bpf: Selftests, improve test_sockmap total bytes counter

The recv thread in test_sockmap waits to receive all bytes from sender but
in the case we use pop data it may wait for more bytes then actually being
sent. This stalls the test harness for multiple seconds. Because this
happens in multiple tests it slows time to run the selftest.

Fix by doing a better job of accounting for total bytes when pop helpers
are used.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/bpf/158939726542.15176.5964532245173539540.stgit@john-Precision-5820-Tower

tools/testing/selftests/bpf/test_sockmap.c

@@ -502,9 +502,10 @@ static int msg_loop(int fd, int iov_count, int iov_length, int cnt,
 		 * paths.
 		 */
 		total_bytes = (float)iov_count * (float)iov_length * (float)cnt;
-		txmsg_pop_total = txmsg_pop;
 		if (txmsg_apply)
-			txmsg_pop_total *= (total_bytes / txmsg_apply);
+			txmsg_pop_total = txmsg_pop * (total_bytes / txmsg_apply);
+		else
+			txmsg_pop_total = txmsg_pop * cnt;
 		total_bytes -= txmsg_pop_total;
 		err = clock_gettime(CLOCK_MONOTONIC, &s->start);
 		if (err < 0)
@@ -638,8 +639,12 @@ static int sendmsg_test(struct sockmap_options *opt)
 
 	rxpid = fork();
 	if (rxpid == 0) {
+		iov_buf -= (txmsg_pop - txmsg_start_pop + 1);
 		if (opt->drop_expected)
-			exit(0);
+			_exit(0);
+
+		if (!iov_buf) /* zero bytes sent case */
+			_exit(0);
 
 		if (opt->sendpage)
 			iov_count = 1;