Re: [PATCH mptcp-next] mptcp: refine mptcp_cleanup_rbuf

[Mat Martineau <mathew.j.martineau@linux.intel.com>]

On Fri, 4 Jun 2021, Paolo Abeni wrote:

> The current cleanup rbuf tries a bit too hard to avoid acquiring
> the subflow socket lock. We may end-up delaying the needed ack,
> or skip acking a blocked subflow.
>
> Address the above extending the conditions used to trigger the cleanup
> to reflect more closely what TCP does and invoking tcp_cleanup_rbuf()
> on all the active subflows.
>
> Note that we can't replicate the exact tests implemented in
> tcp_cleanup_rbuf(), as MPTCP lacks some of the required info - e.g.
> ping-pong mode.
>
> Signed-off-by: Paolo Abeni <pabeni@redhat.com>
> ---
> This addresses the low tput I mesured in tests where BHs and U/S
> processes runs on the same CPU, see scenario 4) in:
>
> https://lore.kernel.org/mptcp/9af85be18465b0f9595459764013568456453ce9.camel@redhat.com/
>
> notes:
>
> -  this does not fix completely:
>
> https://github.com/multipath-tcp/mptcp_net-next/issues/137
>
> but figures are now more stable.
>
> - while perfing all the above spotted:
>
> https://github.com/multipath-tcp/mptcp_net-next/issues/200
>
> which is bad!
>
> ---
> net/mptcp/protocol.c | 44 ++++++++++++++++----------------------------
> net/mptcp/protocol.h |  1 -
> 2 files changed, 16 insertions(+), 29 deletions(-)
>
> diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
> index f9fd905d2d3b..40e97dd6c09f 100644
> --- a/net/mptcp/protocol.c
> +++ b/net/mptcp/protocol.c
> @@ -457,34 +457,29 @@ static bool mptcp_subflow_cleanup_rbuf(struct sock *ssk)
>
> static void mptcp_cleanup_rbuf(struct mptcp_sock *msk)
> {
> -	struct sock *ack_hint = READ_ONCE(msk->ack_hint);
> 	int old_space = READ_ONCE(msk->old_wspace);
> 	struct mptcp_subflow_context *subflow;
> 	struct sock *sk = (struct sock *)msk;
> -	bool cleanup;
> +	int space =  __mptcp_space(sk);
> +	bool cleanup, rx_empty;
>
> -	/* this is a simple superset of what tcp_cleanup_rbuf() implements
> -	 * so that we don't have to acquire the ssk socket lock most of the time
> -	 * to do actually nothing
> -	 */
> -	cleanup = __mptcp_space(sk) - old_space >= max(0, old_space);
> -	if (!cleanup)
> -		return;
> +	cleanup = (space > 0) && (space >= (old_space << 1));
> +	rx_empty = !atomic_read(&sk->sk_rmem_alloc);
>
> -	/* if the hinted ssk is still active, try to use it */
> -	if (likely(ack_hint)) {
> -		mptcp_for_each_subflow(msk, subflow) {
> -			struct sock *ssk = mptcp_subflow_tcp_sock(subflow);
> +	mptcp_for_each_subflow(msk, subflow) {
> +		struct sock *ssk = mptcp_subflow_tcp_sock(subflow);
> +		const struct inet_connection_sock *icsk;
> +		const struct tcp_sock *tp = tcp_sk(sk);
>
> -			if (ack_hint == ssk && mptcp_subflow_cleanup_rbuf(ssk))
> -				return;
> -		}
> +		icsk = inet_csk(ssk);
> +		if (cleanup ||
> +		    (inet_csk_ack_scheduled(sk) &&

Is this intended to check sk or ssk?        ^^

It doesn't look like icsk_ack.pending is ever updated for the mptcp sock, 
which makes me think 'ssk' was the intent. If that's the case, note that 
the inet_csk_ack_scheduled() helper doesn't use READ_ONCE().


-Mat


> +		     ((READ_ONCE(tp->rcv_nxt) - READ_ONCE(tp->rcv_wup) >
> +		       READ_ONCE(icsk->icsk_ack.rcv_mss)) ||
> +		       (rx_empty && READ_ONCE(icsk->icsk_ack.pending) &
> +				    (ICSK_ACK_PUSHED2|ICSK_ACK_PUSHED)))))
> +			mptcp_subflow_cleanup_rbuf(ssk);
> 	}
> -
> -	/* otherwise pick the first active subflow */
> -	mptcp_for_each_subflow(msk, subflow)
> -		if (mptcp_subflow_cleanup_rbuf(mptcp_subflow_tcp_sock(subflow)))
> -			return;
> }
>
> static bool mptcp_check_data_fin(struct sock *sk)
> @@ -629,7 +624,6 @@ static bool __mptcp_move_skbs_from_subflow(struct mptcp_sock *msk,
> 			break;
> 		}
> 	} while (more_data_avail);
> -	WRITE_ONCE(msk->ack_hint, ssk);
>
> 	*bytes += moved;
> 	return done;
> @@ -1912,7 +1906,6 @@ static bool __mptcp_move_skbs(struct mptcp_sock *msk)
> 		__mptcp_update_rmem(sk);
> 		done = __mptcp_move_skbs_from_subflow(msk, ssk, &moved);
> 		mptcp_data_unlock(sk);
> -		tcp_cleanup_rbuf(ssk, moved);
>
> 		if (unlikely(ssk->sk_err))
> 			__mptcp_error_report(sk);
> @@ -1928,7 +1921,6 @@ static bool __mptcp_move_skbs(struct mptcp_sock *msk)
> 		ret |= __mptcp_ofo_queue(msk);
> 		__mptcp_splice_receive_queue(sk);
> 		mptcp_data_unlock(sk);
> -		mptcp_cleanup_rbuf(msk);
> 	}
> 	if (ret)
> 		mptcp_check_data_fin((struct sock *)msk);
> @@ -2177,9 +2169,6 @@ static void __mptcp_close_ssk(struct sock *sk, struct sock *ssk,
> 	if (ssk == msk->last_snd)
> 		msk->last_snd = NULL;
>
> -	if (ssk == msk->ack_hint)
> -		msk->ack_hint = NULL;
> -
> 	if (ssk == msk->first)
> 		msk->first = NULL;
>
> @@ -2394,7 +2383,6 @@ static int __mptcp_init_sock(struct sock *sk)
> 	msk->rmem_released = 0;
> 	msk->tx_pending_data = 0;
>
> -	msk->ack_hint = NULL;
> 	msk->first = NULL;
> 	inet_csk(sk)->icsk_sync_mss = mptcp_sync_mss;
> 	WRITE_ONCE(msk->csum_enabled, mptcp_is_checksum_enabled(sock_net(sk)));
> diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
> index 160c2ab09f19..9dea0734808e 100644
> --- a/net/mptcp/protocol.h
> +++ b/net/mptcp/protocol.h
> @@ -240,7 +240,6 @@ struct mptcp_sock {
> 	bool		use_64bit_ack; /* Set when we received a 64-bit DSN */
> 	bool		csum_enabled;
> 	spinlock_t	join_list_lock;
> -	struct sock	*ack_hint;
> 	struct work_struct work;
> 	struct sk_buff  *ooo_last_skb;
> 	struct rb_root  out_of_order_queue;
> -- 
> 2.26.3
>
>
>

--
Mat Martineau
Intel