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

[Yuval Mintz]

> hello Jiri and Yotam,
> 
> On Thu, 2017-09-28 at 19:34 +0200, Jiri Pirko wrote:
> > From: Yotam Gigi 
> >
> > 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 {
> > __u8remcsum_offload:1;
> >  #ifdef CONFIG_NET_SWITCHDEV
> > __u8offload_fwd_mark:1;
> > +   __u8offload_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.

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

[Davide Caratti]

hello Jiri and Yotam,

On Thu, 2017-09-28 at 19:34 +0200, Jiri Pirko wrote:
> From: Yotam Gigi 
> 
> 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 {
>   __u8remcsum_offload:1;
>  #ifdef CONFIG_NET_SWITCHDEV
>   __u8offload_fwd_mark:1;
> + __u8offload_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

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

[Jiri Pirko]

Thu, Sep 28, 2017 at 07:49:03PM CEST, and...@lunn.ch wrote:
>On Thu, Sep 28, 2017 at 07:34:09PM +0200, Jiri Pirko wrote:
>> From: Yotam Gigi 
>> 
>> Similarly to the offload_fwd_mark field, the offload_mr_fwd_mark field is
>> used to allow partial offloading of MFC multicast routes.
>
>> The reason why the already existing "offload_fwd_mark" bit cannot be used
>> is that a switchdev driver would want to make the distinction between a
>> packet that has already gone through L2 forwarding but did not go through
>> multicast forwarding, and a packet that has already gone through both L2
>> and multicast forwarding.
>
>Hi Jiri
>
>So we are talking about l2 vs l3. So why not call this
>offload_l3_fwd_mark?
>
>Is there anything really specific to multicast here?

Currently it is, not sure if it is going to be used for anything else
later on. In case it will be, it could be renamed very easily.


>
>   Thanks
>  Andrew

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

[Andrew Lunn]

On Thu, Sep 28, 2017 at 07:34:09PM +0200, Jiri Pirko wrote:
> From: Yotam Gigi 
> 
> Similarly to the offload_fwd_mark field, the offload_mr_fwd_mark field is
> used to allow partial offloading of MFC multicast routes.

> The reason why the already existing "offload_fwd_mark" bit cannot be used
> is that a switchdev driver would want to make the distinction between a
> packet that has already gone through L2 forwarding but did not go through
> multicast forwarding, and a packet that has already gone through both L2
> and multicast forwarding.

Hi Jiri

So we are talking about l2 vs l3. So why not call this
offload_l3_fwd_mark?

Is there anything really specific to multicast here?

   Thanks
  Andrew