drm/i915/gt: Allow temporary suspension of inflight requests

In order to support out-of-line error capture, we need to remove the
active request from HW and put it to one side while a worker compresses
and stores all the details associated with that request. (As that
compression may take an arbitrary user-controlled amount of time, we
want to let the engine continue running on other workloads while the
hanging request is dumped.) Not only do we need to remove the active
request, but we also have to remove its context and all requests that
were dependent on it (both in flight, queued and future submission).

Finally once the capture is complete, we need to be able to resubmit the
request and its dependents and allow them to execute.

v2: Replace stack recursion with a simple list.
v3: Check all the parents, not just the first, when searching for a
stuck ancestor!

References: https://gitlab.freedesktop.org/drm/intel/issues/738
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/20200116184754.2860848-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_engine_cs.c

@@ -671,6 +671,7 @@ void
 intel_engine_init_active(struct intel_engine_cs *engine, unsigned int subclass)
 {
 	INIT_LIST_HEAD(&engine->active.requests);
+	INIT_LIST_HEAD(&engine->active.hold);
 
 	spin_lock_init(&engine->active.lock);
 	lockdep_set_subclass(&engine->active.lock, subclass);
@@ -1422,6 +1423,17 @@ static void print_request_ring(struct drm_printer *m, struct i915_request *rq)
 	}
 }
 
+static unsigned long list_count(struct list_head *list)
+{
+	struct list_head *pos;
+	unsigned long count = 0;
+
+	list_for_each(pos, list)
+		count++;
+
+	return count;
+}
+
 void intel_engine_dump(struct intel_engine_cs *engine,
 		       struct drm_printer *m,
 		       const char *header, ...)
@@ -1491,6 +1503,7 @@ void intel_engine_dump(struct intel_engine_cs *engine,
 			hexdump(m, rq->context->lrc_reg_state, PAGE_SIZE);
 		}
 	}
+	drm_printf(m, "\tOn hold?: %lu\n", list_count(&engine->active.hold));
 	spin_unlock_irqrestore(&engine->active.lock, flags);
 
 	drm_printf(m, "\tMMIO base:  0x%08x\n", engine->mmio_base);

drivers/gpu/drm/i915/gt/intel_engine_types.h

@@ -295,6 +295,7 @@ struct intel_engine_cs {
 	struct {
 		spinlock_t lock;
 		struct list_head requests;
+		struct list_head hold; /* ready requests, but on hold */
 	} active;
 
 	struct llist_head barrier_tasks;

drivers/gpu/drm/i915/gt/intel_lrc.c

@@ -1635,8 +1635,8 @@ static void defer_request(struct i915_request *rq, struct list_head * const pl)
 				   !i915_request_completed(rq));
 
 			GEM_BUG_ON(i915_request_is_active(w));
-			if (list_empty(&w->sched.link))
-				continue; /* Not yet submitted; unready */
+			if (!i915_request_is_ready(w))
+				continue;
 
 			if (rq_prio(w) < rq_prio(rq))
 				continue;
@@ -2354,6 +2354,145 @@ static void __execlists_submission_tasklet(struct intel_engine_cs *const engine)
 	}
 }
 
