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Date: Fri, 4 Sep 2020 09:19:39 +0200 From: Jesper Dangaard Brouer <brouer@...hat.com> To: Alexei Starovoitov <alexei.starovoitov@...il.com> Cc: Lorenzo Bianconi <lorenzo@...nel.org>, netdev@...r.kernel.org, bpf@...r.kernel.org, davem@...emloft.net, lorenzo.bianconi@...hat.com, echaudro@...hat.com, sameehj@...zon.com, kuba@...nel.org, john.fastabend@...il.com, daniel@...earbox.net, ast@...nel.org, shayagr@...zon.com, brouer@...hat.com, David Ahern <dsahern@...nel.org> Subject: Re: [PATCH v2 net-next 1/9] xdp: introduce mb in xdp_buff/xdp_frame On Thu, 3 Sep 2020 18:07:05 -0700 Alexei Starovoitov <alexei.starovoitov@...il.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@...nel.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
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