> hello Jiri and Yotam,
>
> On Thu, 2017-09-28 at 19:34 +0200, Jiri Pirko wrote:
> > From: Yotam Gigi <yot...@mellanox.com>
> >
> > diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> > index 19e64bf..ada8214 100644
> > --- a/include/linux/skbuff.h
> > +++ b/include/linux/skbuff.h
> > @@ -772,6 +772,7 @@ struct sk_buff {
> > __u8 remcsum_offload:1;
> > #ifdef CONFIG_NET_SWITCHDEV
> > __u8 offload_fwd_mark:1;
> > + __u8 offload_mr_fwd_mark:1;
>
> I had a look at the pahole output:
>
> $ make allyesconfig
> $ make net/core/skbuff.o
> $ pahole net/core/skbuff.o | grep -C7 tc_from_ingress
>
>
> __u8 ipvs_property:1; /* 147: 7 1 */
> __u8 inner_protocol_type:1; /* 147: 6 1 */
> __u8 remcsum_offload:1; /* 147: 5 1 */
> __u8 offload_fwd_mark:1; /* 147: 4 1 */
> __u8 tc_skip_classify:1; /* 147: 3 1 */
> __u8 tc_at_ingress:1; /* 147: 2 1 */
> __u8 tc_redirected:1; /* 147: 1 1 */
> __u8 tc_from_ingress:1; /* 147: 0 1 */
> __u16 tc_index; /* 148 2 */
>
> /* XXX 2 bytes hole, try to pack */
>
> union {
> __wsum csum; /* 4 */
> struct {
>
> apparently there are no more spare bits to use at that offset: therefore,
> adding 'offload_mr_fwd_mark' before 'tc_skip_classify' will make
> 'tc_from_ingress' slip at offset 148, and tc_index at offset 150.
> I think you can use that 2-bytes hole below tc_index, and also move the
> offload_fwd_mark bit there, as we use both when
> CONFIG_NET_SWITCHDEV is
> enabled. This way we will also gain one spare bit, without changing the
> struct size or worsening the cacheline alignments.
>
> what do you think?
Your pahole output still shows a 2B hole until the following union
which is 4B-aligned.
While it's true tc_index moves to offset 150, the union will not move
[I.e., stay at offset 152] so the layout doesn't really change [greatly]
nor the size of the struct. And we have the benefit of all the bits
remaining consecutive.
