netdev - Re: [PATCH net-next 2/3] net: gro: minor optimization for dev_gro

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Date:   Thu, 03 Feb 2022 18:44:50 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     Alexander H Duyck <alexander.duyck@...il.com>,
        netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexander Lobakin <alexandr.lobakin@...el.com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next 2/3] net: gro: minor optimization for
 dev_gro_receive()

On Thu, 2022-02-03 at 08:39 -0800, Alexander H Duyck wrote:
> On Thu, 2022-02-03 at 16:48 +0100, Paolo Abeni wrote:
> > 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 clearing several
> > fields at once using an u32 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
> > 
> > RFC -> v1:
> >  - use __struct_group to delimt the zeroed area (Alexander)
> > 
> > Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> > ---
> >  include/net/gro.h | 52 +++++++++++++++++++++++++--------------------
> > --
> >  net/core/gro.c    | 18 +++++++---------
> >  2 files changed, 35 insertions(+), 35 deletions(-)
> > 
> > diff --git a/include/net/gro.h b/include/net/gro.h
> > index 8f75802d50fd..fa1bb0f0ad28 100644
> > --- a/include/net/gro.h
> > +++ b/include/net/gro.h
> > @@ -29,46 +29,50 @@ 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 */
> > +       __struct_group(/* no tag */, zeroed, /* no attrs */,
> 
> Any specific reason for using __struct_group here rather than the
> struct_group macro instead?

Just sheer ignorance on my side. I'll fix on the next iteration.

Thanks!

Paolo