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

[Alexander Lobakin <alexandr.lobakin@intel.com>]

From: Paolo Abeni <pabeni@redhat.com>
Date: Tue, 18 Jan 2022 17:31:00 +0100

> 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()?

Oof, sorry, I saw start/end and overlooked that it's only for
a single assignment.
Then it shouldn't cause warnings, but maybe use an anonymous
union instead?

	union {
		u32 zeroed;
		struct {
			u16 gro_remcsum_start;
			...
		};
	};
	__wsum csum;

Use can still use a BUILD_BUG_ON() in this case, like
sizeof(zeroed) != offsetof(csum) - offsetof(zeroed).

> 
> Thanks!
> 
> Paolo

Thanks,
Al