[PATCH net-next] neigh: speedup neigh_resolve_output()

Eric Dumazet
2010-10-07T20:44:13Z

Le jeudi 07 octobre 2010 à 14:53 +0200, Eric Dumazet a écrit :

> Further improvements would need to use a seqlock instead of an rwlock to
> protect neigh->ha[], to not dirty neigh too often and remove two atomic
> ops.
>

I implemented this idea in following patch, on top on previous one.

[PATCH net-next] neigh: speedup neigh_resolve_output()

Add a seqlock in struct neighbour to protect neigh->ha[], and avoid
dirtying neighbour in stress situation (many different flows / dsts)

Dirtying takes place because of read_lock(&n->lock) and n->used writes.

Switching to a seqlock, and writing n->used only on jiffies changes
permits less dirtying.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
---
include/net/neighbour.h | 16 ++++++++++++
net/core/neighbour.c | 47 ++++++++++++++++++++++++--------------
net/ipv4/arp.c | 6 +---
net/sched/sch_teql.c | 8 +++---
4 files changed, 51 insertions(+), 26 deletions(-)

diff --git a/include/net/neighbour.h b/include/net/neighbour.h
index a4538d5..f04e7a2 100644
--- a/include/net/neighbour.h
+++ b/include/net/neighbour.h
@@ -105,6 +105,7 @@ struct neighbour {
atomic_t refcnt;
atomic_t probes;
rwlock_t lock;
+ seqlock_t ha_lock;
unsigned char ha[ALIGN(MAX_ADDR_LEN, sizeof(unsigned long))];
struct hh_cache *hh;
int (*output)(struct sk_buff *skb);
@@ -302,7 +303,10 @@ static inline void neigh_confirm(struct neighbour *neigh)

static inline int neigh_event_send(struct neighbour *neigh, struct sk_buff *skb)
{
- neigh->used = jiffies;
+ unsigned long now = ACCESS_ONCE(jiffies);
+
+ if (neigh->used != now)
+ neigh->used = now;
if (!(neigh->nud_state&(NUD_CONNECTED|NUD_DELAY|NUD_PROBE)))
return __neigh_event_send(neigh, skb);
return 0;
@@ -373,4 +377,14 @@ struct neighbour_cb {

#define NEIGH_CB(skb) ((struct neighbour_cb *)(skb)->cb)

+static inline void neigh_ha_snapshot(char *dst, const struct neighbour *n,
+ const struct net_device *dev)
+{
+ unsigned int seq;
+
+ do {
+ seq = read_seqbegin(&n->ha_lock);
+ memcpy(dst, n->ha, dev->addr_len);
+ } while (read_seqretry(&n->ha_lock, seq));
+}
#endif
diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index 53cc548..54aef9c 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -294,6 +294,7 @@ static struct neighbour *neigh_alloc(struct neigh_table *tbl)

skb_queue_head_init(&n->arp_queue);
rwlock_init(&n->lock);
+ seqlock_init(&n->ha_lock);
n->updated = n->used = now;
n->nud_state = NUD_NONE;
n->output = neigh_blackhole;
@@ -1015,7 +1016,7 @@ out_unlock_bh:
}
EXPORT_SYMBOL(__neigh_event_send);

-static void neigh_update_hhs(struct neighbour *neigh)
+static void neigh_update_hhs(const struct neighbour *neigh)
{
struct hh_cache *hh;
void (*update)(struct hh_cache*, const struct net_device*, const unsigned char *)
@@ -1151,7 +1152,9 @@ int neigh_update(struct neighbour *neigh, const u8 *lladdr, u8 new,
}

if (lladdr != neigh->ha) {
+ write_seqlock(&neigh->ha_lock);
memcpy(&neigh->ha, lladdr, dev->addr_len);
+ write_sequnlock(&neigh->ha_lock);
neigh_update_hhs(neigh);
if (!(new & NUD_CONNECTED))
neigh->confirmed = jiffies -
@@ -1214,6 +1217,7 @@ static inline bool neigh_hh_lookup(struct neighbour *n, struct dst_entry *dst,
{
struct hh_cache *hh;

+ smp_rmb(); /* paired with smp_wmb() in neigh_hh_init() */
for (hh = n->hh; hh; hh = hh->hh_next) {
if (hh->hh_type == protocol) {
atomic_inc(&hh->hh_refcnt);
@@ -1248,8 +1252,8 @@ static void neigh_hh_init(struct neighbour *n, struct dst_entry *dst,
kfree(hh);
return;
}
- read_unlock(&n->lock);
- write_lock(&n->lock);
+
+ write_lock_bh(&n->lock);

