[Intel-gfx] [PATCH v3 12/14] drm/i915/scheduler: Support user-defined priorities

[Chris Wilson]

Use a priority stored in the context as the initial value when
submitting a request. This allows us to change the default priority on a
per-context basis, allowing different contexts to be favoured with GPU
time at the expense of lower importance work. The user can adjust the
context's priority via I915_CONTEXT_PARAM_PRIORITY, with more positive
values being higher priority (they will be serviced earlier, after their
dependencies have been resolved). Any prerequisite work for an execbuf
will have its priority raised to match the new request as required.

Normal users can specify any value in the range of -1023 to 0 [default],
i.e. they can reduce the priority of their workloads (and temporarily
boost it back to normal if so desired).

Privileged users can specify any value in the range of -1023 to 1023,
[default is 0], i.e. they can raise their priority above all overs and
so potentially starve the system.

Note that the existing schedulers are not fair, nor load balancing, the
execution is strictly by priority on a first-come, first-served basis,
and the driver may choose to boost some requests above the range
available to users.

This priority was originally based around nice(2), but evolved to allow
clients to adjust their priority within a small range, and allow for a
privileged high priority range.

For example, this can be used to implement EGL_IMG_context_priority
https://www.khronos.org/registry/egl/extensions/IMG/EGL_IMG_context_priority.txt

	EGL_CONTEXT_PRIORITY_LEVEL_IMG determines the priority level of
        the context to be created. This attribute is a hint, as an
        implementation may not support multiple contexts at some
        priority levels and system policy may limit access to high
        priority contexts to appropriate system privilege level. The
        default value for EGL_CONTEXT_PRIORITY_LEVEL_IMG is
        EGL_CONTEXT_PRIORITY_MEDIUM_IMG."

so we can map

	PRIORITY_HIGH -> 1023 [privileged, will failback to 0]
	PRIORITY_MED -> 0 [default]
	PRIORITY_LOW -> -1023

They also map onto the priorities used by VkQueue (and a VkQueue is
essentially a timeline, our i915_gem_context under full-ppgtt).

Testcase: igt/gem_exec_schedule
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_gem_context.c | 20 ++++++++++++++++++++
 include/uapi/drm/i915_drm.h             |  3 +++
 2 files changed, 23 insertions(+)

diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
index 1f94b8d6d83d..1f74ab266f6b 100644
--- a/drivers/gpu/drm/i915/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/i915_gem_context.c
@@ -1101,6 +1101,9 @@ int i915_gem_context_getparam_ioctl(struct drm_device *dev, void *data,
 	case I915_CONTEXT_PARAM_NO_ERROR_CAPTURE:
 		args->value = !!(ctx->flags & CONTEXT_NO_ERROR_CAPTURE);
 		break;
+	case I915_CONTEXT_PARAM_PRIORITY:
+		args->value = ctx->priority;
+		break;
 	default:
 		ret = -EINVAL;
 		break;
@@ -1156,6 +1159,23 @@ int i915_gem_context_setparam_ioctl(struct drm_device *dev, void *data,
 				ctx->flags &= ~CONTEXT_NO_ERROR_CAPTURE;
 		}
 		break;
+
+	case I915_CONTEXT_PARAM_PRIORITY:
+		{
+			int priority = args->value;
+
+			if (args->size)
+				ret = -EINVAL;
+			else if (priority >= I915_PRIORITY_MAX ||
+				 priority <= -I915_PRIORITY_MAX)
+				ret = -EINVAL;
+			else if (priority > 0 && !capable(CAP_SYS_ADMIN))
+				ret = -EPERM;
+			else
+				ctx->priority = priority;
+		}
+		break;
+
 	default:
 		ret = -EINVAL;
 		break;
diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
index 1c12a350eca3..47901a8ad682 100644
--- a/include/uapi/drm/i915_drm.h
+++ b/include/uapi/drm/i915_drm.h
@@ -391,6 +391,8 @@ typedef struct drm_i915_irq_wait {
 
 /* Query whether DRM_I915_GEM_EXECBUFFER2 supports user defined execution
  * priorities and the driver will attempt to execute batches in priority order.
+ * The initial priority for each batch is supplied by the context and is
+ * controlled via I915_CONTEXT_PARAM_PRIORITY.
  */
 #define I915_PARAM_HAS_SCHEDULER	 41
 
@@ -1224,6 +1226,7 @@ struct drm_i915_gem_context_param {
 #define I915_CONTEXT_PARAM_NO_ZEROMAP	0x2
 #define I915_CONTEXT_PARAM_GTT_SIZE	0x3
 #define I915_CONTEXT_PARAM_NO_ERROR_CAPTURE	0x4
+#define I915_CONTEXT_PARAM_PRIORITY	0x5
 	__u64 value;
 };
 
-- 
2.10.2