linux-kernel - Re: [PATCH v2 1/1] gro: decrease size of CB

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Date:   Wed, 31 May 2023 17:15:58 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Richard Gobert <richardbgobert@...il.com>
Cc:     davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com,
        lixiaoyan@...gle.com, alexanderduyck@...com, lucien.xin@...il.com,
        linyunsheng@...wei.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] gro: decrease size of CB

On Wed, May 31, 2023 at 4:30 PM Richard Gobert <richardbgobert@...il.com> wrote:
>
> The GRO control block (NAPI_GRO_CB) is currently at its maximum size.
> This commit reduces its size by putting two groups of fields that are
> used only at different times into a union.
>
> Specifically, the fields frag0 and frag0_len are the fields that make up
> the frag0 optimisation mechanism, which is used during the initial
> parsing of the SKB.
>
> The fields last and age are used after the initial parsing, while the
> SKB is stored in the GRO list, waiting for other packets to arrive.
>
> There was one location in dev_gro_receive that modified the frag0 fields
> after setting last and age. I changed this accordingly without altering
> the code behaviour.
>
> Signed-off-by: Richard Gobert <richardbgobert@...il.com>
> ---
>  include/net/gro.h | 26 ++++++++++++++++----------
>  net/core/gro.c    | 18 +++++++++++-------
>  2 files changed, 27 insertions(+), 17 deletions(-)
>
> diff --git a/include/net/gro.h b/include/net/gro.h
> index a4fab706240d..7b47dd6ce94f 100644
> --- a/include/net/gro.h
> +++ b/include/net/gro.h
> @@ -11,11 +11,23 @@
>  #include <net/udp.h>
>
>  struct napi_gro_cb {
> -       /* Virtual address of skb_shinfo(skb)->frags[0].page + offset. */
> -       void    *frag0;
> +       union {
> +               struct {
> +                       /* Virtual address of skb_shinfo(skb)->frags[0].page + offset. */
> +                       void    *frag0;
>
> -       /* Length of frag0. */
> -       unsigned int frag0_len;
> +                       /* Length of frag0. */
> +                       unsigned int frag0_len;
> +               };
> +
> +               struct {
> +                       /* used in skb_gro_receive() slow path */
> +                       struct sk_buff *last;
> +
> +                       /* jiffies when first packet was created/queued */
> +                       unsigned long age;
> +               };
> +       };
>
>         /* This indicates where we are processing relative to skb->data. */
>         int     data_offset;
> @@ -32,9 +44,6 @@ struct napi_gro_cb {
>         /* Used in ipv6_gro_receive() and foo-over-udp */
>         u16     proto;
>
> -       /* jiffies when first packet was created/queued */
> -       unsigned long age;
> -
>  /* Used in napi_gro_cb::free */
>  #define NAPI_GRO_FREE             1
>  #define NAPI_GRO_FREE_STOLEN_HEAD 2
> @@ -77,9 +86,6 @@ struct napi_gro_cb {
>
>         /* used to support CHECKSUM_COMPLETE for tunneling protocols */
>         __wsum  csum;
> -
> -       /* used in skb_gro_receive() slow path */
> -       struct sk_buff *last;
>  };
>
>  #define NAPI_GRO_CB(skb) ((struct napi_gro_cb *)(skb)->cb)
> diff --git a/net/core/gro.c b/net/core/gro.c
> index 2d84165cb4f1..c6955ef9ca99 100644
> --- a/net/core/gro.c
> +++ b/net/core/gro.c
> @@ -460,6 +460,14 @@ static void gro_pull_from_frag0(struct sk_buff *skb, int grow)
>         }
>  }
>
> +static void gro_try_pull_from_frag0(struct sk_buff *skb)
> +{
> +       int grow = skb_gro_offset(skb) - skb_headlen(skb);
> +
> +       if (grow > 0)
> +               gro_pull_from_frag0(skb, grow);
> +}
> +
>  static void gro_flush_oldest(struct napi_struct *napi, struct list_head *head)
>  {
>         struct sk_buff *oldest;
> @@ -489,7 +497,6 @@ static enum gro_result dev_gro_receive(struct napi_struct *napi, struct sk_buff
>         struct sk_buff *pp = NULL;
>         enum gro_result ret;
>         int same_flow;
> -       int grow;
>
>         if (netif_elide_gro(skb->dev))
>                 goto normal;
> @@ -564,17 +571,13 @@ static enum gro_result dev_gro_receive(struct napi_struct *napi, struct sk_buff
>         else
>                 gro_list->count++;
>

Could you add a comment here or in front of gro_try_pull_from_frag0() ?

/* Must be called before setting NAPI_GRO_CB(skb)->{age|last} (/

> +       gro_try_pull_from_frag0(skb);
>         NAPI_GRO_CB(skb)->age = jiffies;
>         NAPI_GRO_CB(skb)->last = skb;
>         if (!skb_is_gso(skb))
>                 skb_shinfo(skb)->gso_size = skb_gro_len(skb);
>         list_add(&skb->list, &gro_list->list);
>         ret = GRO_HELD;

Thanks.