/* must check if another thread already did the insert */
if (neigh_hh_lookup(n, dst, protocol)) {
@@ -1263,13 +1267,13 @@ static void neigh_hh_init(struct neighbour *n, struct dst_entry *dst,
hh->hh_output = n->ops->output;

hh->hh_next = n->hh;
+ smp_wmb(); /* paired with smp_rmb() in neigh_hh_lookup() */
n->hh = hh;

if (unlikely(cmpxchg(&dst->hh, NULL, hh) != NULL))
hh_cache_put(hh);
end:
- write_unlock(&n->lock);
- read_lock(&n->lock);
+ write_unlock_bh(&n->lock);
}

/* This function can be used in contexts, where only old dev_queue_xmit
@@ -1308,16 +1312,18 @@ int neigh_resolve_output(struct sk_buff *skb)
if (!neigh_event_send(neigh, skb)) {
int err;
struct net_device *dev = neigh->dev;
+ unsigned int seq;

- read_lock_bh(&neigh->lock);
if (dev->header_ops->cache &&
!dst->hh &&
!(dst->flags & DST_NOCACHE))
neigh_hh_init(neigh, dst, dst->ops->protocol);

- err = dev_hard_header(skb, dev, ntohs(skb->protocol),
- neigh->ha, NULL, skb->len);
- read_unlock_bh(&neigh->lock);
+ do {
+ seq = read_seqbegin(&neigh->ha_lock);
+ err = dev_hard_header(skb, dev, ntohs(skb->protocol),
+ neigh->ha, NULL, skb->len);
+ } while (read_seqretry(&neigh->ha_lock, seq));

if (err >= 0)
rc = neigh->ops->queue_xmit(skb);
@@ -1344,13 +1350,16 @@ int neigh_connected_output(struct sk_buff *skb)
struct dst_entry *dst = skb_dst(skb);
struct neighbour *neigh = dst->neighbour;
struct net_device *dev = neigh->dev;
+ unsigned int seq;

__skb_pull(skb, skb_network_offset(skb));

- read_lock_bh(&neigh->lock);
- err = dev_hard_header(skb, dev, ntohs(skb->protocol),
- neigh->ha, NULL, skb->len);
- read_unlock_bh(&neigh->lock);
+ do {
+ seq = read_seqbegin(&neigh->ha_lock);
+ err = dev_hard_header(skb, dev, ntohs(skb->protocol),
+ neigh->ha, NULL, skb->len);
+ } while (read_seqretry(&neigh->ha_lock, seq));
+
if (err >= 0)
err = neigh->ops->queue_xmit(skb);
else {
@@ -2148,10 +2157,14 @@ static int neigh_fill_info(struct sk_buff *skb, struct neighbour *neigh,

read_lock_bh(&neigh->lock);
ndm->ndm_state = neigh->nud_state;
- if ((neigh->nud_state & NUD_VALID) &&
- nla_put(skb, NDA_LLADDR, neigh->dev->addr_len, neigh->ha) < 0) {
- read_unlock_bh(&neigh->lock);
- goto nla_put_failure;
+ if (neigh->nud_state & NUD_VALID) {
+ char haddr[MAX_ADDR_LEN];
+
+ neigh_ha_snapshot(haddr, neigh, neigh->dev);
+ if (nla_put(skb, NDA_LLADDR, neigh->dev->addr_len, haddr) < 0) {
+ read_unlock_bh(&neigh->lock);
+ goto nla_put_failure;
+ }
}

ci.ndm_used = jiffies_to_clock_t(now - neigh->used);
diff --git a/net/ipv4/arp.c b/net/ipv4/arp.c
index f353095..d8e540c 100644
--- a/net/ipv4/arp.c
+++ b/net/ipv4/arp.c
@@ -502,10 +502,8 @@ int arp_find(unsigned char *haddr, struct sk_buff *skb)

if (n) {
n->used = jiffies;
- if (n->nud_state&NUD_VALID || neigh_event_send(n, skb) == 0) {
- read_lock_bh(&n->lock);
- memcpy(haddr, n->ha, dev->addr_len);
- read_unlock_bh(&n->lock);
+ if (n->nud_state & NUD_VALID || neigh_event_send(n, skb) == 0) {
+ neigh_ha_snapshot(haddr, n, dev);
neigh_release(n);
return 0;
}
diff --git a/net/sched/sch_teql.c b/net/sched/sch_teql.c
index feaabc1..401af95 100644
--- a/net/sched/sch_teql.c
+++ b/net/sched/sch_teql.c
@@ -241,11 +241,11 @@ __teql_resolve(struct sk_buff *skb, struct sk_buff *skb_res, struct net_device *
}
if (neigh_event_send(n, skb_res) == 0) {
int err;
+ char haddr[MAX_ADDR_LEN];

- read_lock(&n->lock);
- err = dev_hard_header(skb, dev, ntohs(skb->protocol),
- n->ha, NULL, skb->len);
- read_unlock(&n->lock);
+ neigh_ha_snapshot(haddr, n, dev);
+ err = dev_hard_header(skb, dev, ntohs(skb->protocol), haddr,
+ NULL, skb->len);

if (err < 0) {
neigh_release(n);