[PATCH] drm/i915/execlists: Suppress mere WAIT preemption

From: Chris Wilson <chris@chris-wilson.co.uk>
To: intel-gfx@lists.freedesktop.org
Subject: [PATCH] drm/i915/execlists: Suppress mere WAIT preemption
Date: Mon,  4 Feb 2019 10:49:52 +0000	[thread overview]
Message-ID: <20190204104952.3200-1-chris@chris-wilson.co.uk> (raw)
In-Reply-To: <20190204084116.3013-2-chris@chris-wilson.co.uk>

WAIT is occasionally suppressed by virtue of preempted requests being
promoted to NEWCLIENT if they have not all ready received that boost.
Make this consistent for all WAIT boosts that they are not allowed to
preempt executing contexts and are merely granted the right to be at the
front of the queue for the next execution slot. This is in keeping with
the desire that the WAIT boost be a minor tweak that does not give
excessive promotion to its user and open ourselves to trivial abuse.

The problem with the inconsistent WAIT preemption becomes more apparent
as the preemption is propagated across the engines, where one engine may
preempt and the other not, and we be relying on the exact execution
order being consistent across engines (e.g. using HW semaphores to
coordinate parallel execution).

v2: Also protect GuC submission from false preemption loops.
v3: Build bug safeguards and better debug messages for st.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
---
 drivers/gpu/drm/i915/i915_request.c          |  12 ++
 drivers/gpu/drm/i915/i915_scheduler.h        |   2 +
 drivers/gpu/drm/i915/intel_lrc.c             |   9 +-
 drivers/gpu/drm/i915/selftests/igt_spinner.c |   9 +-
 drivers/gpu/drm/i915/selftests/intel_lrc.c   | 160 +++++++++++++++++++
 5 files changed, 190 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
index 9ed5baf157a3..d14a1b225f47 100644
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -377,12 +377,24 @@ void __i915_request_submit(struct i915_request *request)
 
 	/* We may be recursing from the signal callback of another i915 fence */
 	spin_lock_nested(&request->lock, SINGLE_DEPTH_NESTING);
+
 	GEM_BUG_ON(test_bit(I915_FENCE_FLAG_ACTIVE, &request->fence.flags));
 	set_bit(I915_FENCE_FLAG_ACTIVE, &request->fence.flags);
+
 	request->global_seqno = seqno;
 	if (test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &request->fence.flags) &&
 	    !i915_request_enable_breadcrumb(request))
 		intel_engine_queue_breadcrumbs(engine);
+
+	/*
+	 * As we do not allow WAIT to preempt inflight requests,
+	 * once we have executed a request, along with triggering
+	 * any execution callbacks, we must preserve its ordering
+	 * within the non-preemptible FIFO.
+	 */
+	BUILD_BUG_ON(__NO_PREEMPTION & ~I915_PRIORITY_MASK); /* only internal */
+	request->sched.attr.priority |= __NO_PREEMPTION;
+
 	spin_unlock(&request->lock);
 
 	engine->emit_fini_breadcrumb(request,
diff --git a/drivers/gpu/drm/i915/i915_scheduler.h b/drivers/gpu/drm/i915/i915_scheduler.h
index dbe9cb7ecd82..54bd6c89817e 100644
--- a/drivers/gpu/drm/i915/i915_scheduler.h
+++ b/drivers/gpu/drm/i915/i915_scheduler.h
@@ -33,6 +33,8 @@ enum {
 #define I915_PRIORITY_WAIT	((u8)BIT(0))
 #define I915_PRIORITY_NEWCLIENT	((u8)BIT(1))
 
+#define __NO_PREEMPTION (I915_PRIORITY_WAIT)
+
 struct i915_sched_attr {
 	/**
 	 * @priority: execution and service priority
diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index a9eb0211ce77..773df0bd685b 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -188,6 +188,12 @@ static inline int rq_prio(const struct i915_request *rq)
 	return rq->sched.attr.priority;
 }
 
