Docs/admin-guide/cgroup-v2: note blkcg_debug_stats gates io.latency stats

The io.stat section says that enabling the io.latency controller exposes
the depth, avg_lat and win stats in addition to the normal ones. However,
these io.latency-specific stats are debug stats and are only emitted when
the blkcg_debug_stats module parameter is enabled, which is disabled by
default.

Make this explicit so users do not expect these fields to appear in
io.stat by default, and qualify the usage text that suggests using
avg_lat to pick an io.latency target.

Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: Guopeng Zhang

Documentation/admin-guide/cgroup-v2.rst

@@ -2257,10 +2257,11 @@ groups D and F will influence each other.  Group G will influence nobody::
 So the ideal way to configure this is to set io.latency in groups A, B, and C.
 Generally you do not want to set a value lower than the latency your device
 supports.  Experiment to find the value that works best for your workload.
-Start at higher than the expected latency for your device and watch the
-avg_lat value in io.stat for your workload group to get an idea of the
-latency you see during normal operation.  Use the avg_lat value as a basis for
-your real setting, setting at 10-15% higher than the value in io.stat.
+Start at higher than the expected latency for your device and, with
+blkcg_debug_stats enabled, watch the avg_lat value in io.stat for your
+workload group to get an idea of the latency you see during normal operation.
+Use the avg_lat value as a basis for your real setting, setting at 10-15%
+higher than the value in io.stat.
 
 How IO Latency Throttling Works
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -2298,7 +2299,9 @@ IO Latency Interface Files
 
   io.stat
 	If the controller is enabled you will see extra stats in io.stat in
-	addition to the normal ones.
+	addition to the normal ones.  These debug stats are only emitted when
+	the blkcg_debug_stats module parameter is enabled (it is disabled by
+	default).
 
 	  depth
 		This is the current queue depth for the group.