On Fri, Mar 25, 2016 at 3:29 PM, Eric Dumazet <eduma...@google.com> wrote:
> We want a generic way to insert an RCU grace period before socket
> freeing for cases where RCU_SLAB_DESTROY_BY_RCU is adding too
> much overhead.
>
> SLAB_DESTROY_BY_RCU strict rules force us to take a reference
> on the socket sk_refcnt, and it is a performance problem for UDP
> encapsulation, or TCP synflood behavior, as many CPUs might
> attempt the atomic operations on a shared sk_refcnt
>
> UDP sockets and TCP listeners can set SOCK_RCU_FREE so that their
> lookup can use traditional RCU rules, without refcount changes.
> They can set the flag only once hashed and visible by other cpus.
>
> Signed-off-by: Eric Dumazet <eduma...@google.com>
> Cc: Tom Herbert <t...@herbertland.com>
> ---
> include/net/sock.h | 2 ++
> net/core/sock.c | 14 +++++++++++++-
> 2 files changed, 15 insertions(+), 1 deletion(-)
>
> diff --git a/include/net/sock.h b/include/net/sock.h
> index 255d3e03727b..c88785a3e76c 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -438,6 +438,7 @@ struct sock {
> struct sk_buff *skb);
> void (*sk_destruct)(struct sock *sk);
> struct sock_reuseport __rcu *sk_reuseport_cb;
> + struct rcu_head sk_rcu;
> };
>
> #define __sk_user_data(sk) ((*((void __rcu **)&(sk)->sk_user_data)))
> @@ -720,6 +721,7 @@ enum sock_flags {
> */
> SOCK_FILTER_LOCKED, /* Filter cannot be changed anymore */
> SOCK_SELECT_ERR_QUEUE, /* Wake select on error queue */
> + SOCK_RCU_FREE, /* wait rcu grace period in sk_destruct() */
> };
>
> #define SK_FLAGS_TIMESTAMP ((1UL << SOCK_TIMESTAMP) | (1UL <<
> SOCK_TIMESTAMPING_RX_SOFTWARE))
> diff --git a/net/core/sock.c b/net/core/sock.c
> index b67b9aedb230..238a94f879ca 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -1418,8 +1418,12 @@ struct sock *sk_alloc(struct net *net, int family,
> gfp_t priority,
> }
> EXPORT_SYMBOL(sk_alloc);
>
> -void sk_destruct(struct sock *sk)
> +/* Sockets having SOCK_RCU_FREE will call this function after one RCU
> + * grace period. This is the case for UDP sockets and TCP listeners.
> + */
> +static void __sk_destruct(struct rcu_head *head)
> {
> + struct sock *sk = container_of(head, struct sock, sk_rcu);
> struct sk_filter *filter;
>
> if (sk->sk_destruct)
> @@ -1448,6 +1452,14 @@ void sk_destruct(struct sock *sk)
> sk_prot_free(sk->sk_prot_creator, sk);
> }
>
> +void sk_destruct(struct sock *sk)
> +{
> + if (sock_flag(sk, SOCK_RCU_FREE))
> + call_rcu(&sk->sk_rcu, __sk_destruct);
> + else
> + __sk_destruct(&sk->sk_rcu);
> +}
> +
Very nice!
> static void __sk_free(struct sock *sk)
> {
> if (unlikely(sock_diag_has_destroy_listeners(sk) &&
> sk->sk_net_refcnt))
> --
> 2.8.0.rc3.226.g39d4020
>
