netdev - Re: [PATCH net-next 2/3] net: move sk_drops out of sock_write

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Message-ID: <CAAVpQUBRV2jEArNrBJumKaUep4V5uL7ez2iSKgvd0L51-1Co8w@mail.gmail.com>
Date: Mon, 25 Aug 2025 13:17:21 -0700
From: Kuniyuki Iwashima <kuniyu@...gle.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, netdev@...r.kernel.org, 
	eric.dumazet@...il.com, Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next 2/3] net: move sk_drops out of sock_write_rx group

On Mon, Aug 25, 2025 at 12:59 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> Move sk_drops into a dedicated cache line.
>
> When a packet flood hits one or more sockets, many cpus
> have to update sk->sk_drops.
>
> This slows down consumers, because currently
> sk_drops is in sock_write_rx group.
>
> Moving sk->sk_drops into a dedicated cache line
> makes sure that consumers no longer suffer from
> false sharing if/when producers only change sk->sk_drops.
>
> Tested with the following stress test, sending about 11 Mpps:
>
> super_netperf 20 -t UDP_STREAM -H DUT -l10 -- -n -P,1000 -m 120
> Note: due to socket lookup, only one UDP socket will receive
> packets on DUT.
>
> Then measure receiver (DUT) behavior. We can see both
> consumer and BH handlers can process more packets per second.
>
> Before:
>
> nstat -n ; sleep 1 ; nstat | grep Udp
> Udp6InDatagrams                 615091             0.0
> Udp6InErrors                    3904277            0.0
> Udp6RcvbufErrors                3904277            0.0
>
> After:
> nstat -n ; sleep 1 ; nstat | grep Udp
> Udp6InDatagrams                 855592             0.0
> Udp6InErrors                    5621467            0.0
> Udp6RcvbufErrors                5621467            0.0
>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Reviewed-by: Kuniyuki Iwashima <kuniyu@...gle.com>


> ---
>  include/net/sock.h | 6 +++---
>  net/core/sock.c    | 1 -
>  2 files changed, 3 insertions(+), 4 deletions(-)
>
> diff --git a/include/net/sock.h b/include/net/sock.h
> index 34d7029eb622773e40e7c4ebd422d33b1c0a7836..f40e3c4883be32c8282694ab215bcf79eb87cbd7 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -390,7 +390,6 @@ struct sock {
>
>         __cacheline_group_begin(sock_write_rx);
>
> -       atomic_t                sk_drops;
>         __s32                   sk_peek_off;
>         struct sk_buff_head     sk_error_queue;
>         struct sk_buff_head     sk_receive_queue;
> @@ -564,13 +563,14 @@ struct sock {
>  #ifdef CONFIG_BPF_SYSCALL
>         struct bpf_local_storage __rcu  *sk_bpf_storage;
>  #endif
> -       struct rcu_head         sk_rcu;
> -       netns_tracker           ns_tracker;
>         struct xarray           sk_user_frags;
>
>  #if IS_ENABLED(CONFIG_PROVE_LOCKING) && IS_ENABLED(CONFIG_MODULES)
>         struct module           *sk_owner;
>  #endif
> +       atomic_t                sk_drops ____cacheline_aligned_in_smp;
> +       struct rcu_head         sk_rcu;
> +       netns_tracker           ns_tracker;
>  };
>
>  struct sock_bh_locked {
> diff --git a/net/core/sock.c b/net/core/sock.c
> index 75368823969a7992a55a6f40d87ffb8886de2f39..cd7c7ed7ff51070d20658684ff796c58c8c09995 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -4436,7 +4436,6 @@ EXPORT_SYMBOL(sk_ioctl);
>
>  static int __init sock_struct_check(void)
>  {
> -       CACHELINE_ASSERT_GROUP_MEMBER(struct sock, sock_write_rx, sk_drops);
>         CACHELINE_ASSERT_GROUP_MEMBER(struct sock, sock_write_rx, sk_peek_off);
>         CACHELINE_ASSERT_GROUP_MEMBER(struct sock, sock_write_rx, sk_error_queue);
>         CACHELINE_ASSERT_GROUP_MEMBER(struct sock, sock_write_rx, sk_receive_queue);
> --
> 2.51.0.261.g7ce5a0a67e-goog
>