[RFC PATCH v12 4/4] docs: cgroup-v2: document the iocost BPF cost model attachment

[Tao Cui <cui.tao@linux.dev>]

From: Tao Cui <cuitao@kylinos.cn>

Document the BPF cost model attachment in the io.cost.model section
of the cgroup v2 documentation: attaching an iocost_model_ops
struct_ops to a device by its major:minor, the model=bpf readback
while attached (ctrl keeps describing the coefficients), the builtin
model coming back on detach, and ctrl writes never selecting a model.

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Tao Cui <cuitao@kylinos.cn>
---
 Documentation/admin-guide/cgroup-v2.rst | 45 ++++++++++++++++++++-----
 1 file changed, 37 insertions(+), 8 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 9e3c67b02ae2..0965645b1286 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -2056,8 +2056,9 @@ IO Interface Files
 	currently implements "io.weight" proportional control.  Lines
 	are keyed by $MAJ:$MIN device numbers and not ordered.  The
 	line for a given device is populated on the first write for
-	the device on "io.cost.qos" or "io.cost.model".  The following
-	nested keys are defined.
+	the device on "io.cost.qos" or "io.cost.model", or when a BPF
+	cost model is attached to it.  The following nested keys are
+	defined.
 
 	  ======	=====================================
 	  enable	Weight-based control enable
@@ -2111,13 +2112,40 @@ IO Interface Files
 	implements "io.weight" proportional control.  Lines are keyed
 	by $MAJ:$MIN device numbers and not ordered.  The line for a
 	given device is populated on the first write for the device on
-	"io.cost.qos" or "io.cost.model".  The following nested keys
-	are defined.
+	"io.cost.qos" or "io.cost.model", or when a BPF cost model is
+	attached to it.  The following nested keys are defined.
 
-	  =====		================================
+	  =====		==========================================
 	  ctrl		"auto" or "user"
-	  model		The cost model in use - "linear"
-	  =====		================================
+	  model		The cost model in use - "linear" or "bpf"
+	  =====		==========================================
+
+	When BPF cost model support is available, a BPF cost model
+	can be bound to a device by attaching an
+	"iocost_model_ops" struct_ops carrying the whole disk's
+	major:minor as encoded by makedev(3) in its "dev" member (a
+	partition's major:minor is rejected).  Attaching creates the
+	controller if needed, like
+	an io.cost.model write does, switches the device's pricing to
+	the model and populates the device's line in this file and in
+	"io.cost.qos" if it did not exist yet.  Attaching fails with
+	-EBUSY if the device already has a model attached.  The
+	attached model prices IOs only while the controller is
+	enabled through "io.cost.qos"; attaching does not enable it.
+	IOs issued directly by the root cgroup bypass the model.
+
+	While a model is attached, "model" selects between it and the
+	builtin model: "model=bpf" switches to the attached model and
+	"model=linear" switches back to the builtin model, and neither
+	detaches the struct_ops.
+
+	Detaching removes the model, after which "model=bpf"
+	fails and the device prices with the builtin model again.
+	Device removal ejects the attached model as well, while the
+	struct_ops link remains until userspace destroys it.  "ctrl"
+	accepts only "auto" and "user" and never selects a model.  It
+	keeps describing the builtin coefficients, which take effect
+	again when switched back.
 
 	When "ctrl" is "auto", the kernel may change all parameters
 	dynamically.  When "ctrl" is set to "user" or any other
@@ -2125,7 +2153,8 @@ IO Interface Files
 	automatic changes are disabled.
 
 	When "model" is "linear", the following model parameters are
-	defined.
+	defined.  They remain readable and writable while "model" is
+	"bpf".
 
 	  =============	========================================
 	  [r|w]bps	The maximum sequential IO throughput
-- 
2.53.0