Set SO_BUSY_POLL (busy_poll_us) on each paired socket via
bpf_setsockopt() so the splice receiver busy-polls the ring instead of
parking - without net.core.busy_read or an application setsockopt.

The sock_ops prog runs for both the active and passive established
callbacks, so each endpoint sets its own socket. This is done before the
peer-not-found early return: pairing is asymmetric (only the second side
to establish finds a peer and calls bpf_sock_splice_pair), so setting it
only on the pairing side would leave the other end without busy-poll.
bpf_setsockopt acts on skops->sk; the peer sets itself on its own
callback. Busy polling is a receive-path optimization
(splice_busy_loop() in tcp_bpf_splice_recvmsg()); TCP is full-duplex so
both ends are receivers and both need it, which the per-endpoint setting
provides.

Assisted-by: Claude:claude-opus-4.8
Signed-off-by: Cong Wang <cwang@multikernel.io>
---
 .../selftests/bpf/progs/test_tcp_splice.c     | 24 +++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/test_tcp_splice.c b/tools/testing/selftests/bpf/progs/test_tcp_splice.c
index 09c7f0f9e311..da43f00046c0 100644
--- a/tools/testing/selftests/bpf/progs/test_tcp_splice.c
+++ b/tools/testing/selftests/bpf/progs/test_tcp_splice.c
@@ -9,6 +9,13 @@
 #include <bpf/bpf_helpers.h>
 #include <bpf/bpf_endian.h>
 
+#ifndef SOL_SOCKET
+#define SOL_SOCKET 1
+#endif
+#ifndef SO_BUSY_POLL
+#define SO_BUSY_POLL 46
+#endif
+
 struct flow_key {
 	__u32	saddr;
 	__u32	daddr;
@@ -29,6 +36,8 @@ void *bpf_cast_to_kern_ctx(void *obj) __ksym;
 __u32 pair_ok;
 __u32 pair_other_err;
 
+__u32 busy_poll_us;
+
 /* IPv4 only: the verifier doesn't accept memcpy from sock_ops ctx
  * because it lowers to "ctx + reg" pointer arithmetic. IPv6 support
  * would need explicit field-by-field reads of local_ip6[i] /
@@ -71,6 +80,21 @@ int sockops_splice(struct bpf_sock_ops *skops)
 	if (skops->family != 2 /* AF_INET */)
 		return 0;
 
+	/* Enable busy-poll on this socket. Both endpoints run this callback,
+	 * so each sets its own socket; this must happen here, before the
+	 * peer-not-found early return below, because pairing is asymmetric -
+	 * only the second side to establish finds a peer and calls
+	 * bpf_sock_splice_pair. Setting it only on the pairing side would
+	 * leave the other side without busy-poll. bpf_setsockopt acts on
+	 * skops->sk only - there is no variant to set the peer - but the peer
+	 * sets itself when its own ESTABLISHED callback fires.
+	 */
+	if (busy_poll_us) {
+		int us = busy_poll_us;
+
+		bpf_setsockopt(skops, SOL_SOCKET, SO_BUSY_POLL, &us, sizeof(us));
+	}
+
 	mk_key(skops, &self_key, 0);
 	mk_key(skops, &peer_key, 1);
 
-- 
2.43.0