Re: [PATCH] net: memcg: late association of sock to memcg

[Eric Dumazet <edumazet@google.com>]

On Fri, Feb 21, 2020 at 5:05 PM Shakeel Butt <shakeelb@google.com> wrote:
>
> If a TCP socket is allocated in IRQ context or cloned from unassociated
> (i.e. not associated to a memcg) in IRQ context then it will remain
> unassociated for its whole life. Almost half of the TCPs created on the
> system are created in IRQ context, so, memory used by suck sockets will
> not be accounted by the memcg.
>
> This issue is more widespread in cgroup v1 where network memory
> accounting is opt-in but it can happen in cgroup v2 if the source socket
> for the cloning was created in root memcg.
>
> To fix the issue, just do the late association of the unassociated
> sockets at accept() time in the process context and then force charge
> the memory buffer already reserved by the socket.
>
> Signed-off-by: Shakeel Butt <shakeelb@google.com>
> ---
>  net/ipv4/inet_connection_sock.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
> index a4db79b1b643..df9c8ef024a2 100644
> --- a/net/ipv4/inet_connection_sock.c
> +++ b/net/ipv4/inet_connection_sock.c
> @@ -482,6 +482,13 @@ struct sock *inet_csk_accept(struct sock *sk, int flags, int *err, bool kern)
>                 }
>                 spin_unlock_bh(&queue->fastopenq.lock);
>         }
> +
> +       if (mem_cgroup_sockets_enabled && !newsk->sk_memcg) {
> +               mem_cgroup_sk_alloc(newsk);
> +               if (newsk->sk_memcg)
> +                       mem_cgroup_charge_skmem(newsk->sk_memcg,
> +                                       sk_mem_pages(newsk->sk_forward_alloc));

I am not sure what you  are trying to do here.

sk->sk_forward_alloc is not the total amount of memory used by a TCP socket.
It is only some part that has been reserved, but not yet consumed.

For example, every skb that has been stored in TCP receive queue or
out-of-order queue might have
used memory.

I guess that if we assume that  a not yet accepted socket can not have
any outstanding data in its transmit queue,
you need to use sk->sk_rmem_alloc as well.

To test this patch, make sure to add a delay before accept(), so that
2MB worth of data can be queued before accept() happens.

Thanks.