[PATCH 1/1] drm/i915: Inherit submitter nice when scheduling requests

[Tvrtko Ursulin]

From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>

Inherit submitter nice at point of request submission to account for long
running processes getting either externally or self re-niced.

This accounts for the current processing landscape where computational
pipelines are composed of CPU and GPU parts working in tandem.

Nice value will only apply to requests which originate from user contexts
and have default context priority. This is to avoid disturbing any
application made choices of low and high (batch processing and latency
sensitive compositing). In this case nice value adjusts the effective
priority in the narrow band of -19 to +20 around
I915_CONTEXT_DEFAULT_PRIORITY.

This means that userspace using the context priority uapi directly has a
wider range of possible adjustments (in practice that only applies to
execlists platforms - with GuC there are only three priority buckets), but
in all cases nice adjustment has the expected effect: positive nice
lowering the scheduling priority and negative nice raising it.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
---
 drivers/gpu/drm/i915/i915_request.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
index 582770360ad1..e5cfa073d8f0 100644
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -1811,8 +1811,17 @@ void i915_request_add(struct i915_request *rq)
 	/* XXX placeholder for selftests */
 	rcu_read_lock();
 	ctx = rcu_dereference(rq->context->gem_context);
-	if (ctx)
+	if (ctx) {
 		attr = ctx->sched;
+		/*
+		 * Inherit process nice when scheduling user contexts but only
+		 * if context has the default priority to avoid touching
+		 * contexts where GEM uapi has been used to explicitly lower
+		 * or elevate it.
+		 */
+		if (attr.priority == I915_CONTEXT_DEFAULT_PRIORITY)
+			attr.priority = -task_nice(current);
+	}
 	rcu_read_unlock();
 
 	__i915_request_queue(rq, &attr);
-- 
2.32.0