[PATCH net-next] net: minor optimization in __alloc_skb()

[PATCH net-next] net: minor optimization in __alloc_skb()

[Eric Dumazet]

TCP allocates 'fast clones' skbs for packets in tx queues.

Currently, __alloc_skb() initializes the companion fclone
field to SKB_FCLONE_CLONE, and leaves other fields untouched.

It makes sense to defer this init much later in skb_clone(),
because all fclone fields are copied and hot in cpu caches
at that time.

This removes one cache line miss in __alloc_skb(), cost seen
on an host with 256 cpus all competing on memory accesses.

Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 net/core/skbuff.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index c62e42d0c5310caaa03298a779974050b8e58658..c4a7517815815d136dab5576d6b8fa52057002b1 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -454,8 +454,6 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
 
 		skb->fclone = SKB_FCLONE_ORIG;
 		refcount_set(&fclones->fclone_ref, 1);
-
-		fclones->skb2.fclone = SKB_FCLONE_CLONE;
 	}
 
 	return skb;
@@ -1513,6 +1511,7 @@ struct sk_buff *skb_clone(struct sk_buff *skb, gfp_t gfp_mask)
 	    refcount_read(&fclones->fclone_ref) == 1) {
 		n = &fclones->skb2;
 		refcount_set(&fclones->fclone_ref, 2);
+		n->fclone = SKB_FCLONE_CLONE;
 	} else {
 		if (skb_pfmemalloc(skb))
 			gfp_mask |= __GFP_MEMALLOC;
-- 
2.37.0.rc0.161.g10f37bed90-goog

Re: [PATCH net-next] net: minor optimization in __alloc_skb()

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net-next.git (master)
by David S. Miller <davem@davemloft.net>:

On Thu,  7 Jul 2022 19:18:46 +0000 you wrote:
> TCP allocates 'fast clones' skbs for packets in tx queues.
> 
> Currently, __alloc_skb() initializes the companion fclone
> field to SKB_FCLONE_CLONE, and leaves other fields untouched.
> 
> It makes sense to defer this init much later in skb_clone(),
> because all fclone fields are copied and hot in cpu caches
> at that time.
> 
> [...]

Here is the summary with links:
  - [net-next] net: minor optimization in __alloc_skb()
    https://git.kernel.org/netdev/net-next/c/c2dd4059dc31

You are awesome, thank you!
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