                                uatomic_dec(&crdp->futex);
@@ -317,7 +320,7 @@ static void call_rcu_data_init(struct call_rcu_data **crdpp,
        if (crdp == NULL)
                urcu_die(errno);
        memset(crdp, '\0', sizeof(*crdp));
-       cds_wfq_init(&crdp->cbs);
+       cds_wfcq_init(&crdp->cbs_head, &crdp->cbs_tail);
        crdp->qlen = 0;
        crdp->futex = 0;
        crdp->flags = flags;
@@ -590,12 +593,12 @@ void call_rcu(struct rcu_head *head,
 {
        struct call_rcu_data *crdp;
 
-       cds_wfq_node_init(&head->next);
+       cds_wfcq_node_init(&head->next);
        head->func = func;
        /* Holding rcu read-side lock across use of per-cpu crdp */
        rcu_read_lock();
        crdp = get_call_rcu_data();
-       cds_wfq_enqueue(&crdp->cbs, &head->next);
+       cds_wfcq_enqueue(&crdp->cbs_head, &crdp->cbs_tail, &head->next);
        uatomic_inc(&crdp->qlen);
        wake_call_rcu_thread(crdp);
        rcu_read_unlock();
@@ -625,10 +628,6 @@ void call_rcu(struct rcu_head *head,
  */
 void call_rcu_data_free(struct call_rcu_data *crdp)
 {
-       struct cds_wfq_node *cbs;
-       struct cds_wfq_node **cbs_tail;
-       struct cds_wfq_node **cbs_endprev;
-
        if (crdp == NULL || crdp == default_call_rcu_data) {
                return;
        }
@@ -638,17 +637,12 @@ void call_rcu_data_free(struct call_rcu_data *crdp)
                while ((uatomic_read(&crdp->flags) & URCU_CALL_RCU_STOPPED) == 0)
                        poll(NULL, 0, 1);
        }
-       if (&crdp->cbs.head != _CMM_LOAD_SHARED(crdp->cbs.tail)) {
-               while ((cbs = _CMM_LOAD_SHARED(crdp->cbs.head)) == NULL)
-                       poll(NULL, 0, 1);
-               _CMM_STORE_SHARED(crdp->cbs.head, NULL);
-               cbs_tail = (struct cds_wfq_node **)
-                       uatomic_xchg(&crdp->cbs.tail, &crdp->cbs.head);
+       if (!cds_wfcq_empty(&crdp->cbs_head, &crdp->cbs_tail)) {
                /* Create default call rcu data if need be */
                (void) get_default_call_rcu_data();
-               cbs_endprev = (struct cds_wfq_node **)
-                       uatomic_xchg(&default_call_rcu_data, cbs_tail);
-               *cbs_endprev = cbs;
+               __cds_wfcq_splice_blocking(&default_call_rcu_data->cbs_head,
+                       &default_call_rcu_data->cbs_tail,
+                       &crdp->cbs_head, &crdp->cbs_tail);
                uatomic_add(&default_call_rcu_data->qlen,
                            uatomic_read(&crdp->qlen));
                wake_call_rcu_thread(default_call_rcu_data);

diff --git a/urcu-call-rcu.h b/urcu-call-rcu.h
index f7eac8de964d56d717cfc5b48432f57c16fc32f9..1dad0e2c46cc88d691827208c38c6be092f2bf1e 100644 (file)
--- a/urcu-call-rcu.h
+++ b/urcu-call-rcu.h
@@ -32,7 +32,7 @@
 #include <stdlib.h>
 #include <pthread.h>
 
-#include <urcu/wfqueue.h>
+#include <urcu/wfcqueue.h>
 
 #ifdef __cplusplus
 extern "C" {
@@ -55,7 +55,7 @@ struct call_rcu_data;
  */
 
 struct rcu_head {
-       struct cds_wfq_node next;
+       struct cds_wfcq_node next;
        void (*func)(struct rcu_head *head);
 };