selftests/bpf: Fix possible/online index mismatch in perf_buffer test

The perf_buffer fails on system with offline cpus:

  # test_progs -t perf_buffer
  serial_test_perf_buffer:PASS:nr_cpus 0 nsec
  serial_test_perf_buffer:PASS:nr_on_cpus 0 nsec
  serial_test_perf_buffer:PASS:skel_load 0 nsec
  serial_test_perf_buffer:PASS:attach_kprobe 0 nsec
  serial_test_perf_buffer:PASS:perf_buf__new 0 nsec
  serial_test_perf_buffer:PASS:epoll_fd 0 nsec
  skipping offline CPU #4
  serial_test_perf_buffer:PASS:perf_buffer__poll 0 nsec
  serial_test_perf_buffer:PASS:seen_cpu_cnt 0 nsec
  serial_test_perf_buffer:PASS:buf_cnt 0 nsec
  ...
  serial_test_perf_buffer:PASS:fd_check 0 nsec
  serial_test_perf_buffer:PASS:drain_buf 0 nsec
  serial_test_perf_buffer:PASS:consume_buf 0 nsec
  serial_test_perf_buffer:FAIL:cpu_seen cpu 5 not seen
  #88 perf_buffer:FAIL
  Summary: 0/0 PASSED, 0 SKIPPED, 1 FAILED

If the offline cpu is from the middle of the possible set,
we get mismatch with possible and online cpu buffers.

The perf buffer test calls perf_buffer__consume_buffer for
all 'possible' cpus, but the library holds only 'online'
cpu buffers and perf_buffer__consume_buffer returns them
based on index.

Adding extra (online) index to keep track of online buffers,
we need the original (possible) index to trigger trace on
proper cpu.
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20211021114132.8196-3-jolsa@kernel.org

tools/testing/selftests/bpf/prog_tests/perf_buffer.c

@@ -45,7 +45,7 @@ int trigger_on_cpu(int cpu)
 
 void serial_test_perf_buffer(void)
 {
-	int err, on_len, nr_on_cpus = 0, nr_cpus, i;
+	int err, on_len, nr_on_cpus = 0, nr_cpus, i, j;
 	struct perf_buffer_opts pb_opts = {};
 	struct test_perf_buffer *skel;
 	cpu_set_t cpu_seen;
@@ -111,15 +111,15 @@ void serial_test_perf_buffer(void)
 		  "got %zu, expected %d\n", perf_buffer__buffer_cnt(pb), nr_on_cpus))
 		goto out_close;
 
-	for (i = 0; i < nr_cpus; i++) {
+	for (i = 0, j = 0; i < nr_cpus; i++) {
 		if (i >= on_len || !online[i])
 			continue;
 
-		fd = perf_buffer__buffer_fd(pb, i);
+		fd = perf_buffer__buffer_fd(pb, j);
 		CHECK(fd < 0 || last_fd == fd, "fd_check", "last fd %d == fd %d\n", last_fd, fd);
 		last_fd = fd;
 
-		err = perf_buffer__consume_buffer(pb, i);
+		err = perf_buffer__consume_buffer(pb, j);
 		if (CHECK(err, "drain_buf", "cpu %d, err %d\n", i, err))
 			goto out_close;
 
@@ -127,12 +127,13 @@ void serial_test_perf_buffer(void)
 		if (trigger_on_cpu(i))
 			goto out_close;
 
-		err = perf_buffer__consume_buffer(pb, i);
-		if (CHECK(err, "consume_buf", "cpu %d, err %d\n", i, err))
+		err = perf_buffer__consume_buffer(pb, j);
+		if (CHECK(err, "consume_buf", "cpu %d, err %d\n", j, err))
 			goto out_close;
 
 		if (CHECK(!CPU_ISSET(i, &cpu_seen), "cpu_seen", "cpu %d not seen\n", i))
 			goto out_close;
+		j++;
 	}
 
 out_free_pb: