[mptcp-next,1/2] mptcp: implement delegated action infrastructure

On MPTCP-level ack reception, the packet scheduler
may select a subflow other then the current one.

Prior to this commit we rely on the workqueue to trigger
action on such subflow.

This changeset introduce an infrastructure that allows
any MPTCP subflow to schedule actions (MPTCP xmit) on
others subflows without resorting to (multiple) process
reschedule.

A dummy NAPI instance is used instead. When MPTCP needs to
trigger action an a different subflow, it enqueues the target
subflow on the NAPI backlog and schedule such instance as needed.

The dummy NAPI poll method walk the sockets backlog and try
to acquire the (BH) socket lock on each of them. If the socket
is owned by the user space, the action will be completed by
the sock release cb, otherwise push is started.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
help with the commit prose to make this change
more upstream-palatable more then welcome! ;)

RFC -> v1
 - use dummy device
 - move override relate stuff into subflow.c
 - incoming sock fallback at MPC time need undo override
 - deferred -> delegated
---
 net/mptcp/protocol.c | 59 ++++++++++++++++++++++++++++++++++++++++++++
 net/mptcp/protocol.h | 52 ++++++++++++++++++++++++++++++++++++++
 net/mptcp/subflow.c  | 39 +++++++++++++++++++++++++++++
 3 files changed, 150 insertions(+)

On Tue, 12 Jan 2021, Paolo Abeni wrote:

> On MPTCP-level ack reception, the packet scheduler
> may select a subflow other then the current one.
>
> Prior to this commit we rely on the workqueue to trigger
> action on such subflow.
>
> This changeset introduce an infrastructure that allows
> any MPTCP subflow to schedule actions (MPTCP xmit) on
> others subflows without resorting to (multiple) process
> reschedule.
>
> A dummy NAPI instance is used instead. When MPTCP needs to
> trigger action an a different subflow, it enqueues the target
> subflow on the NAPI backlog and schedule such instance as needed.
>
> The dummy NAPI poll method walk the sockets backlog and try
> to acquire the (BH) socket lock on each of them. If the socket
> is owned by the user space, the action will be completed by
> the sock release cb, otherwise push is started.
>
> Signed-off-by: Paolo Abeni <pabeni@redhat.com>
> ---
> help with the commit prose to make this change
> more upstream-palatable more then welcome! ;)
>
> RFC -> v1
> - use dummy device
> - move override relate stuff into subflow.c
> - incoming sock fallback at MPC time need undo override
> - deferred -> delegated

Hi Paolo -

Overall, I'm fine with this approach. I don't have a feel for how others 
will see the "upstream-palatability", but I'll take Florian's word for it! 
The net_device isn't small so could be a possible source of pushback, but 
there's just one instance.

> ---
> net/mptcp/protocol.c | 59 ++++++++++++++++++++++++++++++++++++++++++++
> net/mptcp/protocol.h | 52 ++++++++++++++++++++++++++++++++++++++
> net/mptcp/subflow.c  | 39 +++++++++++++++++++++++++++++
> 3 files changed, 150 insertions(+)
>
> diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
> index d538f5a6810c..bdc37fecd912 100644
> --- a/net/mptcp/protocol.c
> +++ b/net/mptcp/protocol.c
> @@ -45,6 +45,9 @@ static struct percpu_counter mptcp_sockets_allocated;
> static void __mptcp_destroy_sock(struct sock *sk);
> static void __mptcp_check_send_data_fin(struct sock *sk);
>
> +DEFINE_PER_CPU(struct mptcp_delegated_action, mptcp_delegated_actions);
> +static struct net_device mptcp_napi_dev;
> +
> /* If msk has an initial subflow socket, and the MP_CAPABLE handshake has not
>  * completed yet or has failed, return the subflow socket.
>  * Otherwise return NULL.
> @@ -2957,6 +2960,20 @@ static void mptcp_release_cb(struct sock *sk)
> 	}
> }
>
> +void mptcp_subflow_process_delegated(struct sock *ssk)
> +{
> +	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(ssk);
> +	struct sock *sk = subflow->conn;
> +
> +	mptcp_data_lock(sk);
> +	if (!sock_owned_by_user(sk))
> +		__mptcp_subflow_push_pending(sk, ssk);
> +	else
> +		set_bit(MPTCP_PUSH_PENDING, &mptcp_sk(sk)->flags);
> +	mptcp_data_unlock(sk);
> +	mptcp_subflow_delegated_done(subflow);
> +}

