Re: [PATCH net-next] inet: Save one atomic op if no memcg to charge

[Eric Dumazet]

On Mon, Jun 19, 2023 at 10:26 AM Abel Wu <wuyun.abel@xxxxxxxxxxxxx> wrote:
>
> If there is no net-memcg associated with the sock, don't bother
> calculating its memory usage for charge.
>
> Signed-off-by: Abel Wu <wuyun.abel@xxxxxxxxxxxxx>
> ---
>  net/ipv4/inet_connection_sock.c | 18 +++++++++++-------
>  1 file changed, 11 insertions(+), 7 deletions(-)
>
> diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
> index 65ad4251f6fd..73798282c1ef 100644
> --- a/net/ipv4/inet_connection_sock.c
> +++ b/net/ipv4/inet_connection_sock.c
> @@ -706,20 +706,24 @@ struct sock *inet_csk_accept(struct sock *sk, int flags, int *err, bool kern)
>  out:
>         release_sock(sk);
>         if (newsk && mem_cgroup_sockets_enabled) {
> -               int amt;
> +               int amt = 0;
>
>                 /* atomically get the memory usage, set and charge the
>                  * newsk->sk_memcg.
>                  */
>                 lock_sock(newsk);
>
> -               /* The socket has not been accepted yet, no need to look at
> -                * newsk->sk_wmem_queued.
> -                */
> -               amt = sk_mem_pages(newsk->sk_forward_alloc +
> -                                  atomic_read(&newsk->sk_rmem_alloc));
>                 mem_cgroup_sk_alloc(newsk);
> -               if (newsk->sk_memcg && amt)
> +               if (newsk->sk_memcg) {
> +                       /* The socket has not been accepted yet, no need
> +                        * to look at newsk->sk_wmem_queued.
> +                        */
> +                       amt = sk_mem_pages(newsk->sk_forward_alloc +
> +                                          atomic_read(&newsk->sk_rmem_alloc));
> +
> +               }
> +
> +               if (amt)
>                         mem_cgroup_charge_skmem(newsk->sk_memcg, amt,
>                                                 GFP_KERNEL | __GFP_NOFAIL);

This looks correct, but claiming reading an atomic_t is an 'atomic op'
is a bit exaggerated.