[Intel-gfx] [PATCH 2/3] drm/i915/gt: Suppress internal I915_PRIORITY_WAIT for timeslicing

[Chris Wilson]

Make sure we ignore the I915_PRIORITY_WAIT hint when looking at
timeslicing, as we do not treat it as a preemption request but as a soft
ordering hint. If we apply the hint, then when we recompute the ordering
after unwinding for the timeslice, we will often leave the order
unchanged due to the soft-hint. However, if we apply it to all those we
unwind, then the two equivalent levels may be reordered, and since the
dependencies will be replayed in order, we will not change the order of
dependencies.

There is a small issue with the lack of cross-engine priority bumping on
unwind, leaving the total graph slightly unordered; but that will not
result in any misordering of rendering on remote machines as any
signalers will also be live. Though there may be a danger that this will
upset our sanitychecks.

Why keep the I915_PRIORITY_WAIT soft-hint, I hear Tvrtko ask? Despite
the many hairy tricks we play to have the hint and then ignore it, I
still like the concept of codel and the promise that it gives for low
latency of independent queues!

Testcase: igt/gem_exec_fence/submit
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
---
 drivers/gpu/drm/i915/gt/intel_lrc.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
index 10109f661bcb..3606a7946707 100644
--- a/drivers/gpu/drm/i915/gt/intel_lrc.c
+++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
@@ -414,6 +414,12 @@ static inline int rq_prio(const struct i915_request *rq)
 	return READ_ONCE(rq->sched.attr.priority);
 }
 
+static int __effective_prio(int prio)
+{
+	BUILD_BUG_ON(__NO_PREEMPTION & ~I915_PRIORITY_MASK); /* only internal */
+	return prio | __NO_PREEMPTION;
+}
+
 static int effective_prio(const struct i915_request *rq)
 {
 	int prio = rq_prio(rq);
@@ -439,8 +445,7 @@ static int effective_prio(const struct i915_request *rq)
 		prio |= I915_PRIORITY_NOSEMAPHORE;
 
 	/* Restrict mere WAIT boosts from triggering preemption */
-	BUILD_BUG_ON(__NO_PREEMPTION & ~I915_PRIORITY_MASK); /* only internal */
-	return prio | __NO_PREEMPTION;
+	return __effective_prio(prio);
 }
 
 static int queue_prio(const struct intel_engine_execlists *execlists)
@@ -1126,6 +1131,7 @@ __unwind_incomplete_requests(struct intel_engine_cs *engine)
 			continue; /* XXX */
 
 		__i915_request_unsubmit(rq);
+		rq->sched.attr.priority |= __NO_PREEMPTION;
 
 		/*
 		 * Push the request back into the queue for later resubmission.
@@ -1930,7 +1936,7 @@ need_timeslice(const struct intel_engine_cs *engine,
 	if (!list_is_last(&rq->sched.link, &engine->active.requests))
 		hint = max(hint, rq_prio(list_next_entry(rq, sched.link)));
 
-	return hint >= effective_prio(rq);
+	return __effective_prio(hint) >= effective_prio(rq);
 }
 
 static bool
@@ -1965,7 +1971,7 @@ switch_prio(struct intel_engine_cs *engine, const struct i915_request *rq)
 	if (list_is_last(&rq->sched.link, &engine->active.requests))
 		return INT_MIN;
 
-	return rq_prio(list_next_entry(rq, sched.link));
+	return __effective_prio(rq_prio(list_next_entry(rq, sched.link)));
 }
 
 static inline unsigned long
-- 
2.20.1