netdev - Re: [PATCH net-next 07/10] net: group sk_backlog and sk_receive

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Message-ID: <e9695618-74c3-4189-8e2f-6b5a311c487c@kernel.org>
Date: Wed, 17 Sep 2025 09:45:41 -0600
From: David Ahern <dsahern@...nel.org>
To: Eric Dumazet <edumazet@...gle.com>, "David S . Miller"
 <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>
Cc: Simon Horman <horms@...nel.org>, Willem de Bruijn <willemb@...gle.com>,
 Kuniyuki Iwashima <kuniyu@...gle.com>, netdev@...r.kernel.org,
 eric.dumazet@...il.com
Subject: Re: [PATCH net-next 07/10] net: group sk_backlog and sk_receive_queue

On 9/16/25 10:09 AM, Eric Dumazet wrote:
> UDP receivers suffer from sk_rmem_alloc updates,
> currently sharing a cache line with fields that
> need to be read-mostly (sock_read_rx group):
> 
> 1) RFS enabled hosts read sk_napi_id
> from __udpv6_queue_rcv_skb().
> 
> 2) sk->sk_rcvbuf is read from __udp_enqueue_schedule_skb()
> 
> /* --- cacheline 3 boundary (192 bytes) --- */
> struct {
>     atomic_t           rmem_alloc;           /*  0xc0   0x4 */   // Oops
>     int                len;                  /*  0xc4   0x4 */
>     struct sk_buff *   head;                 /*  0xc8   0x8 */
>     struct sk_buff *   tail;                 /*  0xd0   0x8 */
> } sk_backlog;                                /*  0xc0  0x18 */
> __u8                       __cacheline_group_end__sock_write_rx[0]; /*  0xd8     0 */
> __u8                       __cacheline_group_begin__sock_read_rx[0]; /*  0xd8     0 */
> struct dst_entry *         sk_rx_dst;        /*  0xd8   0x8 */
> int                        sk_rx_dst_ifindex;/*  0xe0   0x4 */
> u32                        sk_rx_dst_cookie; /*  0xe4   0x4 */
> unsigned int               sk_ll_usec;       /*  0xe8   0x4 */
> unsigned int               sk_napi_id;       /*  0xec   0x4 */
> u16                        sk_busy_poll_budget;/*  0xf0   0x2 */
> u8                         sk_prefer_busy_poll;/*  0xf2   0x1 */
> u8                         sk_userlocks;     /*  0xf3   0x1 */
> int                        sk_rcvbuf;        /*  0xf4   0x4 */
> struct sk_filter *         sk_filter;        /*  0xf8   0x8 */
> 
> Move sk_error (which is less often dirtied) there.
> 
> Alternative would be to cache align sock_read_rx but
> this has more implications/risks.
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> ---
>  include/net/sock.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 

Reviewed-by: David Ahern <dsahern@...nel.org>