cgroup, docs: cpu controller's interaction with various scheduling policies

The cpu controller interface files account for or affect processes
differently based on their scheduling policy, and the underlying
scheduler used (fair-class vs. BPF scheduler). Document these
differences

Signed-off-by: Shashank Balaji <shashank.mahadasyam@sony.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: shashank-mahadasyam

Documentation/admin-guide/cgroup-v2.rst

@@ -1095,19 +1095,34 @@ realtime processes irrespective of CONFIG_RT_GROUP_SCHED.
 CPU Interface Files
 ~~~~~~~~~~~~~~~~~~~
 
-All time durations are in microseconds.
+The interaction of a process with the cpu controller depends on its scheduling
+policy and the underlying scheduler. From the point of view of the cpu controller,
+processes can be categorized as follows:
+
+* Processes under the fair-class scheduler
+* Processes under a BPF scheduler with the ``cgroup_set_weight`` callback
+* Everything else: ``SCHED_{FIFO,RR,DEADLINE}`` and processes under a BPF scheduler
+  without the ``cgroup_set_weight`` callback
+
+For details on when a process is under the fair-class scheduler or a BPF scheduler,
+check out :ref:`Documentation/scheduler/sched-ext.rst <sched-ext>`.
+
+For each of the following interface files, the above categories
+will be referred to. All time durations are in microseconds.
 
   cpu.stat
 	A read-only flat-keyed file.
 	This file exists whether the controller is enabled or not.
 
-	It always reports the following three stats:
+	It always reports the following three stats, which account for all the
+	processes in the cgroup:
 
 	- usage_usec
 	- user_usec
 	- system_usec
 
-	and the following five when the controller is enabled:
+	and the following five when the controller is enabled, which account for
+	only the processes under the fair-class scheduler:
 
 	- nr_periods
 	- nr_throttled
@@ -1125,6 +1140,10 @@ All time durations are in microseconds.
 	If the cgroup has been configured to be SCHED_IDLE (cpu.idle = 1),
 	then the weight will show as a 0.
 
+	This file affects only processes under the fair-class scheduler and a BPF
+	scheduler with the ``cgroup_set_weight`` callback depending on what the
+	callback actually does.
+
   cpu.weight.nice
 	A read-write single value file which exists on non-root
 	cgroups.  The default is "0".
@@ -1137,6 +1156,10 @@ All time durations are in microseconds.
 	granularity is coarser for the nice values, the read value is
 	the closest approximation of the current weight.
 
+	This file affects only processes under the fair-class scheduler and a BPF
+	scheduler with the ``cgroup_set_weight`` callback depending on what the
+	callback actually does.
+
   cpu.max
 	A read-write two value file which exists on non-root cgroups.
 	The default is "max 100000".
@@ -1149,18 +1172,24 @@ All time durations are in microseconds.
 	$PERIOD duration.  "max" for $MAX indicates no limit.  If only
 	one number is written, $MAX is updated.
 
+	This file affects only processes under the fair-class scheduler.
+
   cpu.max.burst
 	A read-write single value file which exists on non-root
 	cgroups.  The default is "0".
 
 	The burst in the range [0, $MAX].
 
+	This file affects only processes under the fair-class scheduler.
+
   cpu.pressure
 	A read-write nested-keyed file.
 
 	Shows pressure stall information for CPU. See
 	:ref:`Documentation/accounting/psi.rst <psi>` for details.
 
+	This file accounts for all the processes in the cgroup.
+
   cpu.uclamp.min
 	A read-write single value file which exists on non-root cgroups.
 	The default is "0", i.e. no utilization boosting.
@@ -1177,6 +1206,8 @@ All time durations are in microseconds.
 	the current value for the maximum utilization (limit), i.e.
 	`cpu.uclamp.max`.
 
+	This file affects all the processes in the cgroup.
+
   cpu.uclamp.max
 	A read-write single value file which exists on non-root cgroups.
 	The default is "max". i.e. no utilization capping
@@ -1189,6 +1220,8 @@ All time durations are in microseconds.
 	value is used to clamp the task specific maximum utilization clamp,
 	including those of realtime processes.
 
+	This file affects all the processes in the cgroup.
+
   cpu.idle
 	A read-write single value file which exists on non-root cgroups.
 	The default is 0.
@@ -1199,7 +1232,7 @@ All time durations are in microseconds.
 	own relative priorities, but the cgroup itself will be treated as
 	very low priority relative to its peers.
 
-
+	This file affects only processes under the fair-class scheduler.
 
 Memory
 ------