selftests: ublk: improve I/O ordering test with bpftrace

Remove test_generic_01.sh since block layer may reorder I/O, making
the test prone to false positives. Apply the improvements to
test_generic_02.sh instead, which supposes for covering ublk dispatch
io order.

Rework test_generic_02 to verify that ublk dispatch doesn't reorder I/O
by comparing request start order with completion order using bpftrace.

The bpftrace script now:
- Tracks each request's start sequence number in a map keyed by sector
- On completion, verifies the request's start order matches expected
  completion order
- Reports any out-of-order completions detected

The test script:
- Wait bpftrace BEGIN code block is run
- Pins fio to CPU 0 for deterministic behavior
- Uses block_io_start and block_rq_complete tracepoints
- Checks bpftrace output for reordering errors

Reported-and-tested-by: Alexander Atanasov <alex@zazolabs.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Ming Lei

tools/testing/selftests/ublk/Makefile

@@ -7,8 +7,7 @@ endif
 
 LDLIBS += -lpthread -lm -luring
 
-TEST_PROGS := test_generic_01.sh
-TEST_PROGS += test_generic_02.sh
+TEST_PROGS := test_generic_02.sh
 TEST_PROGS += test_generic_03.sh
 TEST_PROGS += test_generic_06.sh
 TEST_PROGS += test_generic_07.sh

tools/testing/selftests/ublk/test_generic_01.sh

@@ -13,23 +13,28 @@ if ! _have_program fio; then
 	exit "$UBLK_SKIP_CODE"
 fi
 
-_prep_test "null" "sequential io order for MQ"
+_prep_test "null" "ublk dispatch won't reorder IO for MQ"
 
 dev_id=$(_add_ublk_dev -t null -q 2)
 _check_add_dev $TID $?
 
 dev_t=$(_get_disk_dev_t "$dev_id")
 bpftrace trace/seq_io.bt "$dev_t" "W" 1 > "$UBLK_TMP" 2>&1 &
 btrace_pid=$!
-sleep 2
 
-if ! kill -0 "$btrace_pid" > /dev/null 2>&1; then
+# Wait for bpftrace probes to be attached (BEGIN block prints BPFTRACE_READY)
+for _ in $(seq 100); do
+	grep -q "BPFTRACE_READY" "$UBLK_TMP" 2>/dev/null && break
+	sleep 0.1
+done
+
+if ! kill -0 "$btrace_pid" 2>/dev/null; then
 	_cleanup_test "null"
 	exit "$UBLK_SKIP_CODE"
 fi
 
-# run fio over this ublk disk
-fio --name=write_seq \
+# run fio over this ublk disk (pinned to CPU 0)
+taskset -c 0 fio --name=write_seq \
     --filename=/dev/ublkb"${dev_id}" \
     --ioengine=libaio --iodepth=16 \
     --rw=write \
@@ -39,8 +44,11 @@ fio --name=write_seq \
 ERR_CODE=$?
 kill "$btrace_pid"
 wait
-if grep -q "io_out_of_order" "$UBLK_TMP"; then
-	cat "$UBLK_TMP"
+
+# Check for out-of-order completions detected by bpftrace
+if grep -q "^out_of_order:" "$UBLK_TMP"; then
+	echo "I/O reordering detected:"
+	grep "^out_of_order:" "$UBLK_TMP"
 	ERR_CODE=255
 fi
 _cleanup_test "null"

tools/testing/selftests/ublk/test_generic_02.sh

@@ -2,23 +2,52 @@
 	$1: 	dev_t
 	$2: 	RWBS
 	$3:     strlen($2)
+
+	Track request order between block_io_start and block_rq_complete.
+	Sequence starts at 1 so 0 means "never seen". On first valid
+	completion, sync complete_seq to handle probe attachment races.
+	block_rq_complete listed first to reduce missed completion window.
 */
+
 BEGIN {
-	@last_rw[$1, str($2)] = (uint64)0;
+	@start_seq = (uint64)1;
+	@complete_seq = (uint64)0;
+	@out_of_order = (uint64)0;
+	@start_order[0] = (uint64)0;
+	delete(@start_order[0]);
+	printf("BPFTRACE_READY\n");
 }
+
 tracepoint:block:block_rq_complete
+/(int64)args.dev == $1 && !strncmp(args.rwbs, str($2), $3)/
 {
-	$dev = $1;
-	if ((int64)args.dev == $1 && !strncmp(args.rwbs, str($2), $3)) {
-		$last = @last_rw[$dev, str($2)];
-		if ((uint64)args.sector != $last) {
-			printf("io_out_of_order: exp %llu actual %llu\n",
-				args.sector, $last);
+	$expected = @start_order[args.sector];
+	if ($expected > 0) {
+		if (@complete_seq == 0) {
+			@complete_seq = $expected;
+		}
+		if ($expected != @complete_seq) {
+			printf("out_of_order: sector %llu started at seq %llu but completed at seq %llu\n",
+				args.sector, $expected, @complete_seq);
+			@out_of_order = @out_of_order + 1;
 		}
-		@last_rw[$dev, str($2)] = (args.sector + args.nr_sector);
+		delete(@start_order[args.sector]);
+		@complete_seq = @complete_seq + 1;
 	}
 }
 
+tracepoint:block:block_io_start
+/(int64)args.dev == $1 && !strncmp(args.rwbs, str($2), $3)/
+{
+	@start_order[args.sector] = @start_seq;
+	@start_seq = @start_seq + 1;
+}
+
 END {
-	clear(@last_rw);
+	printf("total_start: %llu total_complete: %llu out_of_order: %llu\n",
+		@start_seq - 1, @complete_seq, @out_of_order);
+	clear(@start_order);
+	clear(@start_seq);
+	clear(@complete_seq);
+	clear(@out_of_order);
 }