Re: [PATCH v2 net-next 1/9] xdp: introduce mb in xdp_buff/xdp_frame

[Jesper Dangaard Brouer <brouer@redhat.com>]

On Thu, 3 Sep 2020 18:07:05 -0700
Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:

> On Thu, Sep 03, 2020 at 10:58:45PM +0200, Lorenzo Bianconi wrote:
> > Introduce multi-buffer bit (mb) in xdp_frame/xdp_buffer to specify
> > if shared_info area has been properly initialized for non-linear
> > xdp buffers
> > 
> > Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
> > ---
> >  include/net/xdp.h | 8 ++++++--
> >  net/core/xdp.c    | 1 +
> >  2 files changed, 7 insertions(+), 2 deletions(-)
> > 
> > diff --git a/include/net/xdp.h b/include/net/xdp.h
> > index 3814fb631d52..42f439f9fcda 100644
> > --- a/include/net/xdp.h
> > +++ b/include/net/xdp.h
> > @@ -72,7 +72,8 @@ struct xdp_buff {
> >  	void *data_hard_start;
> >  	struct xdp_rxq_info *rxq;
> >  	struct xdp_txq_info *txq;
> > -	u32 frame_sz; /* frame size to deduce data_hard_end/reserved tailroom*/
> > +	u32 frame_sz:31; /* frame size to deduce data_hard_end/reserved tailroom*/
> > +	u32 mb:1; /* xdp non-linear buffer */
> >  };
> >  
> >  /* Reserve memory area at end-of data area.
> > @@ -96,7 +97,8 @@ struct xdp_frame {
> >  	u16 len;
> >  	u16 headroom;
> >  	u32 metasize:8;
> > -	u32 frame_sz:24;
> > +	u32 frame_sz:23;
> > +	u32 mb:1; /* xdp non-linear frame */  
> 
> Hmm. Last time I checked compilers were generating ugly code with bitfields.
> Not performant and not efficient.
> frame_sz is used in the fast path.
> I suspect the first hunk alone will cause performance degradation.
> Could you use normal u8 or u32 flag field?

For struct xdp_buff sure we can do this.  For struct xdp_frame, I'm not
sure, as it is a state compressed version of xdp_buff + extra
information.  The xdp_frame have been called skb-light, and I know
people (e.g Ahern) wants to add more info to this, vlan, RX-hash, csum,
and we must keep this to 1-cache-line, for performance reasons.

You do make a good point, that these bit-fields might hurt performance
more.  I guess, we need to test this.  As I constantly worry that we
will slowly kill XDP performance with a 1000 paper-cuts.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer