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

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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>
This commit is contained in:
Guopeng Zhang 2026-07-07 18:21:48 +08:00 committed by Tejun Heo
parent 451a746757
commit 97fef60258
1 changed files with 8 additions and 5 deletions
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13
Documentation/admin-guide/cgroup-v2.rst
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@ -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.