[lttng-dev] [URCU PATCH 3/3] call_rcu: use wfcqueue, eliminate false-sharing
Lai Jiangshan
laijs at cn.fujitsu.com
Sun Oct 7 23:09:29 EDT 2012
On 10/02/2012 10:16 PM, Mathieu Desnoyers wrote:
> Eliminate false-sharing between call_rcu (enqueuer) and worker threads
> on the queue head and tail.
>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
> ---
> diff --git a/tests/Makefile.am b/tests/Makefile.am
> index 81718bb..c92bbe6 100644
> --- 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 13b24ff..cf65992 100644
> --- 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;
wrong here. In this code, cbs_tail and cbs_head are in the same cache line.
---
struct call_rcu_data {
struct cds_wfcq_tail cbs_tail;
struct cds_wfcq_head __attribute__((aligned(CAA_CACHE_LINE_SIZE))) cbs_head;
/* other fields, can move some fields up to use the room between tail and head */
};
# cat test.c
struct a {
int __attribute__((aligned(64))) i;
int j;
};
struct b {
int i;
int __attribute__((aligned(64))) j;
};
void main(void)
{
printf("%d,%d\n", sizeof(struct a), sizeof(struct b));
}
# ./a.out
64,128
> 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 f7eac8d..1dad0e2 100644
> --- 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);
> };
>
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