There are a couple of bits of code (like this) that include some more of 
the MPTCP logic, compared to just the infrastructure mentioned in the 
commit message. The second patch is fairly small, does it make more sense 
to squash the two? Or is it better to be more strict about separating the 
infrastructure from the packet scheduling code? I don't think it's a big 
deal, just trying to keep the patches close to the descriptions and 
(hopefully) bisectable.

Thanks,

Mat

> +
> static int mptcp_hash(struct sock *sk)
> {
> 	/* should never be called,
> @@ -3373,13 +3390,55 @@ static struct inet_protosw mptcp_protosw = {
> #define MPTCP_USE_SLAB		1
> #endif
>
> +static int mptcp_napi_poll(struct napi_struct *napi, int budget)
> +{
> +	struct mptcp_delegated_action *delegated;
> +	struct mptcp_subflow_context *subflow;
> +	int work_done = 0;
> +
> +	delegated = container_of(napi, struct mptcp_delegated_action, napi);
> +	while ((subflow = mptcp_subflow_delegated_next(delegated)) != NULL) {
> +		struct sock *ssk = mptcp_subflow_tcp_sock(subflow);
> +
> +		bh_lock_sock_nested(ssk);
> +		if (!sock_owned_by_user(ssk))
> +			mptcp_subflow_process_delegated(ssk);
> +
> +		/* if the sock is locked the delegated status will be cleared
> +		 * by tcp_release_cb_override
> +		 */
> +		bh_unlock_sock(ssk);
> +
> +		if (++work_done == budget)
> +			return budget;
> +	}
> +
> +	/* always provide a 0 'work_done' argument, so that napi_complete_done
> +	 * will not try accessing the NULL napi->dev ptr
> +	 */
> +	napi_complete_done(napi, 0);
> +	return work_done;
> +}
> +
> void __init mptcp_proto_init(void)
> {
> +	struct mptcp_delegated_action *delegated;
> +	int cpu;
> +
> 	mptcp_prot.h.hashinfo = tcp_prot.h.hashinfo;
>
> 	if (percpu_counter_init(&mptcp_sockets_allocated, 0, GFP_KERNEL))
> 		panic("Failed to allocate MPTCP pcpu counter\n");
>
> +	init_dummy_netdev(&mptcp_napi_dev);
> +	for_each_possible_cpu(cpu) {
> +		delegated = per_cpu_ptr(&mptcp_delegated_actions, cpu);
> +		INIT_LIST_HEAD(&delegated->head);
> +		netif_tx_napi_add(&mptcp_napi_dev, &delegated->napi, mptcp_napi_poll,
> +				  NAPI_POLL_WEIGHT);
> +		napi_enable(&delegated->napi);
> +	}
> +
> 	mptcp_subflow_init();
> 	mptcp_pm_init();
> 	mptcp_token_init();
> diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
> index 871534df6140..7fa2ac3f2f4e 100644
> --- a/net/mptcp/protocol.h
> +++ b/net/mptcp/protocol.h
> @@ -378,6 +378,13 @@ enum mptcp_data_avail {
> 	MPTCP_SUBFLOW_OOO_DATA
> };
>
> +struct mptcp_delegated_action {
> +	struct napi_struct napi;
> +	struct list_head head;
> +};
> +
> +DECLARE_PER_CPU(struct mptcp_delegated_action, mptcp_delegated_actions);
> +
> /* MPTCP subflow context */
> struct mptcp_subflow_context {
> 	struct	list_head node;/* conn_list of subflows */
> @@ -415,6 +422,9 @@ struct mptcp_subflow_context {
> 	u8	local_id;
