[RFC 1/6] gpu: rfc: Proposal for a GPU cgroup controller

[Hridya Valsaraju]

This patch adds a proposal for a new GPU cgroup controller for
accounting/limiting GPU and GPU-related memory allocations.
The proposed controller is based on the DRM cgroup controller[1] and
follows the design of the RDMA cgroup controller.

The new cgroup controller would:
* Allow setting per-cgroup limits on the total size of buffers charged
  to it.
* Allow setting per-device limits on the total size of buffers
  allocated by device within a cgroup.
* Expose a per-device/allocator breakdown of the buffers charged to a
  cgroup.

The prototype in the following patches are only for memory accounting
using the GPU cgroup controller and does not implement limit setting.

[1]: https://lore.kernel.org/amd-gfx/20210126214626.16260-1-brian.welty@xxxxxxxxx/

Signed-off-by: Hridya Valsaraju <hridya@xxxxxxxxxx>
---

Hi all,

Here is the RFC documentation for the GPU cgroup controller that we
talked about at LPC 2021 along with a prototype. I reached out to Tejun
with the idea recently and he mentioned that cgroup-aware BPF(by Kenny
Ho) or the new misc cgroup controller can also be considered as
alternatives to track GPU resources. I am sending the RFC to the list to
give everyone else a chance to chime in with their thoughts as well so
that we can reach an agreement on how to proceed. Thanks in advance!

Regards,
Hridya


 Documentation/gpu/rfc/gpu-cgroup.rst | 192 +++++++++++++++++++++++++++
 Documentation/gpu/rfc/index.rst      |   4 +
 2 files changed, 196 insertions(+)
 create mode 100644 Documentation/gpu/rfc/gpu-cgroup.rst

