[PATCH 2/3] drm/i915/execlists: Suppress preempting self

[Chris Wilson <chris@chris-wilson.co.uk>]

In order to avoid preempting ourselves, we currently refuse to schedule
the tasklet if we reschedule an inflight context. However, this glosses
over a few issues such as what happens after a CS completion event and
we then preempt the newly executing context with itself, or if something
else causes a tasklet_schedule triggering the same evaluation to
preempt the active context with itself.

To avoid the extra complications, after deciding that we have
potentially queued a request with higher priority than the currently
executing request, inspect the head of the queue to see if it is indeed
higher priority from another context.

v2: We can simplify a bunch of tests based on the knowledge that PI will
ensure that earlier requests along the same context will have the highest
priority.

References: a2bf92e8cc16 ("drm/i915/execlists: Avoid kicking priority on the current context")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
---
 drivers/gpu/drm/i915/i915_scheduler.c | 20 ++++--
 drivers/gpu/drm/i915/intel_lrc.c      | 91 ++++++++++++++++++++++++---
 2 files changed, 100 insertions(+), 11 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_scheduler.c b/drivers/gpu/drm/i915/i915_scheduler.c
index 0da718ceab43..7db1255665a8 100644
--- a/drivers/gpu/drm/i915/i915_scheduler.c
+++ b/drivers/gpu/drm/i915/i915_scheduler.c
@@ -238,6 +238,18 @@ sched_lock_engine(struct i915_sched_node *node, struct intel_engine_cs *locked)
 	return engine;
 }
 
+static bool inflight(const struct i915_request *rq,
+		     const struct intel_engine_cs *engine)
+{
+	const struct i915_request *active;
+
+	if (!rq->global_seqno)
+		return false;
+
+	active = port_request(engine->execlists.port);
+	return active->hw_context == rq->hw_context;
+}
+
 static void __i915_schedule(struct i915_request *rq,
 			    const struct i915_sched_attr *attr)
 {
@@ -327,6 +339,7 @@ static void __i915_schedule(struct i915_request *rq,
 		INIT_LIST_HEAD(&dep->dfs_link);
 
 		engine = sched_lock_engine(node, engine);
+		lockdep_assert_held(&engine->timeline.lock);
 
 		/* Recheck after acquiring the engine->timeline.lock */
 		if (prio <= node->attr.priority || node_signaled(node))
@@ -355,17 +368,16 @@ static void __i915_schedule(struct i915_request *rq,
 		if (prio <= engine->execlists.preempt_priority_hint)
 			continue;
 
+		engine->execlists.preempt_priority_hint = prio;
+
 		/*
 		 * If we are already the currently executing context, don't
 		 * bother evaluating if we should preempt ourselves.
 		 */
-		if (node_to_request(node)->global_seqno &&
-		    i915_seqno_passed(port_request(engine->execlists.port)->global_seqno,
-				      node_to_request(node)->global_seqno))
+		if (inflight(node_to_request(node), engine))
 			continue;
 
 		/* Defer (tasklet) submission until after all of our updates. */
-		engine->execlists.preempt_priority_hint = prio;
 		tasklet_hi_schedule(&engine->execlists.tasklet);
 	}
 
diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index 71006b031f54..b44db7d49584 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -182,13 +182,89 @@ static inline int rq_prio(const struct i915_request *rq)
 	return rq->sched.attr.priority;
 }
 
+static int queue_prio(const struct intel_engine_execlists *execlists)
+{
+	struct i915_priolist *p;
+	struct rb_node *rb;
+
+	rb = rb_first_cached(&execlists->queue);
+	if (!rb)
+		return INT_MIN;
+
+	/*
+	 * As the priolist[] are inverted, with the highest priority in [0],
+	 * we have to flip the index value to become priority.
+	 */
+	p = to_priolist(rb);
+	return ((p->priority + 1) << I915_USER_PRIORITY_SHIFT) - ffs(p->used);
+}
+
 static inline bool need_preempt(const struct intel_engine_cs *engine,
-				const struct i915_request *last,
-				int prio)
+				const struct i915_request *rq,
+				int hint)
 {
-	return (intel_engine_has_preemption(engine) &&
-		__execlists_need_preempt(prio, rq_prio(last)) &&
-		!i915_request_completed(last));
+	const struct intel_context *ctx = rq->hw_context;
+	const int last_prio = rq_prio(rq);
+
+	if (!intel_engine_has_preemption(engine))
+		return false;
+
+	if (i915_request_completed(rq))
+		return false;
+
+	/*
+	 * Check if the current priority hint merits a preemption attempt.
+	 *
+	 * However, the priority hint is a mere hint that we may need to
+	 * preempt. If that hint is stale or we may be trying to preempt
+	 * ourselves, ignore the request.
+	 */
+	if (!__execlists_need_preempt(hint, last_prio))
+		return false;
+
+	/*
+	 * Check against the first request in ELSP[1], it will, thanks to the
+	 * power of PI, be the highest priority of that context.
+	 */
+	if (!list_is_last(&rq->link, &engine->timeline.requests)) {
+		rq = list_next_entry(rq, link);
+		GEM_BUG_ON(rq->hw_context == ctx);
+		if (rq_prio(rq) > last_prio)
+			return true;
+	}
+
+	/*
+	 * If the inflight context did not trigger the preemption, then maybe
+	 * it was the set of queued requests? Pick the highest priority in
+	 * the queue (the first active priolist) and see if it deserves to be
+	 * running instead of ELSP[0].
+	 *
+	 * The highest priority request in the queue can not be either
+	 * ELSP[0] or ELSP[1] as, thanks again to PI, if it was the same
+	 * context, it's priority would not exceed ELSP[0] aka last_prio.
+	 */
+	return queue_prio(&engine->execlists) > last_prio;
+}
+
+__maybe_unused static inline bool
+assert_priority_queue(const struct intel_engine_execlists *execlists,
+		      const struct i915_request *prev,
+		      const struct i915_request *next)
+{
+	if (!prev)
+		return true;
+
+	/*
+	 * Without preemption, the prev may refer to the still active element
+	 * which we refuse to let go.
+	 *
+	 * Even with preemption, there are times when we think it is better not
+	 * to preempt and leave an ostensibly lower priority request in flight.
+	 */
+	if (port_request(execlists->port) == prev)
+		return true;
+
+	return rq_prio(prev) >= rq_prio(next);
 }
 
 /*
@@ -630,8 +706,7 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 		int i;
 
 		priolist_for_each_request_consume(rq, rn, p, i) {
-			GEM_BUG_ON(last &&
-				   need_preempt(engine, last, rq_prio(rq)));
+			GEM_BUG_ON(!assert_priority_queue(execlists, last, rq));
 
 			/*
 			 * Can we combine this request with the current port?
@@ -876,6 +951,8 @@ static void process_csb(struct intel_engine_cs *engine)
 	const u32 * const buf = execlists->csb_status;
 	u8 head, tail;
 
+	lockdep_assert_held(&engine->timeline.lock);
+
 	/*
 	 * Note that csb_write, csb_status may be either in HWSP or mmio.
 	 * When reading from the csb_write mmio register, we have to be
-- 
2.20.1

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