Re: [PATCH 05/10] net: move destructor_arg to the front of sk_buff.

Eric Dumazet
2012-04-10T15:05:09Z

On Tue, 2012-04-10 at 15:26 +0100, Ian Campbell wrote:
> As of the previous patch we align the end (rather than the start) of the struct
> to a cache line and so, with 32 and 64 byte cache lines and the shinfo size
> increase from the next patch, the first 8 bytes of the struct end up on a
> different cache line to the rest of it so make sure it is something relatively
> unimportant to avoid hitting an extra cache line on hot operations such as
> kfree_skb.
>
> Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: Eric Dumazet <eric.dumazet@gmail.com>
> ---
> include/linux/skbuff.h | 15 ++++++++++-----
> net/core/skbuff.c | 5 ++++-
> 2 files changed, 14 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 0ad6a46..f0ae39c 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -265,6 +265,15 @@ struct ubuf_info {
> * the end of the header data, ie. at skb->end.
> */
> struct skb_shared_info {
> + /* Intermediate layers must ensure that destructor_arg
> + * remains valid until skb destructor */
> + void *destructor_arg;
> +
> + /*
> + * Warning: all fields from here until dataref are cleared in
> + * __alloc_skb()
> + *
> + */
> unsigned char nr_frags;
> __u8 tx_flags;
> unsigned short gso_size;
> @@ -276,14 +285,10 @@ struct skb_shared_info {
> __be32 ip6_frag_id;
>
> /*
> - * Warning : all fields before dataref are cleared in __alloc_skb()
> + * Warning: all fields before dataref are cleared in __alloc_skb()
> */
> atomic_t dataref;
>
> - /* Intermediate layers must ensure that destructor_arg
> - * remains valid until skb destructor */
> - void * destructor_arg;
> -
> /* must be last field, see pskb_expand_head() */
> skb_frag_t frags[MAX_SKB_FRAGS];
> };
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index d4e139e..b8a41d6 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -214,7 +214,10 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
>
> /* make sure we initialize shinfo sequentially */
> shinfo = skb_shinfo(skb);
> - memset(shinfo, 0, offsetof(struct skb_shared_info, dataref));
> +
> + memset(&shinfo->nr_frags, 0,
> + offsetof(struct skb_shared_info, dataref)
> + - offsetof(struct skb_shared_info, nr_frags));
> atomic_set(&shinfo->dataref, 1);
> kmemcheck_annotate_variable(shinfo->destructor_arg);
>

Not sure if we can do the same in build_skb()