selftests: xsk: Decrease sending speed

Decrease sending speed to avoid potentially overflowing some buffers
in the skb case that leads to dropped packets we cannot control (and
thus the tests may generate false negatives). Decrease batch size and
introduce a usleep in the transmit thread to not overflow the
receiver.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210825093722.10219-12-magnus.karlsson@gmail.com

tools/testing/selftests/bpf/xdpxceiver.c

@@ -637,6 +637,7 @@ static void tx_only_all(struct ifobject *ifobject)
 
 		tx_only(ifobject->xsk, &frame_nb, batch_size);
 		pkt_cnt += batch_size;
+		usleep(10);
 	}
 
 	complete_tx_only_all(ifobject);

tools/testing/selftests/bpf/xdpxceiver.h

@@ -36,7 +36,7 @@
 #define UDP_PKT_DATA_SIZE (UDP_PKT_SIZE - sizeof(struct udphdr))
 #define USLEEP_MAX 10000
 #define SOCK_RECONF_CTR 10
-#define BATCH_SIZE 64
+#define BATCH_SIZE 8
 #define POLL_TMOUT 1000
 #define DEFAULT_PKT_CNT (4 * 1024)
 #define RX_FULL_RXQSIZE 32