Subject: Re: [patch net-next 1/7] skbuff: Add the offload_mr_fwd_mark field

From: Davide Caratti <[email-masked]>
Date: Fri, 29 Sep 2017 13:14:46 +0200

hello Jiri and Yotam,

On Thu, 2017-09-28 at 19:34 +0200, Jiri Pirko wrote:
> From: Yotam Gigi <[email-masked]>
> 
> 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?

regards,
-- 
davide