+static int effective_prio(const struct i915_request *rq)
+{
+	/* Restrict mere WAIT boosts from triggering preemption */
+	return rq_prio(rq) | __NO_PREEMPTION;
+}
+
 static int queue_prio(const struct intel_engine_execlists *execlists)
 {
 	struct i915_priolist *p;
@@ -208,7 +214,7 @@ static int queue_prio(const struct intel_engine_execlists *execlists)
 static inline bool need_preempt(const struct intel_engine_cs *engine,
 				const struct i915_request *rq)
 {
-	const int last_prio = rq_prio(rq);
+	int last_prio;
 
 	if (!intel_engine_has_preemption(engine))
 		return false;
@@ -228,6 +234,7 @@ static inline bool need_preempt(const struct intel_engine_cs *engine,
 	 * preempt. If that hint is stale or we may be trying to preempt
 	 * ourselves, ignore the request.
 	 */
+	last_prio = effective_prio(rq);
 	if (!__execlists_need_preempt(engine->execlists.queue_priority_hint,
 				      last_prio))
 		return false;
diff --git a/drivers/gpu/drm/i915/selftests/igt_spinner.c b/drivers/gpu/drm/i915/selftests/igt_spinner.c
index 9ebd9225684e..86354e51bdd3 100644
--- a/drivers/gpu/drm/i915/selftests/igt_spinner.c
+++ b/drivers/gpu/drm/i915/selftests/igt_spinner.c
@@ -142,10 +142,17 @@ igt_spinner_create_request(struct igt_spinner *spin,
 	*batch++ = upper_32_bits(vma->node.start);
 	*batch++ = MI_BATCH_BUFFER_END; /* not reached */
 
-	i915_gem_chipset_flush(spin->i915);
+	if (engine->emit_init_breadcrumb &&
+	    rq->timeline->has_initial_breadcrumb) {
+		err = engine->emit_init_breadcrumb(rq);
+		if (err)
+			goto cancel_rq;
+	}
 
 	err = engine->emit_bb_start(rq, vma->node.start, PAGE_SIZE, 0);
 
+	i915_gem_chipset_flush(spin->i915);
+
 cancel_rq:
 	if (err) {
 		i915_request_skip(rq, err);
diff --git a/drivers/gpu/drm/i915/selftests/intel_lrc.c b/drivers/gpu/drm/i915/selftests/intel_lrc.c
index fb35f53c9ce3..16037a841146 100644
--- a/drivers/gpu/drm/i915/selftests/intel_lrc.c
+++ b/drivers/gpu/drm/i915/selftests/intel_lrc.c
@@ -405,6 +405,165 @@ static int live_suppress_self_preempt(void *arg)
 	goto err_client_b;
 }
 
