ipv4: PKTINFO doesnt need dst reference
Le lundi 07 novembre 2011 à 15:33 +0100, Eric Dumazet a écrit :
> At least, in recent kernels we dont change dst->refcnt in forwarding
> patch (usinf NOREF skb->dst)
>
> One particular point is the atomic_inc(dst->refcnt) we have to perform
> when queuing an UDP packet if socket asked PKTINFO stuff (for example a
> typical DNS server has to setup this option)
>
> I have one patch somewhere that stores the information in skb->cb[] and
> avoid the atomic_{inc|dec}(dst->refcnt).
>
OK I found it, I did some extra tests and believe its ready.
[PATCH net-next] ipv4: IP_PKTINFO doesnt need dst reference
When a socket uses IP_PKTINFO notifications, we currently force a dst
reference for each received skb. Reader has to access dst to get needed
information (rt_iif & rt_spec_dst) and must release dst reference.
We also forced a dst reference if skb was put in socket backlog, even
without IP_PKTINFO handling. This happens under stress/load.
We can instead store the needed information in skb->cb[], so that only
softirq handler really access dst, improving cache hit ratios.
This removes two atomic operations per packet, and false sharing as
well.
On a benchmark using a mono threaded receiver (doing only recvmsg()
calls), I can reach 720.000 pps instead of 570.000 pps.
IP_PKTINFO is typically used by DNS servers, and any multihomed aware
UDP application.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
index 09ff51b..80d5fa4 100644
--- a/net/ipv4/ip_sockglue.c
+++ b/net/ipv4/ip_sockglue.c
@@ -55,20 +55,13 @@
/*
* SOL_IP control messages.
*/
+#define PKTINFO_SKB_CB(__skb) ((struct in_pktinfo *)((__skb)->cb))
static void ip_cmsg_recv_pktinfo(struct msghdr *msg, struct sk_buff *skb)
{
- struct in_pktinfo info;
- struct rtable *rt = skb_rtable(skb);
+ struct in_pktinfo info = *PKTINFO_SKB_CB(skb);
info.ipi_addr.s_addr = ip_hdr(skb)->daddr;
- if (rt) {
- info.ipi_ifindex = rt->rt_iif;
- info.ipi_spec_dst.s_addr = rt->rt_spec_dst;
- } else {
- info.ipi_ifindex = 0;
- info.ipi_spec_dst.s_addr = 0;
- }
put_cmsg(msg, SOL_IP, IP_PKTINFO, sizeof(info), &info);
}
@@ -992,20 +985,28 @@
}
/**
- * ip_queue_rcv_skb - Queue an skb into sock receive queue
+ * ipv4_pktinfo_prepare - transfert some info from rtable to skb
* @sk: socket
* @skb: buffer
*
- * Queues an skb into socket receive queue. If IP_CMSG_PKTINFO option
- * is not set, we drop skb dst entry now, while dst cache line is hot.
+ * To support IP_CMSG_PKTINFO option, we store rt_iif and rt_spec_dst
+ * in skb->cb[] before dst drop.
+ * This way, receiver doesnt make cache line misses to read rtable.
*/
-int ip_queue_rcv_skb(struct sock *sk, struct sk_buff *skb)
+void ipv4_pktinfo_prepare(struct sk_buff *skb)
{
- if (!(inet_sk(sk)->cmsg_flags & IP_CMSG_PKTINFO))
- skb_dst_drop(skb);
- return sock_queue_rcv_skb(sk, skb);
+ struct in_pktinfo *pktinfo = PKTINFO_SKB_CB(skb);
+ const struct rtable *rt = skb_rtable(skb);
+
+ if (rt) {
+ pktinfo->ipi_ifindex = rt->rt_iif;
+ pktinfo->ipi_spec_dst.s_addr = rt->rt_spec_dst;
+ } else {
+ pktinfo->ipi_ifindex = 0;
+ pktinfo->ipi_spec_dst.s_addr = 0;
+ }
+ skb_dst_drop(skb);
}
-EXPORT_SYMBOL(ip_queue_rcv_skb);
int ip_setsockopt(struct sock *sk, int level,
int optname, char __user *optval, unsigned int optlen)