Re: [RFC PATCH 2/3] net: gro: minor optimization for dev_gro_receive()

[Paolo Abeni <pabeni@redhat.com>]

On Tue, 2022-01-18 at 16:56 +0100, Alexander Lobakin wrote:
> From: Paolo Abeni <pabeni@redhat.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@redhat.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