netdev - Re: [PATCH net-next v7 2/3] net: gro: move L3 flush checks to tcp_gro_receive and udp_gro_receive

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Message-ID: <a36bf0b117f7786bbf028494d68185486025777d.camel@redhat.com>
Date: Tue, 16 Apr 2024 11:21:48 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Richard Gobert <richardbgobert@...il.com>, davem@...emloft.net, 
	edumazet@...gle.com, kuba@...nel.org, dsahern@...nel.org, 
	willemdebruijn.kernel@...il.com, shuah@...nel.org, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH net-next v7 2/3] net: gro: move L3 flush checks to
 tcp_gro_receive and udp_gro_receive_segment

On Fri, 2024-04-12 at 17:55 +0200, Richard Gobert wrote:
> {inet,ipv6}_gro_receive functions perform flush checks (ttl, flags,
> iph->id, ...) against all packets in a loop. These flush checks are used
> currently in all tcp flows and in some UDP flows in GRO.
> 
> These checks need to be done only once and only against the found p skb,
> since they only affect flush and not same_flow.
> 
> Leveraging the previous commit in the series, in which correct network
> header offsets are saved for both outer and inner network headers -
> allowing these checks to be done only once, in tcp_gro_receive and
> udp_gro_receive_segment. As a result, NAPI_GRO_CB(p)->flush is not used at
> all. In addition, flush_id checks are more declarative and contained in
> inet_gro_flush, thus removing the need for flush_id in napi_gro_cb.
> 
> This results in less parsing code for UDP flows and non-loop flush tests
> for TCP flows.
> 
> To make sure results are not within noise range - I've made netfilter drop
> all TCP packets, and measured CPU performance in GRO (in this case GRO is
> responsible for about 50% of the CPU utilization).
> 
> L3 flush/flush_id checks are not relevant to UDP connections where
> skb_gro_receive_list is called. The only code change relevant to this flow
> is inet_gro_receive. The rest of the code parsing this flow stays the
> same.
> 
> All concurrent connections tested are with the same ip srcaddr and
> dstaddr.
> 
> perf top while replaying 64 concurrent IP/UDP connections (UDP fwd flow):
> net-next:
>         3.03%  [kernel]  [k] inet_gro_receive
> 
> patch applied:
>         2.78%  [kernel]  [k] inet_gro_receive

Why there are no figures for
udp_gro_receive_segment()/gro_network_flush() here?

Also you should be able to observer a very high amount of CPU usage by
GRO even with TCP with very high speed links, keeping the BH/GRO on a
CPU and the user-space/data copy on a different one (or using rx zero
copy).

Thanks,

Paolo