drm/i915/execlists: Suppress preempting self

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.

However, when we avoid preempting ELSP[0], we still retain the preemption
value as it may match a second preemption request within the same time period
that we need to resolve after the next CS event. However, since we only
store the maximum preemption priority seen, it may not match the
subsequent event and so we should double check whether or not we
actually do need to trigger a preempt-to-idle by comparing the top
priorities from each queue. Later, this gives us a hook for finer
control over deciding whether the preempt-to-idle is justified.

The sequence of events where we end up preempting for no avail is:

1. Queue requests/contexts A, B
2. Priority boost A; no preemption as it is executing, but keep hint
3. After CS switch, B is less than hint, force preempt-to-idle
4. Resubmit B after idling

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.
v3: Demonstrate the stale preemption hint with a selftest

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>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190129185452.20989-4-chris@chris-wilson.co.uk

1 files changed, 138 insertions, 0 deletions

diff --git a/drivers/gpu/drm/i915/selftests/intel_lrc.c b/drivers/gpu/drm/i915/selftests/intel_lrc.c
index 2b2ecd76c2ac..fb35f53c9ce3 100644
--- a/drivers/gpu/drm/i915/selftests/intel_lrc.c
+++ b/drivers/gpu/drm/i915/selftests/intel_lrc.c
@@ -268,6 +268,143 @@ err_wedged:
 	goto err_ctx_lo;
 }
 
+struct preempt_client {
+	struct igt_spinner spin;
+	struct i915_gem_context *ctx;
+};
+
+static int preempt_client_init(struct drm_i915_private *i915,
+			       struct preempt_client *c)
+{
+	c->ctx = kernel_context(i915);
+	if (!c->ctx)
+		return -ENOMEM;
+
+	if (igt_spinner_init(&c->spin, i915))
+		goto err_ctx;
+
+	return 0;
+
+err_ctx:
+	kernel_context_close(c->ctx);
+	return -ENOMEM;
+}
+
+static void preempt_client_fini(struct preempt_client *c)
+{
+	igt_spinner_fini(&c->spin);
+	kernel_context_close(c->ctx);
+}
+
+static int live_suppress_self_preempt(void *arg)
+{
+	struct drm_i915_private *i915 = arg;
+	struct intel_engine_cs *engine;
+	struct i915_sched_attr attr = {
+		.priority = I915_USER_PRIORITY(I915_PRIORITY_MAX)
+	};
+	struct preempt_client a, b;
+	enum intel_engine_id id;
+	intel_wakeref_t wakeref;
+	int err = -ENOMEM;
+
+	/*
+	 * Verify that if a preemption request does not cause a change in
+	 * the current execution order, the preempt-to-idle injection is
+	 * skipped and that we do not accidentally apply it after the CS
+	 * completion event.
+	 */
+
+	if (!HAS_LOGICAL_RING_PREEMPTION(i915))
+		return 0;
+
+	if (USES_GUC_SUBMISSION(i915))
+		return 0; /* presume black blox */
+
+	mutex_lock(&i915->drm.struct_mutex);
+	wakeref = intel_runtime_pm_get(i915);
+
+	if (preempt_client_init(i915, &a))
+		goto err_unlock;
+	if (preempt_client_init(i915, &b))
+		goto err_client_a;
+
+	for_each_engine(engine, i915, id) {
+		struct i915_request *rq_a, *rq_b;
+		int depth;
+
+		engine->execlists.preempt_hang.count = 0;
+
+		rq_a = igt_spinner_create_request(&a.spin,
+						  a.ctx, engine,
+						  MI_NOOP);
+		if (IS_ERR(rq_a)) {
+			err = PTR_ERR(rq_a);
+			goto err_client_b;
+		}
+
+		i915_request_add(rq_a);
+		if (!igt_wait_for_spinner(&a.spin, rq_a)) {
+			pr_err("First client failed to start\n");
+			goto err_wedged;
+		}
+
+		for (depth = 0; depth < 8; depth++) {
+			rq_b = igt_spinner_create_request(&b.spin,
+							  b.ctx, engine,
+							  MI_NOOP);
+			if (IS_ERR(rq_b)) {
+				err = PTR_ERR(rq_b);
+				goto err_client_b;
+			}
+			i915_request_add(rq_b);
+
+			GEM_BUG_ON(i915_request_completed(rq_a));
+			engine->schedule(rq_a, &attr);
+			igt_spinner_end(&a.spin);
+
+			if (!igt_wait_for_spinner(&b.spin, rq_b)) {
+				pr_err("Second client failed to start\n");
+				goto err_wedged;
+			}
+
+			swap(a, b);
+			rq_a = rq_b;
+		}
+		igt_spinner_end(&a.spin);
+
+		if (engine->execlists.preempt_hang.count) {
+			pr_err("Preemption recorded x%d, depth %d; should have been suppressed!\n",
+			       engine->execlists.preempt_hang.count,
+			       depth);
+			err = -EINVAL;
+			goto err_client_b;
+		}
+
+		if (igt_flush_test(i915, I915_WAIT_LOCKED))
+			goto err_wedged;
+	}
+
+	err = 0;
+err_client_b:
+	preempt_client_fini(&b);
+err_client_a:
+	preempt_client_fini(&a);
+err_unlock:
+	if (igt_flush_test(i915, I915_WAIT_LOCKED))
+		err = -EIO;
+	intel_runtime_pm_put(i915, wakeref);
+	mutex_unlock(&i915->drm.struct_mutex);
+	return err;
+
+err_wedged:
+	igt_spinner_end(&b.spin);
+	igt_spinner_end(&a.spin);
+	i915_gem_set_wedged(i915);
+	err = -EIO;
+	goto err_client_b;
+}
+
 static int live_preempt_hang(void *arg)
 {
 	struct drm_i915_private *i915 = arg;
@@ -647,6 +784,7 @@ int intel_execlists_live_selftests(struct drm_i915_private *i915)
 		SUBTEST(live_sanitycheck),
 		SUBTEST(live_preempt),
 		SUBTEST(live_late_preempt),
+		SUBTEST(live_suppress_self_preempt),
 		SUBTEST(live_preempt_hang),
 		SUBTEST(live_preempt_smoke),
 	};