[RFC] Exclusive gpu access for SteamVR usecases

[Andres Rodriguez]

When multiple environments are running simultaneously on a system, e.g.
an X desktop + a SteamVR game session, it may be useful to sacrifice
performance in one environment in order to boost it on the other.

This series provides a mechanism for a DRM_MASTER to provide exclusive
gpu access to a group of processes.

Note: This series is built on the assumption that the drm lease patch series
will extend DRM_MASTER status to lesees.

The libdrm we intend to provide is as follows:

/**
 * Set the priority of all contexts in a process
 *
 * This function will change the priority of all contexts owned by
 * the process identified by fd.
 *
 * \param dev             - \c [in] device handle
 * \param fd              - \c [in] fd from target process
 * \param priority        - \c [in] target priority AMDGPU_CTX_PRIORITY_*
 *
 * \return  0 on success\n
 *         <0 - Negative POSIX error code
 *
 * \notes @fd can be *any* file descriptor from the target process.
 * \notes this function requires DRM_MASTER
 */
int amdgpu_sched_process_priority_set(amdgpu_device_handle dev,
				      int fd, int32_t priority);

/**
 * Request to raise the minimum required priority to schedule a gpu job
 *
 * Submit a request to increase the minimum required priority to schedule
 * a gpu job. Once this function returns, the gpu scheduler will no longer
 * consider jobs from contexts with priority lower than @priority.
 *
 * The minimum priority considered by the scheduler will be the highest from
 * all currently active requests.
 *
 * Requests are refcounted, and must be balanced using
 * amdgpu_sched_min_priority_put()
 *
 * \param dev             - \c [in] device handle
 * \param priority        - \c [in] target priority AMDGPU_CTX_PRIORITY_*
 *
 * \return  0 on success\n
 *         <0 - Negative POSIX error code
 *
 * \notes this function requires DRM_MASTER
 */
int amdgpu_sched_min_priority_get(amdgpu_device_handle dev,
				  int32_t priority);

/**
 * Drop a request to raise the minimum required scheduler priority
 *
 * This call balances amdgpu_sched_min_priority_get()
 *
 * If no other active requests exists for @priority, the minimum required
 * priority will decay to a lower level until one is reached with an active
 * request or the lowest priority is reached.
 *
 * \param dev             - \c [in] device handle
 * \param priority        - \c [in] target priority AMDGPU_CTX_PRIORITY_*
 *
 * \return  0 on success\n
 *         <0 - Negative POSIX error code
 *
 * \notes this function requires DRM_MASTER
 */
int amdgpu_sched_min_priority_put(amdgpu_device_handle dev,
				  int32_t priority);

Using this app, VRComposer can raise the priority of the VRapp and itself. Then
it can restrict the minimum scheduler priority in order to become exclusive gpu
clients.

One of the areas I'd like feedback is the following scenario. If a VRapp opens
a new fd and creates a new context after a call to set_priority, this specific
context will be lower priority than the rest. If the minimum required priority
is then raised, it is possible that this new context will be starved and
deadlock the VRapp.

One solution I had in mind to address this situation, is to make set_priority
also raise the priority of future contexts created by the VRapp. However, that
would require keeping track of the requested priority on a per-process data
structure. The current design appears to steer clean of keeping any process
specific data, and everything instead of stored on a per-file basis. Which is
why I did not pursue this approach. But if this is something you'd like me to
implement let me know.

One could also argue that preventing an application deadlock should be handled
between the VRComposer and the VRApp. It is not the kernel's responsibility to
babysit userspace applications and prevent themselves from shooting themselves
in the foot. The same could be achieved by improper usage of shared fences
between processes.

Thoughts/feedback/comments on this issue, or others, are appreciated.

Regards,
Andres