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Eric Dumazet <eric.dumazet@xxxxxxxxx> writes:
> 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.
So what I am really reading it for is ip_local_out which I change to
take a struct net a few patches later in the series. The patches that
changes everything is noticably cleaner and easier to review with these
couple of patches pulling struct net into it's own variable ahead of
time, and ip_build_and_send_pkt does call ip_local_out unconditionally.
I am in the process of figuring out how to compute net once in the
output path and just passing it through so I don't need to compute net
from dst->dev. As when the dust settles I hope to allow for a dst->dev
in another network namespace. So that routes with a destination device
in another network namespace will allow for something simpler and faster
than ipvlan that achieves a very similar effect.
In this case to achieve what you are looking for, for cache line
friendliness I believe we would need to pass net in from
tcp_v4_send_synack, and it's cousins in dccp.
skc_net does seem firmly in the first cache line of sockets so it does
look like any of the the reads to inet_sock that we do perform would
hit the same cache line.
To recap. I store net in a variable because I start using it
unconditionally a few patches later. The only way I can see to avoid
hitting the cold cache line is to pass net into ip_build_and_send_pkt.
Do you think passing net into ip_build_and_send_pkt is the sensible way
to address your performance concern? Or do you have issues with my
passing of net through the output path?
> 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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