+static void __execlists_hold(struct i915_request *rq)
+{
+	LIST_HEAD(list);
+
+	do {
+		struct i915_dependency *p;
+
+		if (i915_request_is_active(rq))
+			__i915_request_unsubmit(rq);
+
+		RQ_TRACE(rq, "on hold\n");
+		clear_bit(I915_FENCE_FLAG_PQUEUE, &rq->fence.flags);
+		list_move_tail(&rq->sched.link, &rq->engine->active.hold);
+		i915_request_set_hold(rq);
+
+		list_for_each_entry(p, &rq->sched.waiters_list, wait_link) {
+			struct i915_request *w =
+				container_of(p->waiter, typeof(*w), sched);
+
+			/* Leave semaphores spinning on the other engines */
+			if (w->engine != rq->engine)
+				continue;
+
+			if (!i915_request_is_ready(w))
+				continue;
+
+			if (i915_request_completed(w))
+				continue;
+
+			if (i915_request_on_hold(rq))
+				continue;
+
+			list_move_tail(&w->sched.link, &list);
+		}
+
+		rq = list_first_entry_or_null(&list, typeof(*rq), sched.link);
+	} while (rq);
+}
+
+__maybe_unused
+static void execlists_hold(struct intel_engine_cs *engine,
+			   struct i915_request *rq)
+{
+	spin_lock_irq(&engine->active.lock);
+
+	/*
+	 * Transfer this request onto the hold queue to prevent it
+	 * being resumbitted to HW (and potentially completed) before we have
+	 * released it. Since we may have already submitted following
+	 * requests, we need to remove those as well.
+	 */
+	GEM_BUG_ON(i915_request_on_hold(rq));
+	GEM_BUG_ON(rq->engine != engine);
+	__execlists_hold(rq);
+
+	spin_unlock_irq(&engine->active.lock);
+}
+
+static bool hold_request(const struct i915_request *rq)
+{
+	struct i915_dependency *p;
+
+	/*
+	 * If one of our ancestors is on hold, we must also be on hold,
+	 * otherwise we will bypass it and execute before it.
+	 */
+	list_for_each_entry(p, &rq->sched.signalers_list, signal_link) {
+		const struct i915_request *s =
+			container_of(p->signaler, typeof(*s), sched);
+
+		if (s->engine != rq->engine)
+			continue;
+
+		if (i915_request_on_hold(s))
+			return true;
+	}
+
+	return false;
+}
+
+static void __execlists_unhold(struct i915_request *rq)
+{
+	LIST_HEAD(list);
+
+	do {
+		struct i915_dependency *p;
+
+		GEM_BUG_ON(!i915_request_on_hold(rq));
+		GEM_BUG_ON(!i915_sw_fence_signaled(&rq->submit));
+
+		i915_request_clear_hold(rq);
+		list_move_tail(&rq->sched.link,
+			       i915_sched_lookup_priolist(rq->engine,
+							  rq_prio(rq)));
+		set_bit(I915_FENCE_FLAG_PQUEUE, &rq->fence.flags);
+		RQ_TRACE(rq, "hold release\n");
+
+		/* Also release any children on this engine that are ready */
+		list_for_each_entry(p, &rq->sched.waiters_list, wait_link) {
+			struct i915_request *w =
+				container_of(p->waiter, typeof(*w), sched);
+
+			if (w->engine != rq->engine)
+				continue;
+
+			if (!i915_request_on_hold(rq))
+				continue;
+
+			/* Check that no other parents are also on hold */
+			if (hold_request(rq))
+				continue;
+
+			list_move_tail(&w->sched.link, &list);
+		}
+
+		rq = list_first_entry_or_null(&list, typeof(*rq), sched.link);
+	} while (rq);
+}
+
+__maybe_unused
+static void execlists_unhold(struct intel_engine_cs *engine,
+			     struct i915_request *rq)
+{
+	spin_lock_irq(&engine->active.lock);
+
+	/*
+	 * Move this request back to the priority queue, and all of its
+	 * children and grandchildren that were suspended along with it.
+	 */
+	__execlists_unhold(rq);
+
+	if (rq_prio(rq) > engine->execlists.queue_priority_hint) {
+		engine->execlists.queue_priority_hint = rq_prio(rq);
+		tasklet_hi_schedule(&engine->execlists.tasklet);
+	}
+
+	spin_unlock_irq(&engine->active.lock);
+}
+
 static noinline void preempt_reset(struct intel_engine_cs *engine)
 {
 	const unsigned int bit = I915_RESET_ENGINE + engine->id;
@@ -2466,6 +2605,13 @@ static void submit_queue(struct intel_engine_cs *engine,
 	__submit_queue_imm(engine);
 }
 
+static bool ancestor_on_hold(const struct intel_engine_cs *engine,
+			     const struct i915_request *rq)
+{
+	GEM_BUG_ON(i915_request_on_hold(rq));
+	return !list_empty(&engine->active.hold) && hold_request(rq);
+}
+
 static void execlists_submit_request(struct i915_request *request)
 {
 	struct intel_engine_cs *engine = request->engine;
@@ -2474,12 +2620,17 @@ static void execlists_submit_request(struct i915_request *request)
 	/* Will be called from irq-context when using foreign fences. */
 	spin_lock_irqsave(&engine->active.lock, flags);
 