> 	u8	remote_id;
>
> +	long	delegated_status;
> +	struct	list_head delegated_node;
> +
> 	struct	sock *tcp_sock;	    /* tcp sk backpointer */
> 	struct	sock *conn;	    /* parent mptcp_sock */
> 	const	struct inet_connection_sock_af_ops *icsk_af_ops;
> @@ -463,6 +473,48 @@ static inline void mptcp_add_pending_subflow(struct mptcp_sock *msk,
> 	spin_unlock_bh(&msk->join_list_lock);
> }
>
> +void mptcp_subflow_process_delegated(struct sock *ssk);
> +
> +static inline void mptcp_subflow_delegate(struct mptcp_subflow_context *subflow)
> +{
> +	struct mptcp_delegated_action *delegated;
> +	bool schedule;
> +
> +	if (!test_and_set_bit(1, &subflow->delegated_status)) {
> +		local_bh_disable();
> +		delegated = this_cpu_ptr(&mptcp_delegated_actions);
> +		schedule = list_empty(&delegated->head);
> +		list_add_tail(&subflow->delegated_node, &delegated->head);
> +		if (schedule)
> +			napi_schedule(&delegated->napi);
> +		local_bh_enable();
> +	}
> +}
> +
> +static inline struct mptcp_subflow_context *
> +mptcp_subflow_delegated_next(struct mptcp_delegated_action *delegated)
> +{
> +	struct mptcp_subflow_context *ret;
> +
> +	if (list_empty(&delegated->head))
> +		return NULL;
> +
> +	ret = list_first_entry(&delegated->head, struct mptcp_subflow_context, delegated_node);
> +	list_del_init(&ret->delegated_node);
> +	return ret;
> +}
> +
> +static inline bool mptcp_subflow_has_delegated_action(const struct mptcp_subflow_context *subflow)
> +{
> +	return !test_bit(1, &subflow->delegated_status);
> +}
> +
> +static inline void mptcp_subflow_delegated_done(struct mptcp_subflow_context *subflow)
> +{
> +	clear_bit(1, &subflow->delegated_status);
> +	list_del_init(&subflow->delegated_node);
> +}
> +
> int mptcp_is_enabled(struct net *net);
> unsigned int mptcp_get_add_addr_timeout(struct net *net);
> void mptcp_subflow_fully_established(struct mptcp_subflow_context *subflow,
> diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
> index 31cc362a4638..0a34d21e4d1f 100644
> --- a/net/mptcp/subflow.c
> +++ b/net/mptcp/subflow.c
> @@ -18,6 +18,7 @@
> #include <net/tcp.h>
> #if IS_ENABLED(CONFIG_MPTCP_IPV6)
> #include <net/ip6_route.h>
> +#include <net/transp_v6.h>
> #endif
> #include <net/mptcp.h>
> #include <uapi/linux/mptcp.h>
> @@ -428,6 +429,7 @@ static int subflow_v4_conn_request(struct sock *sk, struct sk_buff *skb)
> static struct tcp_request_sock_ops subflow_request_sock_ipv6_ops;
> static struct inet_connection_sock_af_ops subflow_v6_specific;
> static struct inet_connection_sock_af_ops subflow_v6m_specific;
> +static struct proto tcpv6_prot_override;
>
> static int subflow_v6_conn_request(struct sock *sk, struct sk_buff *skb)
> {
> @@ -509,6 +511,14 @@ static void subflow_ulp_fallback(struct sock *sk,
> 	icsk->icsk_ulp_ops = NULL;
> 	rcu_assign_pointer(icsk->icsk_ulp_data, NULL);
> 	tcp_sk(sk)->is_mptcp = 0;
