Re: [PATCH net-next] udp: handle gro_receive only when necessary

[Paolo Abeni <pabeni@redhat.com>]

Hi,

On Mon, 2017-12-18 at 12:11 +0800, zhangliping wrote:
> From: zhangliping <zhangliping02@baidu.com>
> 
> Under our udp pressure performance test, after gro is disabled, rx rate
> will be improved from ~2500kpps to ~2800kpps. We can find some difference
> from perf report:
> 1. gro is enabled:
>   24.23%   [kernel]       [k] udp4_lib_lookup2
>    5.42%   [kernel]       [k] __memcpy
>    3.87%   [kernel]       [k] fib_table_lookup
>    3.76%   [kernel]       [k] __netif_receive_skb_core
>    3.68%   [kernel]       [k] ip_rcv
> 
> 2. gro is disabled:
>    9.66%   [kernel]       [k] udp4_lib_lookup2
>    9.47%   [kernel]       [k] __memcpy
>    4.75%   [kernel]       [k] fib_table_lookup
>    4.71%   [kernel]       [k] __netif_receive_skb_core
>    3.90%   [kernel]       [k] virtnet_poll
> 
> So if there's no udp tunnel(such as vxlan) configured, we can skip
> the udp gro processing.

I tested something similar some time ago, but I measured a much smaller
gain. Also the topmost perf offenders looks quite different from what I
see here, can you please share more details about the test case?

> Signed-off-by: zhangliping <zhangliping02@baidu.com>
> ---
>  include/net/udp.h      |  2 ++
>  net/ipv4/udp_offload.c |  7 +++++++
>  net/ipv4/udp_tunnel.c  | 11 ++++++++++-
>  3 files changed, 19 insertions(+), 1 deletion(-)
> 
> diff --git a/include/net/udp.h b/include/net/udp.h
> index 6c759c8594e2..c503f8b06845 100644
> --- a/include/net/udp.h
> +++ b/include/net/udp.h
> @@ -188,6 +188,8 @@ static inline struct udphdr *udp_gro_udphdr(struct sk_buff *skb)
>  	return uh;
>  }
>  
> +extern struct static_key_false udp_gro_needed;
> +
>  /* hash routines shared between UDPv4/6 and UDP-Litev4/6 */
>  static inline int udp_lib_hash(struct sock *sk)
>  {
> diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
> index 01801b77bd0d..9cb11a833964 100644
> --- a/net/ipv4/udp_offload.c
> +++ b/net/ipv4/udp_offload.c
> @@ -10,10 +10,14 @@
>   *	UDPv4 GSO support
>   */
>  
> +#include <linux/static_key.h>
>  #include <linux/skbuff.h>
>  #include <net/udp.h>
>  #include <net/protocol.h>
>  
> +DEFINE_STATIC_KEY_FALSE(udp_gro_needed);
> +EXPORT_SYMBOL_GPL(udp_gro_needed);
> +

I think that adding a new static key is not required, as we can
probably reuse 'udp_encap_needed' and 'udpv6_encap_needed'. The latter
choice allows earlier branching (in
udp4_gro_receive()/udp6_gro_receive() instead of udp_gro_receive().

Cheers,

Paolo