netdev - Re: [patch net-next 1/7] skbuff: Add the offload_mr_fwd

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Message-ID: <1506683686.2980.44.camel@redhat.com>
Date:   Fri, 29 Sep 2017 13:14:46 +0200
From:   Davide Caratti <dcaratti@...hat.com>
To:     Jiri Pirko <jiri@...nulli.us>, netdev@...r.kernel.org
Cc:     davem@...emloft.net, yotamg@...lanox.com, idosch@...lanox.com,
        mlxsw@...lanox.com, nikolay@...ulusnetworks.com, andrew@...n.ch,
        dsa@...ulusnetworks.com, edumazet@...gle.com, willemb@...gle.com,
        johannes.berg@...el.com, pabeni@...hat.com, daniel@...earbox.net,
        f.fainelli@...il.com, fw@...len.de, gfree.wind@....163.com
Subject: Re: [patch net-next 1/7] skbuff: Add the offload_mr_fwd_mark field

hello Jiri and Yotam,

On Thu, 2017-09-28 at 19:34 +0200, Jiri Pirko wrote:
> From: Yotam Gigi <yotamg@...lanox.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?

regards,
-- 
davide