> +
> +	/* undo override */
> +#if IS_ENABLED(CONFIG_MPTCP_IPV6)
> +	if (sk->sk_prot == &tcpv6_prot_override)
> +		sk->sk_prot = &tcpv6_prot;
> +	else
> +#endif
> +		sk->sk_prot = &tcp_prot;
> }
>
> static void subflow_drop_ctx(struct sock *ssk)
> @@ -682,6 +692,7 @@ static struct sock *subflow_syn_recv_sock(const struct sock *sk,
> }
>
> static struct inet_connection_sock_af_ops subflow_specific;
> +static struct proto tcp_prot_override;
>
> enum mapping_status {
> 	MAPPING_OK,
> @@ -1206,6 +1217,16 @@ static void mptcp_attach_cgroup(struct sock *parent, struct sock *child)
> #endif /* CONFIG_SOCK_CGROUP_DATA */
> }
>
> +static void mptcp_subflow_ops_override(struct sock *ssk)
> +{
> +#if IS_ENABLED(CONFIG_MPTCP_IPV6)
> +	if (ssk->sk_prot == &tcpv6_prot)
> +		ssk->sk_prot = &tcpv6_prot_override;
> +	else
> +#endif
> +		ssk->sk_prot = &tcp_prot_override;
> +}
> +
> int mptcp_subflow_create_socket(struct sock *sk, struct socket **new_sock)
> {
> 	struct mptcp_subflow_context *subflow;
> @@ -1261,6 +1282,7 @@ int mptcp_subflow_create_socket(struct sock *sk, struct socket **new_sock)
> 	*new_sock = sf;
> 	sock_hold(sk);
> 	subflow->conn = sk;
> +	mptcp_subflow_ops_override(sf->sk);
>
> 	return 0;
> }
> @@ -1277,6 +1299,7 @@ static struct mptcp_subflow_context *subflow_create_ctx(struct sock *sk,
>
> 	rcu_assign_pointer(icsk->icsk_ulp_data, ctx);
> 	INIT_LIST_HEAD(&ctx->node);
> +	INIT_LIST_HEAD(&ctx->delegated_node);
>
> 	pr_debug("subflow=%p", ctx);
>
> @@ -1442,6 +1465,16 @@ static void subflow_ulp_clone(const struct request_sock *req,
> 	}
> }
>
> +static void tcp_release_cb_override(struct sock *ssk)
> +{
> +	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(ssk);
> +
> +	if (mptcp_subflow_has_delegated_action(subflow))
> +		mptcp_subflow_process_delegated(ssk);
> +
> +	tcp_release_cb(ssk);
> +}
> +
> static struct tcp_ulp_ops subflow_ulp_ops __read_mostly = {
> 	.name		= "mptcp",
> 	.owner		= THIS_MODULE,
> @@ -1482,6 +1515,9 @@ void __init mptcp_subflow_init(void)
> 	subflow_specific.syn_recv_sock = subflow_syn_recv_sock;
> 	subflow_specific.sk_rx_dst_set = subflow_finish_connect;
>
> +	tcp_prot_override = tcp_prot;
> +	tcp_prot_override.release_cb = tcp_release_cb_override;
> +
> #if IS_ENABLED(CONFIG_MPTCP_IPV6)
> 	subflow_request_sock_ipv6_ops = tcp_request_sock_ipv6_ops;
> 	subflow_request_sock_ipv6_ops.route_req = subflow_v6_route_req;
> @@ -1497,6 +1533,9 @@ void __init mptcp_subflow_init(void)
> 	subflow_v6m_specific.net_header_len = ipv4_specific.net_header_len;
> 	subflow_v6m_specific.mtu_reduced = ipv4_specific.mtu_reduced;
> 	subflow_v6m_specific.net_frag_header_len = 0;
> +
> +	tcpv6_prot_override = tcpv6_prot;
> +	tcpv6_prot_override.release_cb = tcp_release_cb_override;
> #endif
>
> 	mptcp_diag_subflow_init(&subflow_ulp_ops);
> -- 
> 2.26.2

--
Mat Martineau
Intel