This patch implements the backup flag test scheduler, named bpf_bkup,
which picks the first non-backup subflow to send data.

Signed-off-by: Geliang Tang <geliang.tang@suse.com>
---
 tools/testing/selftests/bpf/bpf_tcp_helpers.h |  1 +
 .../selftests/bpf/progs/mptcp_bpf_bkup.c      | 43 +++++++++++++++++++
 2 files changed, 44 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/mptcp_bpf_bkup.c

diff --git a/tools/testing/selftests/bpf/bpf_tcp_helpers.h b/tools/testing/selftests/bpf/bpf_tcp_helpers.h
index 488de4b920ef..1f8addd61f14 100644
--- a/tools/testing/selftests/bpf/bpf_tcp_helpers.h
+++ b/tools/testing/selftests/bpf/bpf_tcp_helpers.h
@@ -234,6 +234,7 @@ extern void tcp_cong_avoid_ai(struct tcp_sock *tp, __u32 w, __u32 acked) __ksym;
 #define MPTCP_SUBFLOWS_MAX	8
 
 struct mptcp_subflow_context {
+	__u32	backup : 1;
 	struct	sock *tcp_sock;	    /* tcp sk backpointer */
 } __attribute__((preserve_access_index));
 
diff --git a/tools/testing/selftests/bpf/progs/mptcp_bpf_bkup.c b/tools/testing/selftests/bpf/progs/mptcp_bpf_bkup.c
new file mode 100644
index 000000000000..2431d5f89150
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/mptcp_bpf_bkup.c
@@ -0,0 +1,43 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2022, SUSE. */
+
+#include <linux/bpf.h>
+#include "bpf_tcp_helpers.h"
+
+char _license[] SEC("license") = "GPL";
+
+SEC("struct_ops/mptcp_sched_bkup_init")
+void BPF_PROG(mptcp_sched_bkup_init, const struct mptcp_sock *msk)
+{
+}
+
+SEC("struct_ops/mptcp_sched_bkup_release")
+void BPF_PROG(mptcp_sched_bkup_release, const struct mptcp_sock *msk)
+{
+}
+
+void BPF_STRUCT_OPS(bpf_bkup_get_subflow, const struct mptcp_sock *msk,
+		    struct mptcp_sched_data *data)
+{
+	int nr = 0;
+
+	for (int i = 0; i < MPTCP_SUBFLOWS_MAX; i++) {
+		if (!data->contexts[i])
+			break;
+
+		if (!BPF_CORE_READ_BITFIELD_PROBED(data->contexts[i], backup)) {
+			nr = i;
+			break;
+		}
+	}
+
+	bpf_mptcp_subflow_set_scheduled(data->contexts[nr]);
+}
+
+SEC(".struct_ops")
+struct mptcp_sched_ops bkup = {
+	.init		= (void *)mptcp_sched_bkup_init,
+	.release	= (void *)mptcp_sched_bkup_release,
+	.get_subflow	= (void *)bpf_bkup_get_subflow,
+	.name		= "bpf_bkup",
+};
-- 
2.34.1