netdev - Re: [RFC PATCH 2/4] net: enable UDP gro on demand.

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Date:   Fri, 14 Sep 2018 13:16:41 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Willem de Bruijn <willemb@...gle.com>,
        steffen.klassert@...unet.com
Subject: Re: [RFC PATCH 2/4] net: enable UDP gro on demand.

On Fri, Sep 14, 2018 at 11:47 AM Paolo Abeni <pabeni@...hat.com> wrote:
>
> Currently, the UDP GRO callback is always invoked, regardless of
> the existence of any actual user (e.g. a UDP tunnel). With retpoline
> enabled, this causes measurable overhead.
>
> This changeset introduces explicit accounting of the sockets requiring
> UDP GRO and updates the UDP offloads at runtime accordingly, so that
> the GRO callback is present (and invoked) only when there is at least
> one socket requiring it.

I have a difference solution both to the UDP socket lookup avoidance
and configurable GRO in general.

The first can be achieved by exporting the udp_encap_needed_key static key:

"
diff --git a/include/net/udp.h b/include/net/udp.h
index 8482a990b0bb..9e82cb391dea 100644
--- a/include/net/udp.h
+++ b/include/net/udp.h
@@ -443,8 +443,10 @@ int udpv4_offload_init(void);

 void udp_init(void);

+DECLARE_STATIC_KEY_FALSE(udp_encap_needed_key);
 void udp_encap_enable(void);

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index f4e35b2ff8b8..bd873a5b8a86 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1889,7 +1889,7 @@ static int __udp_queue_rcv_skb(struct sock *sk,
struct sk_buff *skb)
        return 0;
 }

-static DEFINE_STATIC_KEY_FALSE(udp_encap_needed_key);
+DEFINE_STATIC_KEY_FALSE(udp_encap_needed_key);
 void udp_encap_enable(void)
 {
        static_branch_enable(&udp_encap_needed_key);
diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
index 4f6aa95a9b12..f44fe328aa0f 100644
--- a/net/ipv4/udp_offload.c
+++ b/net/ipv4/udp_offload.c
@@ -405,7 +405,7 @@ static struct sk_buff *udp4_gro_receive(struct
list_head *head,
 {
        struct udphdr *uh = udp_gro_udphdr(skb);

-       if (unlikely(!uh))
+       if (unlikely(!uh) || !static_branch_unlikely(&udp_encap_needed_key))
                goto flush;
 "

.. and same for ipv6.

The second is a larger patchset that converts dev_offload to
net_offload, so that all offloads share the same infrastructure, and a
sysctl interface to be able to disable all gro_receive types, not just
udp.

I've been sitting on it for too long. Let me slightly clean it up and
send it out for discussion sake..

>
> Tested with pktgen vs udpgso_bench_rx
> Before:
> udp rx:     27 MB/s  1613271 calls/s
>
> After:
> udp rx:     30 MB/s  1771537 calls/s
>
> Signed-off-by: Paolo Abeni <pabeni@...hat.com>