-	queue_request(engine, request);
+	if (unlikely(ancestor_on_hold(engine, request))) {
+		list_add_tail(&request->sched.link, &engine->active.hold);
+		i915_request_set_hold(request);
+	} else {
+		queue_request(engine, request);
 
-	GEM_BUG_ON(RB_EMPTY_ROOT(&engine->execlists.queue.rb_root));
-	GEM_BUG_ON(list_empty(&request->sched.link));
+		GEM_BUG_ON(RB_EMPTY_ROOT(&engine->execlists.queue.rb_root));
+		GEM_BUG_ON(list_empty(&request->sched.link));
 
-	submit_queue(engine, request);
+		submit_queue(engine, request);
+	}
 
 	spin_unlock_irqrestore(&engine->active.lock, flags);
 }
@@ -3320,6 +3471,10 @@ static void execlists_reset_cancel(struct intel_engine_cs *engine)
 		i915_priolist_free(p);
 	}
 
+	/* On-hold requests will be flushed to timeline upon their release */
+	list_for_each_entry(rq, &engine->active.hold, sched.link)
+		mark_eio(rq);
+
 	/* Cancel all attached virtual engines */
 	while ((rb = rb_first_cached(&execlists->virtual))) {
 		struct virtual_engine *ve =

drivers/gpu/drm/i915/gt/selftest_lrc.c

@@ -285,6 +285,108 @@ static int live_unlite_preempt(void *arg)
 	return live_unlite_restore(arg, I915_USER_PRIORITY(I915_PRIORITY_MAX));
 }
 
+static int live_hold_reset(void *arg)
+{
+	struct intel_gt *gt = arg;
+	struct intel_engine_cs *engine;
+	enum intel_engine_id id;
+	struct igt_spinner spin;
+	int err = 0;
+
+	/*
+	 * In order to support offline error capture for fast preempt reset,
+	 * we need to decouple the guilty request and ensure that it and its
+	 * descendents are not executed while the capture is in progress.
+	 */
+
+	if (!intel_has_reset_engine(gt))
+		return 0;
+
+	if (igt_spinner_init(&spin, gt))
+		return -ENOMEM;
+
+	for_each_engine(engine, gt, id) {
+		struct intel_context *ce;
+		unsigned long heartbeat;
+		struct i915_request *rq;
+
+		ce = intel_context_create(engine);
+		if (IS_ERR(ce)) {
+			err = PTR_ERR(ce);
+			break;
+		}
+
+		engine_heartbeat_disable(engine, &heartbeat);
+
+		rq = igt_spinner_create_request(&spin, ce, MI_ARB_CHECK);
+		if (IS_ERR(rq)) {
+			err = PTR_ERR(rq);
+			goto out;
+		}
+		i915_request_add(rq);
+
+		if (!igt_wait_for_spinner(&spin, rq)) {
+			intel_gt_set_wedged(gt);
+			err = -ETIME;
+			goto out;
+		}
+
+		/* We have our request executing, now remove it and reset */
+
+		if (test_and_set_bit(I915_RESET_ENGINE + id,
+				     &gt->reset.flags)) {
+			spin_unlock_irq(&engine->active.lock);
+			intel_gt_set_wedged(gt);
+			err = -EBUSY;
+			goto out;
+		}
+		tasklet_disable(&engine->execlists.tasklet);
+
+		engine->execlists.tasklet.func(engine->execlists.tasklet.data);
+		GEM_BUG_ON(execlists_active(&engine->execlists) != rq);
+
+		execlists_hold(engine, rq);
+		GEM_BUG_ON(!i915_request_on_hold(rq));
+
+		intel_engine_reset(engine, NULL);
+		GEM_BUG_ON(rq->fence.error != -EIO);
+
+		tasklet_enable(&engine->execlists.tasklet);
+		clear_and_wake_up_bit(I915_RESET_ENGINE + id,
+				      &gt->reset.flags);
+
+		/* Check that we do not resubmit the held request */
+		i915_request_get(rq);
+		if (!i915_request_wait(rq, 0, HZ / 5)) {
+			pr_err("%s: on hold request completed!\n",
+			       engine->name);
+			i915_request_put(rq);
+			err = -EIO;
+			goto out;
+		}
+		GEM_BUG_ON(!i915_request_on_hold(rq));
+
+		/* But is resubmitted on release */
+		execlists_unhold(engine, rq);
+		if (i915_request_wait(rq, 0, HZ / 5) < 0) {
+			pr_err("%s: held request did not complete!\n",
+			       engine->name);
+			intel_gt_set_wedged(gt);
+			err = -ETIME;
+		}
+		i915_request_put(rq);
+
+out:
+		engine_heartbeat_enable(engine, heartbeat);
+		intel_context_put(ce);
+		if (err)
+			break;
+	}
+
+	igt_spinner_fini(&spin);
+	return err;
+}
+
 static int
 emit_semaphore_chain(struct i915_request *rq, struct i915_vma *vma, int idx)
 {
@@ -3315,6 +3417,7 @@ int intel_execlists_live_selftests(struct drm_i915_private *i915)
 		SUBTEST(live_sanitycheck),
 		SUBTEST(live_unlite_switch),
 		SUBTEST(live_unlite_preempt),
+		SUBTEST(live_hold_reset),
 		SUBTEST(live_timeslice_preempt),
 		SUBTEST(live_timeslice_queue),
 		SUBTEST(live_busywait_preempt),

