neigh: reduce arp latency

Remove the artificial HZ latency on arp resolution.

Instead of firing a timer in one jiffy (up to 10 ms if HZ=100), lets
send the ARP message immediately.

Before patch :

# arp -d 192.168.20.108 ; ping -c 3 192.168.20.108
PING 192.168.20.108 (192.168.20.108) 56(84) bytes of data.
64 bytes from 192.168.20.108: icmp_seq=1 ttl=64 time=9.91 ms
64 bytes from 192.168.20.108: icmp_seq=2 ttl=64 time=0.065 ms
64 bytes from 192.168.20.108: icmp_seq=3 ttl=64 time=0.061 ms

After patch :

$ arp -d 192.168.20.108 ; ping -c 3 192.168.20.108
PING 192.168.20.108 (192.168.20.108) 56(84) bytes of data.
64 bytes from 192.168.20.108: icmp_seq=1 ttl=64 time=0.152 ms
64 bytes from 192.168.20.108: icmp_seq=2 ttl=64 time=0.064 ms
64 bytes from 192.168.20.108: icmp_seq=3 ttl=64 time=0.074 ms

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 26 insertions, 14 deletions

diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index 8fab9b0bb203..4002261f20d1 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -844,6 +844,19 @@ static void neigh_invalidate(struct neighbour *neigh)
         skb_queue_purge(&neigh->arp_queue);
 }
 
+static void neigh_probe(struct neighbour *neigh)
+        __releases(neigh->lock)
+{
+        struct sk_buff *skb = skb_peek(&neigh->arp_queue);
+        /* keep skb alive even if arp_queue overflows */
+        if (skb)
+                skb = skb_copy(skb, GFP_ATOMIC);
+        write_unlock(&neigh->lock);
+        neigh->ops->solicit(neigh, skb);
+        atomic_inc(&neigh->probes);
+        kfree_skb(skb);
+}
+
 /* Called when a timer expires for a neighbour entry. */
 
 static void neigh_timer_handler(unsigned long arg)
@@ -920,14 +933,7 @@ static void neigh_timer_handler(unsigned long arg)
                         neigh_hold(neigh);
         }
         if (neigh->nud_state & (NUD_INCOMPLETE | NUD_PROBE)) {
-                struct sk_buff *skb = skb_peek(&neigh->arp_queue);
-                /* keep skb alive even if arp_queue overflows */
-                if (skb)
-                        skb = skb_copy(skb, GFP_ATOMIC);
-                write_unlock(&neigh->lock);
-                neigh->ops->solicit(neigh, skb);
-                atomic_inc(&neigh->probes);
-                kfree_skb(skb);
+                neigh_probe(neigh);
         } else {
 out:
                 write_unlock(&neigh->lock);
@@ -942,7 +948,7 @@ out:
 int __neigh_event_send(struct neighbour *neigh, struct sk_buff *skb)
 {
         int rc;
-        unsigned long now;
+        bool immediate_probe = false;
 
         write_lock_bh(&neigh->lock);
 
@@ -950,14 +956,16 @@ int __neigh_event_send(struct neighbour *neigh, struct sk_buff *skb)
         if (neigh->nud_state & (NUD_CONNECTED | NUD_DELAY | NUD_PROBE))
                 goto out_unlock_bh;
 
-        now = jiffies;
-
         if (!(neigh->nud_state & (NUD_STALE | NUD_INCOMPLETE))) {
                 if (neigh->parms->mcast_probes + neigh->parms->app_probes) {
+                        unsigned long next, now = jiffies;
+
                         atomic_set(&neigh->probes, neigh->parms->ucast_probes);
                         neigh->nud_state     = NUD_INCOMPLETE;
-                        neigh->updated = jiffies;
-                        neigh_add_timer(neigh, now + 1);
+                        neigh->updated = now;
+                        next = now + max(neigh->parms->retrans_time, HZ/2);
+                        neigh_add_timer(neigh, next);
+                        immediate_probe = true;
                 } else {
                         neigh->nud_state = NUD_FAILED;
                         neigh->updated = jiffies;
@@ -989,7 +997,11 @@ int __neigh_event_send(struct neighbour *neigh, struct sk_buff *skb)
                 rc = 1;
         }
 out_unlock_bh:
-        write_unlock_bh(&neigh->lock);
+        if (immediate_probe)
+                neigh_probe(neigh);
+        else
+                write_unlock(&neigh->lock);
+        local_bh_enable();
         return rc;
 }
 EXPORT_SYMBOL(__neigh_event_send);