This patch implements the round-robin BPF MPTCP scheduler, named bpf_rr,
which always picks the next available subflow to send data. If no such
next subflow available, picks the first one.

Use the helper bpf_mptcp_get_next_subflow() to get the next available
subflow.

Signed-off-by: Geliang Tang <geliang.tang@suse.com>
---
 .../testing/selftests/bpf/bpf_mptcp_helpers.h | 27 ++++++++++++
 .../selftests/bpf/progs/mptcp_bpf_rr.c        | 42 +++++++++++++++++++
 2 files changed, 69 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/mptcp_bpf_rr.c

diff --git a/tools/testing/selftests/bpf/bpf_mptcp_helpers.h b/tools/testing/selftests/bpf/bpf_mptcp_helpers.h
index 5135eb6710e8..972fd2e56ed4 100644
--- a/tools/testing/selftests/bpf/bpf_mptcp_helpers.h
+++ b/tools/testing/selftests/bpf/bpf_mptcp_helpers.h
@@ -26,4 +26,31 @@ struct mptcp_sched_ops {
 	void *owner;
 };
 
+struct mptcp_subflow_context {
+	__u32	token;
+	__u32	request_mptcp : 1,  /* send MP_CAPABLE */
+		request_join : 1,   /* send MP_JOIN */
+		request_bkup : 1,
+		mp_capable : 1,	    /* remote is MPTCP capable */
+		mp_join : 1,	    /* remote is JOINing */
+		fully_established : 1,	    /* path validated */
+		pm_notified : 1,    /* PM hook called for established status */
+		conn_finished : 1,
+		map_valid : 1,
+		map_csum_reqd : 1,
+		map_data_fin : 1,
+		mpc_map : 1,
+		backup : 1,
+		send_mp_prio : 1,
+		send_mp_fail : 1,
+		send_fastclose : 1,
+		send_infinite_map : 1,
+		rx_eof : 1,
+		can_ack : 1,        /* only after processing the remote a key */
+		disposable : 1,	    /* ctx can be free at ulp release time */
+		stale : 1,	    /* unable to snd/rcv data, do not use for xmit */
+		local_id_valid : 1; /* local_id is correctly initialized */
+	struct	sock *tcp_sock;	    /* tcp sk backpointer */
+} __attribute__((preserve_access_index));
+
 #endif
diff --git a/tools/testing/selftests/bpf/progs/mptcp_bpf_rr.c b/tools/testing/selftests/bpf/progs/mptcp_bpf_rr.c
new file mode 100644
index 000000000000..0d0f639c8745
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/mptcp_bpf_rr.c
@@ -0,0 +1,42 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* Copyright (c) 2022, SUSE. */
+
+#include <linux/bpf.h>
+#include <linux/stddef.h>
+#include <linux/tcp.h>
+#include "bpf_mptcp_helpers.h"
+
+char _license[] SEC("license") = "GPL";
+
+extern struct mptcp_subflow_context *
+bpf_mptcp_get_next_subflow(struct mptcp_sock *msk) __ksym;
+
+SEC("struct_ops/mptcp_sched_rr_init")
+void BPF_PROG(mptcp_sched_rr_init, struct mptcp_sock *msk)
+{
+}
+
+SEC("struct_ops/mptcp_sched_rr_release")
+void BPF_PROG(mptcp_sched_rr_release, struct mptcp_sock *msk)
+{
+}
+
+struct sock *BPF_STRUCT_OPS(bpf_rr_get_subflow, struct mptcp_sock *msk)
+{
+	struct mptcp_subflow_context *subflow;
+	struct sock *ssk = msk->first;
+
+	subflow = bpf_mptcp_get_next_subflow(msk);
+	if (subflow)
+		ssk = subflow->tcp_sock;
+
+	return ssk;
+}
+
+SEC(".struct_ops")
+struct mptcp_sched_ops rr = {
+	.init		= (void *)mptcp_sched_rr_init,
+	.release	= (void *)mptcp_sched_rr_release,
+	.get_subflow	= (void *)bpf_rr_get_subflow,
+	.name		= "bpf_rr",
+};
-- 
2.34.1