diff --git a/Documentation/gpu/rfc/gpu-cgroup.rst b/Documentation/gpu/rfc/gpu-cgroup.rst
new file mode 100644
index 000000000000..9bff23007b22
--- /dev/null
+++ b/Documentation/gpu/rfc/gpu-cgroup.rst
@@ -0,0 +1,192 @@
+===================================
+GPU cgroup controller
+===================================
+
+Goals
+=====
+This document intends to outline a plan to create a cgroup v2 controller subsystem
+for the per-cgroup accounting of device and system memory allocated by the GPU
+and related subsystems.
+
+The new cgroup controller would:
+
+* Allow setting per-cgroup limits on the total size of buffers charged to it.
+
+* Allow setting per-device limits on the total size of buffers allocated by a
+  device/allocator within a cgroup.
+
+* Expose a per-device/allocator breakdown of the buffers charged to a cgroup.
+
+Alternatives Considered
+=======================
+
+The following alternatives were considered:
+
+The memory cgroup controller
+____________________________
+
+1. As was noted in [1], memory accounting provided by the GPU cgroup
+controller is not a good fit for integration into memcg due to the
+differences in how accounting is performed. It implements a mechanism
+for the allocator attribution of GPU and GPU-related memory by
+charging each buffer to the cgroup of the process on behalf of which
+the memory was allocated. The buffer stays charged to the cgroup until
+it is freed regardless of whether the process retains any references
+to it. On the other hand, the memory cgroup controller offers a more
+fine-grained charging and uncharging behavior depending on the kind of
+page being accounted.
+
+2. Memcg performs accounting in units of pages. In the DMA-BUF buffer sharing model,
+a process takes a reference to the entire buffer(hence keeping it alive) even if
+it is only accessing parts of it. Therefore, per-page memory tracking for DMA-BUF
+memory accounting would only introduce additional overhead without any benefits.
+
+[1]: https://patchwork.kernel.org/project/dri-devel/cover/20190501140438.9506-1-brian.welty@xxxxxxxxx/#22624705
+
+Userspace service to keep track of buffer allocations and releases
+__________________________________________________________________
+
+1. There is no way for a userspace service to intercept all allocations and releases.
+2. In case the process gets killed or restarted, we lose all accounting so far.
+
+UAPI
+====
+When enabled, the new cgroup controller would create the following files in every cgroup.
+
+::
+
+        gpu.memory.current (R)
+        gpu.memory.max (R/W)
+
+gpu.memory.current is a read-only file and would contain per-device memory allocations
+in a key-value format where key is a string representing the device name
+and the value is the size of memory charged to the device in the cgroup in bytes.
+
+For example:
+
+::
+
+        cat /sys/kernel/fs/cgroup1/gpu.memory.current
+        dev1 4194304
+        dev2 4194304
+
+The string key for each device is set by the device driver when the device registers
+with the GPU cgroup controller to participate in resource accounting(see section
+'Design and Implementation' for more details).
+
+gpu.memory.max is a read/write file. It would show the current total
+size limits on memory usage for the cgroup and the limits on total memory usage
+for each allocator/device.
+
+Setting a total limit for a cgroup can be done as follows:
+
+::
+
+        echo “total 41943040” > /sys/kernel/fs/cgroup1/gpu.memory.max
+
+Setting a total limit for a particular device/allocator can be done as follows:
+
+::
+
+        echo “dev1 4194304” >  /sys/kernel/fs/cgroup1/gpu.memory.max
+
+In this example, 'dev1' is the string key set by the device driver during
+registration.
+
+Design and Implementation
+=========================
+
+The cgroup controller would closely follow the design of the RDMA cgroup controller
+subsystem where each cgroup maintains a list of resource pools.
+Each resource pool contains a struct device and the counter to track current total,
+and the maximum limit set for the device.
+
+The below code block is a preliminary estimation on how the core kernel data structures
+and APIs would look like.
+
+.. code-block:: c
+
+        /**
+         * The GPU cgroup controller data structure.
+         */
+        struct gpucg {
+                struct cgroup_subsys_state css;
+                /* list of all resource pools that belong to this cgroup */
+                struct list_head rpools;
+        };
+
+        struct gpucg_device {
+                /*
+                 * list  of various resource pools in various cgroups that the device is
+                 * part of.
+                 */
+                struct list_head rpools;
+                /* list of all devices registered for GPU cgroup accounting */
+                struct list_head dev_node;
+                /* name to be used as identifier for accounting and limit setting */
+                const char *name;
+        };
+
+        struct gpucg_resource_pool {
+                /* The device whose resource usage is tracked by this resource pool */
+                struct gpucg_device *device;
+
+                /* list of all resource pools for the cgroup */
+                struct list_head cg_node;
+
+                /*
+                 * list maintained by the gpucg_device to keep track of its
+                 * resource pools
+                 */
+                struct list_head dev_node;
+
+                /* tracks memory usage of the resource pool */
+                struct page_counter total;
+        };
+
+        /**
+         * gpucg_register_device - Registers a device for memory accounting using the
+         * GPU cgroup controller.
+         *
+         * @device: The device to register for memory accounting. Must remain valid
+         * after registration.
+         * @name: Pointer to a string literal to denote the name of the device.
+         */
+        void gpucg_register_device(struct gpucg_device *gpucg_dev, const char *name);
+
+        /**
+         * gpucg_try_charge - charge memory to the specified gpucg and gpucg_device.
+         *
+         * @gpucg: The gpu cgroup to charge the memory to.
+         * @device: The device to charge the memory to.
+         * @usage: size of memory to charge in bytes.
+         *
+         * Return: returns 0 if the charging is successful and otherwise returns an
+         * error code.
+         */
+        int gpucg_try_charge(struct gpucg *gpucg, struct gpucg_device *device, u64 usage);
+
+        /**
+         * gpucg_uncharge - uncharge memory from the specified gpucg and gpucg_device.
+         *
+         * @gpucg: The gpu cgroup to uncharge the memory from.
+         * @device: The device to charge the memory from.
+         * @usage: size of memory to uncharge in bytes.
+         */
+        void gpucg_uncharge(struct gpucg *gpucg, struct gpucg_device *device, u64 usage);
+
+Future Work
+===========
+Additional GPU resources can be supported by adding new controller files.
+
+Upstreaming Plan
+================
+* Decide on a UAPI that accommodates all use-cases for the upstream GPU ecosystem
+  as well as for Android.
+
+* Prototype the GPU cgroup controller and integrate its usage into the DMA-BUF
+  system heap.
+
+* Demonstrate its usage from userspace in the Android Open Space Project.
+
+* Send out RFCs to LKML for the GPU cgroup controller and iterate.
diff --git a/Documentation/gpu/rfc/index.rst b/Documentation/gpu/rfc/index.rst
index 91e93a705230..0a9bcd94e95d 100644
--- a/Documentation/gpu/rfc/index.rst
+++ b/Documentation/gpu/rfc/index.rst
@@ -23,3 +23,7 @@ host such documentation:
 .. toctree::
 
     i915_scheduler.rst
+
+.. toctree::
+
+    gpu-cgroup.rst
-- 
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