drivers/gpu/drm/i915/i915_request.h

@@ -90,6 +90,13 @@ enum {
 	 */
 	I915_FENCE_FLAG_SIGNAL,
 
+	/*
+	 * I915_FENCE_FLAG_HOLD - this request is currently on hold
+	 *
+	 * This request has been suspended, pending an ongoing investigation.
+	 */
+	I915_FENCE_FLAG_HOLD,
+
 	/*
 	 * I915_FENCE_FLAG_NOPREEMPT - this request should not be preempted
 	 *
@@ -471,6 +478,27 @@ static inline bool i915_request_is_running(const struct i915_request *rq)
 	return __i915_request_has_started(rq);
 }
 
+/**
+ * i915_request_is_running - check if the request is ready for execution
+ * @rq: the request
+ *
+ * Upon construction, the request is instructed to wait upon various
+ * signals before it is ready to be executed by the HW. That is, we do
+ * not want to start execution and read data before it is written. In practice,
+ * this is controlled with a mixture of interrupts and semaphores. Once
+ * the submit fence is completed, the backend scheduler will place the
+ * request into its queue and from there submit it for execution. So we
+ * can detect when a request is eligible for execution (and is under control
+ * of the scheduler) by querying where it is in any of the scheduler's lists.
+ *
+ * Returns true if the request is ready for execution (it may be inflight),
+ * false otherwise.
+ */
+static inline bool i915_request_is_ready(const struct i915_request *rq)
+{
+	return !list_empty(&rq->sched.link);
+}
+
 static inline bool i915_request_completed(const struct i915_request *rq)
 {
 	if (i915_request_signaled(rq))
@@ -500,6 +528,21 @@ static inline bool i915_request_has_sentinel(const struct i915_request *rq)
 	return unlikely(test_bit(I915_FENCE_FLAG_SENTINEL, &rq->fence.flags));
 }
 
+static inline bool i915_request_on_hold(const struct i915_request *rq)
+{
+	return unlikely(test_bit(I915_FENCE_FLAG_HOLD, &rq->fence.flags));
+}
+
+static inline void i915_request_set_hold(struct i915_request *rq)
+{
+	set_bit(I915_FENCE_FLAG_HOLD, &rq->fence.flags);
+}
+
+static inline void i915_request_clear_hold(struct i915_request *rq)
+{
+	clear_bit(I915_FENCE_FLAG_HOLD, &rq->fence.flags);
+}
+
 static inline struct intel_timeline *
 i915_request_timeline(struct i915_request *rq)
 {