iocost_monitor: exit successfully if interval is zero

to #29361128

commit f4fe3ea636385a51f1dfbb27c387a04b12b919e9 upstream.

This is to help external tools to decide whether iocost_monitor has all its
requirements met or not based on the exit status of an -i0 run.
Signed-off-by: NTejun Heo <tj@kernel.org>
Signed-off-by: NJens Axboe <axboe@kernel.dk>
Signed-off-by: NJiufei Xue <jiufei.xue@linux.alibaba.com>
Reviewed-by: NJoseph Qi <joseph.qi@linux.alibaba.com>

tools/cgroup/iocost_monitor.py

@@ -28,7 +28,8 @@ parser.add_argument('devname', metavar='DEV',
 parser.add_argument('--cgroup', action='append', metavar='REGEX',
                     help='Regex for target cgroups, ')
 parser.add_argument('--interval', '-i', metavar='SECONDS', type=float, default=1,
-                    help='Monitoring interval in seconds')
+                    help='Monitoring interval in seconds (0 exits immediately '
+                    'after checking requirements)')
 parser.add_argument('--json', action='store_true',
                     help='Output in json')
 args = parser.parse_args()
@@ -243,6 +244,9 @@ for i, ptr in radix_tree_for_each(blkcg_root.blkg_tree.address_of_()):
 if ioc is None:
     err(f'Could not find ioc for {devname}');
 
+if interval == 0:
+    sys.exit(0)
+
 # Keep printing
 while True:
     now = time.time()