Docs/admin-guide/cgroup-v2: document io.latency rotational vs non-rotational behavior

io.latency is documented only in terms of average latency and the avg_lat
stat, which matches rotational devices.  On non-rotational devices a group
misses its target once enough of the IOs in the window individually exceed
it, and io.stat reports missed/total rather than avg_lat/win.

Describe both cases: how a miss is detected, note that the avg_lat tuning
guidance is rotational-only, and update the io.stat field list (mark
avg_lat/win as rotational-only, document missed/total).

Acked-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tao Cui <cuitao@kylinos.cn>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 14b8c571c0d14251ac8befd6f67f2b42753ed6ca..aed195a71cbf1ed9c2e250d76d52c49410b07120 100644 (file)
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -2239,9 +2239,12 @@ IO Latency
 ~~~~~~~~~~
 
 This is a cgroup v2 controller for IO workload protection.  You provide a group
-with a latency target, and if the average latency exceeds that target the
-controller will throttle any peers that have a lower latency target than the
-protected workload.
+with a latency target, and if the group misses its target the controller will
+throttle any peers that have a lower latency target than the protected
+workload.  How a miss is detected depends on the device: on rotational devices
+the average latency over the window must exceed the target, while on
+non-rotational devices a miss is counted once enough of the IOs in the window
+individually exceed the target.
 
 The limits are only applied at the peer level in the hierarchy.  This means that
 in the diagram below, only groups A, B, and C will influence each other, and
@@ -2258,10 +2261,12 @@ 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, 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.
+blkcg_debug_stats enabled, observe io.stat for your workload group to get an
+idea of the latency you see during normal operation.  On rotational devices,
+use the avg_lat value as a basis for your real setting, setting it 10-15%
+higher.  On non-rotational devices io.stat reports no average latency; set
+the target based on your device and use the missed/total fields to verify it
+is being met.
 
 How IO Latency Throttling Works
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -2303,19 +2308,36 @@ IO Latency Interface Files
        the blkcg_debug_stats module parameter is enabled (it is disabled by
        default).
 
+       The reported latency fields depend on the device.  Rotational devices
+       report avg_lat and win; non-rotational devices report missed and total
+       instead.  missed and total are live counters for the current window and
+       may change between reads.
+
          depth
                This is the current queue depth for the group.
 
          avg_lat
-               This is an exponential moving average with a decay rate of 1/exp
-               bound by the sampling interval.  The decay rate interval can be
-               calculated by multiplying the win value in io.stat by the
-               corresponding number of samples based on the win value.
+               (Rotational devices only.)  This is an exponential moving
+               average with a decay rate of 1/exp bound by the sampling
+               interval.  The decay rate interval can be calculated by
+               multiplying the win value in io.stat by the corresponding number
+               of samples based on the win value.
 
          win
-               The sampling window size in milliseconds.  This is the minimum
-               duration of time between evaluation events.  Windows only elapse
-               with IO activity.  Idle periods extend the most recent window.
+               (Rotational devices only.)  The sampling window size in
+               milliseconds.  This is the minimum duration of time between
+               evaluation events.  Windows only elapse with IO activity.  Idle
+               periods extend the most recent window.
+
+         missed
+               (Non-rotational devices only.)  The number of IOs in the
+               current window whose latency exceeded the target.  A group is
+               considered to be missing its target once missed reaches a
+               certain ratio of total.
+
+         total
+               (Non-rotational devices only.)  The total number of IOs
+               accounted in the current window.
 
 IO Priority
 ~~~~~~~~~~~