+static int __i915_sw_fence_call
+dummy_notify(struct i915_sw_fence *fence, enum i915_sw_fence_notify state)
+{
+	return NOTIFY_DONE;
+}
+
+static struct i915_request *dummy_request(struct intel_engine_cs *engine)
+{
+	struct i915_request *rq;
+
+	rq = kmalloc(sizeof(*rq), GFP_KERNEL | __GFP_ZERO);
+	if (!rq)
+		return NULL;
+
+	INIT_LIST_HEAD(&rq->active_list);
+	rq->engine = engine;
+
+	i915_sched_node_init(&rq->sched);
+
+	/* mark this request as permanently incomplete */
+	rq->fence.seqno = 1;
+	BUILD_BUG_ON(sizeof(rq->fence.seqno) != 8); /* upper 32b == 0 */
+	rq->hwsp_seqno = (u32 *)&rq->fence.seqno + 1;
+
+	i915_sw_fence_init(&rq->submit, dummy_notify);
+	i915_sw_fence_commit(&rq->submit);
+
+	return rq;
+}
+
+static void dummy_request_free(struct i915_request *dummy)
+{
+	i915_request_mark_complete(dummy);
+	i915_sched_node_fini(dummy->engine->i915, &dummy->sched);
+	kfree(dummy);
+}
+
+static int live_suppress_wait_preempt(void *arg)
+{
+	struct drm_i915_private *i915 = arg;
+	struct preempt_client client[4];
+	struct intel_engine_cs *engine;
+	enum intel_engine_id id;
+	intel_wakeref_t wakeref;
+	int err = -ENOMEM;
+	int i;
+
+	/*
+	 * Waiters are given a little priority nudge, but not enough
+	 * to actually cause any preemption. Double check that we do
+	 * not needlessly generate preempt-to-idle cycles.
+	 */
+
+	if (!HAS_LOGICAL_RING_PREEMPTION(i915))
+		return 0;
+
+	mutex_lock(&i915->drm.struct_mutex);
+	wakeref = intel_runtime_pm_get(i915);
+
+	if (preempt_client_init(i915, &client[0])) /* ELSP[0] */
+		goto err_unlock;
+	if (preempt_client_init(i915, &client[1])) /* ELSP[1] */
+		goto err_client_0;
+	if (preempt_client_init(i915, &client[2])) /* head of queue */
+		goto err_client_1;
+	if (preempt_client_init(i915, &client[3])) /* bystander */
+		goto err_client_2;
+
+	for_each_engine(engine, i915, id) {
+		int depth;
+
+		if (!engine->emit_init_breadcrumb)
+			continue;
+
+		for (depth = 0; depth < ARRAY_SIZE(client); depth++) {
+			struct i915_request *rq[ARRAY_SIZE(client)];
+			struct i915_request *dummy;
+
+			engine->execlists.preempt_hang.count = 0;
+
+			dummy = dummy_request(engine);
+			if (!dummy)
+				goto err_client_3;
+
+			for (i = 0; i < ARRAY_SIZE(client); i++) {
+				rq[i] = igt_spinner_create_request(&client[i].spin,
+								   client[i].ctx, engine,
+								   MI_NOOP);
+				if (IS_ERR(rq[i])) {
+					err = PTR_ERR(rq[i]);
+					goto err_wedged;
+				}
+
+				/* Disable NEWCLIENT promotion */
+				i915_gem_active_set(&rq[i]->timeline->last_request,
+						    dummy);
+				i915_request_add(rq[i]);
+			}
+
+			dummy_request_free(dummy);
+
+			GEM_BUG_ON(i915_request_completed(rq[0]));
+			if (!igt_wait_for_spinner(&client[0].spin, rq[0])) {
+				pr_err("%s: First client failed to start\n",
+				       engine->name);
+				goto err_wedged;
+			}
+			GEM_BUG_ON(!i915_request_started(rq[0]));
+
+			if (i915_request_wait(rq[depth],
+					      I915_WAIT_LOCKED |
+					      I915_WAIT_PRIORITY,
+					      1) != -ETIME) {
+				pr_err("%s: Waiter depth:%d completed!\n",
+				       engine->name, depth);
+				goto err_wedged;
+			}
+
+			for (i = 0; i < ARRAY_SIZE(client); i++)
+				igt_spinner_end(&client[i].spin);
+
+			if (igt_flush_test(i915, I915_WAIT_LOCKED))
+				goto err_wedged;
+
+			if (engine->execlists.preempt_hang.count) {
+				pr_err("%s: Preemption recorded x%d, depth %d; should have been suppressed!\n",
+				       engine->name,
+				       engine->execlists.preempt_hang.count,
+				       depth);
+				err = -EINVAL;
+				goto err_client_3;
+			}
+		}
+	}
+
+	err = 0;
+err_client_3:
+	preempt_client_fini(&client[3]);
+err_client_2:
+	preempt_client_fini(&client[2]);
+err_client_1:
+	preempt_client_fini(&client[1]);
+err_client_0:
+	preempt_client_fini(&client[0]);
+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:
+	for (i = 0; i < ARRAY_SIZE(client); i++)
+		igt_spinner_end(&client[i].spin);
+	i915_gem_set_wedged(i915);
+	err = -EIO;
+	goto err_client_3;
+}
+
 static int live_preempt_hang(void *arg)
 {
 	struct drm_i915_private *i915 = arg;
@@ -785,6 +944,7 @@ int intel_execlists_live_selftests(struct drm_i915_private *i915)
 		SUBTEST(live_preempt),
 		SUBTEST(live_late_preempt),
 		SUBTEST(live_suppress_self_preempt),
+		SUBTEST(live_suppress_wait_preempt),
 		SUBTEST(live_preempt_hang),
 		SUBTEST(live_preempt_smoke),
 	};
-- 
2.20.1

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