linux-kernel - Re: [PATCH net-next v10 2/3] net: gro: move L3 flush checks to tcp_gro_receive and udp_gro_receive

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Date: Tue, 14 May 2024 14:13:21 +0200
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Richard Gobert <richardbgobert@...il.com>
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, 
	pabeni@...hat.com, willemdebruijn.kernel@...il.com, dsahern@...nel.org, 
	alexander.duyck@...il.com, shuah@...nel.org, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH net-next v10 2/3] net: gro: move L3 flush checks to
 tcp_gro_receive and udp_gro_receive_segment

Hi Richard,

On Thu, May 9, 2024 at 9:09 PM Richard Gobert <richardbgobert@...ilcom> wrote:
> {inet,ipv6}_gro_receive functions perform flush checks (ttl, flags,
> iph->id, ...) against all packets in a loop. These flush checks are used in
> all merging UDP and TCP flows.
>
> These checks need to be done only once and only against the found p skb,
> since they only affect flush and not same_flow.
>
> This patch leverages correct network header offsets from the cb 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 non-loop flush tests for TCP and UDP
> 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).
>
> perf top while replaying 64 parallel IP/TCP streams merging in GRO:
> (gro_receive_network_flush is compiled inline to tcp_gro_receive)
> net-next:
>         6.94% [kernel] [k] inet_gro_receive
>         3.02% [kernel] [k] tcp_gro_receive
>
> patch applied:
>         4.27% [kernel] [k] tcp_gro_receive
>         4.22% [kernel] [k] inet_gro_receive
>
> perf top while replaying 64 parallel IP/IP/TCP streams merging in GRO (same
> results for any encapsulation, in this case inet_gro_receive is top
> offender in net-next)
> net-next:
>         10.09% [kernel] [k] inet_gro_receive
>         2.08% [kernel] [k] tcp_gro_receive
>
> patch applied:
>         6.97% [kernel] [k] inet_gro_receive
>         3.68% [kernel] [k] tcp_gro_receive
>
> Signed-off-by: Richard Gobert <richardbgobert@...il.com>

Thanks for your patch, which is now commit 4b0ebbca3e167976 ("net: gro:
move L3 flush checks to tcp_gro_receive and udp_gro_receive_segment")
in net-next/main (next-20240514).

noreply@...erman.id.au reports build failures on m68k, e.g.
http://kisskb.ellerman.id.au/kisskb/buildresult/15168903/

    net/core/gro.c: In function ‘dev_gro_receive’:
    ././include/linux/compiler_types.h:460:38: error: call to
‘__compiletime_assert_654’ declared with attribute error: BUILD_BUG_ON
failed: !IS_ALIGNED(offsetof(struct napi_gro_cb, zeroed), sizeof(u32))

> --- a/include/net/gro.h
> +++ b/include/net/gro.h
> @@ -36,15 +36,15 @@ struct napi_gro_cb {
>         /* This is non-zero if the packet cannot be merged with the new skb. */
>         u16     flush;
>
> -       /* Save the IP ID here and check when we get to the transport layer */
> -       u16     flush_id;
> -
>         /* Number of segments aggregated. */
>         u16     count;
>
>         /* Used in ipv6_gro_receive() and foo-over-udp and esp-in-udp */
>         u16     proto;

On most architectures, there is now a hole of 2 bytes here.
However, on m68k the minimum alignment of __wsum (__u32) below is 2,
hence there is no hole, breaking the assertion.

Probably you just want to make this explicit, by adding

    u16 pad;

here.

>
> +       /* used to support CHECKSUM_COMPLETE for tunneling protocols */
> +       __wsum  csum;

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68korg

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds