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On Tue, 2015-10-06 at 13:53 -0500, Eric W. Biederman wrote:
> Compute net and store it in a variable in the functions
> ip_build_and_send_pkt and ip_queue_xmit so that it does not need to be
> recomputed next time it is needed.
>
> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> ---
> net/ipv4/ip_output.c | 10 ++++++----
> 1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
> index 10366ee03bec..a7012f2fa68a 100644
> --- a/net/ipv4/ip_output.c
> +++ b/net/ipv4/ip_output.c
> @@ -139,6 +139,7 @@ int ip_build_and_send_pkt(struct sk_buff *skb, const
> struct sock *sk,
> {
> struct inet_sock *inet = inet_sk(sk);
> struct rtable *rt = skb_rtable(skb);
> + struct net *net = sock_net(sk);
> struct iphdr *iph;
>
> /* Build the IP header. */
> @@ -157,7 +158,7 @@ int ip_build_and_send_pkt(struct sk_buff *skb, const
> struct sock *sk,
> iph->id = 0;
> } else {
> iph->frag_off = 0;
> - __ip_select_ident(sock_net(sk), iph, 1);
> + __ip_select_ident(net, iph, 1);
> }
>
Note that under normal SYNACK processing, we do not read sock_net(sk)
here.
This patch would slow down the SYNACK path under stress, unless compiler
is smart enough to not care of what you wrote.
Generally speaking, I do not see why storing 'struct net' pointer into a
variable in the stack is very different from sk->sk_net access (sk being
a register in most cases)
Note that I am about to submit following patch, so that you understand
the context : the listener socket is cold in cpu cache at the time we
transmit a SYNACK. It is better to get net from the request_sock which
is very hot at this point.
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 55ed3266b05f..93277bde8dd9 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -3026,7 +3026,7 @@ struct sk_buff *tcp_make_synack(const struct sock *sk,
struct dst_entry *dst,
th->window = htons(min(req->rcv_wnd, 65535U));
tcp_options_write((__be32 *)(th + 1), NULL, &opts);
th->doff = (tcp_header_size >> 2);
- TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_OUTSEGS);
+ TCP_INC_STATS_BH(sock_net(req_to_sk(req)), TCP_MIB_OUTSEGS);
#ifdef CONFIG_TCP_MD5SIG
/* Okay, we have all we need - do the md5 hash if needed */
@@ -3519,9 +3519,11 @@ int tcp_rtx_synack(const struct sock *sk, struct
request_sock *req)
tcp_rsk(req)->txhash = net_tx_rndhash();
res = af_ops->send_synack(sk, NULL, &fl, req, 0, NULL, true);
- if (!res) {
- TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_RETRANSSEGS);
- NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPSYNRETRANS);
+ if (likely(!res)) {
+ struct net *net = sock_net(req_to_sk(req));
+
+ TCP_INC_STATS_BH(net, TCP_MIB_RETRANSSEGS);
+ NET_INC_STATS_BH(net, LINUX_MIB_TCPSYNRETRANS);
}
return res;
}
--
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