From mboxrd@z Thu Jan  1 00:00:00 1970
From: laijs@cn.fujitsu.com (Lai Jiangshan)
Date: Mon, 08 Oct 2012 11:09:29 +0800
Subject: [lttng-dev] [URCU PATCH 3/3] call_rcu: use wfcqueue,
 eliminate false-sharing
In-Reply-To: <20121002141638.GD4057@Krystal>
References: <20121002141307.GA4057@Krystal> <20121002141638.GD4057@Krystal>
Message-ID: <507243E9.8010604@cn.fujitsu.com>

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);
>  };
>