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Date: Fri, 7 Apr 2017 07:28:28 -0700
From: Alexander Duyck <alexander.duyck@...il.com>
To: Steffen Klassert <steffen.klassert@...unet.com>
Cc: David Miller <davem@...emloft.net>, Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 1/1] skbuff: Extend gso_type to unsigned int.
On Mon, Apr 3, 2017 at 1:15 AM, Steffen Klassert
<steffen.klassert@...unet.com> wrote:
> All available gso_type flags are currently in use, so
> extend gso_type from 'unsigned short' to 'unsigned int'
> to be able to add further flags.
>
> We also reorder the struct skb_shared_info to use
> two bytes of the four byte hole before dataref.
> All fields before _unused are cleared now, i.e.
> two bytes more than before the change.
>
> Structure layout on x86-64 before the change:
>
> struct skb_shared_info {
> unsigned char nr_frags; /* 0 1 */
> __u8 tx_flags; /* 1 1 */
> short unsigned int gso_size; /* 2 2 */
> short unsigned int gso_segs; /* 4 2 */
> short unsigned int gso_type; /* 6 2 */
> struct sk_buff * frag_list; /* 8 8 */
> struct skb_shared_hwtstamps hwtstamps; /* 16 8 */
> u32 tskey; /* 24 4 */
> __be32 ip6_frag_id; /* 28 4 */
> atomic_t dataref; /* 32 4 */
>
> /* XXX 4 bytes hole, try to pack */
>
> void * destructor_arg; /* 40 8 */
> skb_frag_t frags[17]; /* 48 272 */
> /* --- cacheline 5 boundary (320 bytes) --- */
>
> /* size: 320, cachelines: 5, members: 12 */
> /* sum members: 316, holes: 1, sum holes: 4 */
> };
>
> Structure layout on x86-64 after the change:
>
> struct skb_shared_info {
> struct sk_buff * frag_list; /* 0 8 */
> struct skb_shared_hwtstamps hwtstamps; /* 8 8 */
> u32 tskey; /* 16 4 */
> __be32 ip6_frag_id; /* 20 4 */
> unsigned int gso_type; /* 24 4 */
> unsigned char nr_frags; /* 28 1 */
> __u8 tx_flags; /* 29 1 */
> short unsigned int gso_size; /* 30 2 */
> short unsigned int gso_segs; /* 32 2 */
> short unsigned int _unused; /* 34 2 */
> atomic_t dataref; /* 36 4 */
> void * destructor_arg; /* 40 8 */
> skb_frag_t frags[17]; /* 48 272 */
> /* --- cacheline 5 boundary (320 bytes) --- */
>
> /* size: 320, cachelines: 5, members: 13 */
> };
>
> Signed-off-by: Steffen Klassert <steffen.klassert@...unet.com>
> ---
> include/linux/skbuff.h | 13 +++++++------
> net/core/skbuff.c | 2 +-
> 2 files changed, 8 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index c776abd..cb36159 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -413,20 +413,21 @@ struct ubuf_info {
> * the end of the header data, ie. at skb->end.
> */
> struct skb_shared_info {
> + struct sk_buff *frag_list;
> + struct skb_shared_hwtstamps hwtstamps;
> + u32 tskey;
> + __be32 ip6_frag_id;
> + unsigned int gso_type;
> unsigned char nr_frags;
> __u8 tx_flags;
> unsigned short gso_size;
> /* Warning: this field is not always filled in (UFO)! */
> unsigned short gso_segs;
> - unsigned short gso_type;
> - struct sk_buff *frag_list;
> - struct skb_shared_hwtstamps hwtstamps;
> - u32 tskey;
> - __be32 ip6_frag_id;
>
> /*
> - * Warning : all fields before dataref are cleared in __alloc_skb()
> + * Warning : all fields before _unused are cleared in __alloc_skb()
> */
> + unsigned short _unused;
You can probably just put a comment about a 2 byte hole here.
In most cases the cost here is the fact that you are adding bytes
beyond 32. So if you are adding 2 bytes or 4 the cost should be about
the same for the memset since it is having to deal with something less
than or equal to a long. This all compiles out into 4 mov operations
on x86_64, and adding this regardless of adding 2, 4, or 8 will just
add one more. So adding _unused is just an unnecessary optimization.
Doing that would make the code changes below unneeded.
> atomic_t dataref;
>
> /* Intermediate layers must ensure that destructor_arg
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 9f78109..8796b93 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -257,7 +257,7 @@ 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, 0, offsetof(struct skb_shared_info, _unused));
> atomic_set(&shinfo->dataref, 1);
> kmemcheck_annotate_variable(shinfo->destructor_arg);
>
> --
> 2.7.4
>
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