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Date: Tue, 18 Jan 2022 17:31:00 +0100 From: Paolo Abeni <pabeni@...hat.com> To: Alexander Lobakin <alexandr.lobakin@...el.com> Cc: netdev@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>, Kees Cook <keescook@...omium.org> Subject: Re: [RFC PATCH 2/3] net: gro: minor optimization for dev_gro_receive() On Tue, 2022-01-18 at 16:56 +0100, Alexander Lobakin wrote: > From: Paolo Abeni <pabeni@...hat.com> > Date: Tue, 18 Jan 2022 16:24:19 +0100 > > > While inspecting some perf report, I noticed that the compiler > > emits suboptimal code for the napi CB initialization, fetching > > and storing multiple times the memory for flags bitfield. > > This is with gcc 10.3.1, but I observed the same with older compiler > > versions. > > > > We can help the compiler to do a nicer work e.g. initially setting > > all the bitfield to 0 using an u16 alias. The generated code is quite > > smaller, with the same number of conditional > > > > Before: > > objdump -t net/core/gro.o | grep " F .text" > > 0000000000000bb0 l F .text 0000000000000357 dev_gro_receive > > > > After: > > 0000000000000bb0 l F .text 000000000000033c dev_gro_receive > > > > Signed-off-by: Paolo Abeni <pabeni@...hat.com> > > --- > > include/net/gro.h | 13 +++++++++---- > > net/core/gro.c | 16 +++++----------- > > 2 files changed, 14 insertions(+), 15 deletions(-) > > > > diff --git a/include/net/gro.h b/include/net/gro.h > > index 8f75802d50fd..a068b27d341f 100644 > > --- a/include/net/gro.h > > +++ b/include/net/gro.h > > @@ -29,14 +29,17 @@ struct napi_gro_cb { > > /* Number of segments aggregated. */ > > u16 count; > > > > - /* Start offset for remote checksum offload */ > > - u16 gro_remcsum_start; > > + /* Used in ipv6_gro_receive() and foo-over-udp */ > > + u16 proto; > > > > /* jiffies when first packet was created/queued */ > > unsigned long age; > > > > - /* Used in ipv6_gro_receive() and foo-over-udp */ > > - u16 proto; > > + /* portion of the cb set to zero at every gro iteration */ > > + u32 zeroed_start[0]; > > + > > + /* Start offset for remote checksum offload */ > > + u16 gro_remcsum_start; > > > > /* This is non-zero if the packet may be of the same flow. */ > > u8 same_flow:1; > > @@ -70,6 +73,8 @@ struct napi_gro_cb { > > /* GRO is done by frag_list pointer chaining. */ > > u8 is_flist:1; > > > > + u32 zeroed_end[0]; > > This should be wrapped in struct_group() I believe, or compilers > will start complaining soon. See [0] for the details. > Adding Kees to the CCs. Thank you for the reference. That really slipped-off my mind. This patch does not use memcpy() or similar, just a single direct assignement. Would that still require struct_group()? Thanks! Paolo
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