netdev - Re: [PATCH net-next] net-device: move gso_partial_features to net_device_read

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Message-ID: <CANn89i+4jOEOrE3=T7XfEypKoQaFaGeYbbPdfXSHQaEHiXgWWw@mail.gmail.com>
Date: Tue, 2 Jan 2024 16:50:28 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Simon Horman <horms@...nel.org>
Cc: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Willem de Bruijn <willemb@...gle.com>, netdev@...r.kernel.org, 
	eric.dumazet@...il.com, Coco Li <lixiaoyan@...gle.com>, 
	David Ahern <dsahern@...nel.org>
Subject: Re: [PATCH net-next] net-device: move gso_partial_features to net_device_read_tx

On Sat, Dec 23, 2023 at 5:16 PM Simon Horman <horms@...nel.org> wrote:
>
> On Thu, Dec 21, 2023 at 02:07:47PM +0000, Eric Dumazet wrote:
> > dev->gso_partial_features is read from tx fast path for GSO packets.
> >
> > Move it to appropriate section to avoid a cache line miss.
> >
> > Fixes: 43a71cd66b9c ("net-device: reorganize net_device fast path variables")
> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> > Cc: Coco Li <lixiaoyan@...gle.com>
> > Cc: David Ahern <dsahern@...nel.org>
>
> Thanks Eric,
>
> FWIIW, this change looks good to me.
>
> Reviewed-by: Simon Horman <horms@...nel.org>
>
> I have a follow-up question below.
>
> ...
>
> > diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> > index 75c7725e5e4fdf59da55923cd803e084956b0fa0..5d1ec780122919c31e4215358d736aef3f8a0acd 100644
> > --- a/include/linux/netdevice.h
> > +++ b/include/linux/netdevice.h
> > @@ -2114,6 +2114,7 @@ struct net_device {
> >       const struct net_device_ops *netdev_ops;
> >       const struct header_ops *header_ops;
> >       struct netdev_queue     *_tx;
> > +     netdev_features_t       gso_partial_features;
> >       unsigned int            real_num_tx_queues;
> >       unsigned int            gso_max_size;
> >       unsigned int            gso_ipv4_max_size;
>
> While looking at this I came to wonder if it would
> be worth adding a 16bit pad a little below this hunk
> so that tc_to_txq sits on it's own cacheline.
>

Hi Simon, thanks for the suggestion.

I am still working on struct net_device layout (after being off for
last ~10 days), I will try to address your feedback soon.


> I'm unsure if the access pattern of tc_to_txq makes this worthwhile.
> But if so it would be a simple tweak.
>
> With such a change in place, on top of your patch, the diff of pahole output
> on x86_64 is:
>
> @@ -7432,10 +7432,9 @@
>         s16                        num_tc;               /*    54     2 */
>         unsigned int               mtu;                  /*    56     4 */
>         short unsigned int         needed_headroom;      /*    60     2 */
> -       struct netdev_tc_txq       tc_to_txq[16];        /*    62    64 */
> -
> -       /* XXX 2 bytes hole, try to pack */
> -
> +       u16                        pad1;                 /*    62     2 */
> +       /* --- cacheline 1 boundary (64 bytes) --- */
> +       struct netdev_tc_txq       tc_to_txq[16];        /*    64    64 */
>         /* --- cacheline 2 boundary (128 bytes) --- */
>         struct xps_dev_maps *      xps_maps[2];          /*   128    16 */
>         struct nf_hook_entries *   nf_hooks_egress;      /*   144     8 */