drm/i915/scheduler: Execute requests in order of priorities

Track the priority of each request and use it to determine the order in
which we submit requests to the hardware via execlists.

The priority of the request is determined by the user (eventually via
the context) but may be overridden at any time by the driver. When we set
the priority of the request, we bump the priority of all of its
dependencies to match - so that a high priority drawing operation is not
stuck behind a background task.

When the request is ready to execute (i.e. we have signaled the submit
fence following completion of all its dependencies, including third
party fences), we put the request into a priority sorted rbtree to be
submitted to the hardware. If the request is higher priority than all
pending requests, it will be submitted on the next context-switch
interrupt as soon as the hardware has completed the current request. We
do not currently preempt any current execution to immediately run a very
high priority request, at least not yet.

One more limitation, is that this is first implementation is for
execlists only so currently limited to gen8/gen9.

v2: Replace recursive priority inheritance bumping with an iterative
depth-first search list.
v3: list_next_entry() for walking lists
v4: Explain how the dfs solves the recursion problem with PI.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161114204105.29171-8-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_debugfs.c

@@ -631,8 +631,9 @@ static void print_request(struct seq_file *m,
 			  struct drm_i915_gem_request *rq,
 			  const char *prefix)
 {
-	seq_printf(m, "%s%x [%x:%x] @ %d: %s\n", prefix,
+	seq_printf(m, "%s%x [%x:%x] prio=%d @ %dms: %s\n", prefix,
 		   rq->global_seqno, rq->ctx->hw_id, rq->fence.seqno,
+		   rq->priotree.priority,
 		   jiffies_to_msecs(jiffies - rq->emitted_jiffies),
 		   rq->timeline->common->name);
 }
@@ -3218,6 +3219,7 @@ static int i915_engine_info(struct seq_file *m, void *unused)
 
 		if (i915.enable_execlists) {
 			u32 ptr, read, write;
+			struct rb_node *rb;
 
 			seq_printf(m, "\tExeclist status: 0x%08x %08x\n",
 				   I915_READ(RING_EXECLIST_STATUS_LO(engine)),
@@ -3257,7 +3259,8 @@ static int i915_engine_info(struct seq_file *m, void *unused)
 			rcu_read_unlock();
 
 			spin_lock_irq(&engine->timeline->lock);
-			list_for_each_entry(rq, &engine->execlist_queue, execlist_link) {
+			for (rb = engine->execlist_first; rb; rb = rb_next(rb)) {
+				rq = rb_entry(rb, typeof(*rq), priotree.node);
 				print_request(m, rq, "\t\tQ ");
 			}
 			spin_unlock_irq(&engine->timeline->lock);

drivers/gpu/drm/i915/i915_gem.c

@@ -2721,10 +2721,11 @@ static void i915_gem_cleanup_engine(struct intel_engine_cs *engine)
 
 		spin_lock_irqsave(&engine->timeline->lock, flags);
 
-		INIT_LIST_HEAD(&engine->execlist_queue);
 		i915_gem_request_put(engine->execlist_port[0].request);
 		i915_gem_request_put(engine->execlist_port[1].request);
 		memset(engine->execlist_port, 0, sizeof(engine->execlist_port));
+		engine->execlist_queue = RB_ROOT;
+		engine->execlist_first = NULL;
 
 		spin_unlock_irqrestore(&engine->timeline->lock, flags);
 	}

drivers/gpu/drm/i915/i915_gem_request.c

@@ -132,6 +132,7 @@ __i915_priotree_add_dependency(struct i915_priotree *pt,
 			       struct i915_dependency *dep,
 			       unsigned long flags)
 {
+	INIT_LIST_HEAD(&dep->dfs_link);
 	list_add(&dep->wait_link, &signal->waiters_list);
 	list_add(&dep->signal_link, &pt->signalers_list);
 	dep->signaler = signal;
@@ -158,6 +159,8 @@ i915_priotree_fini(struct drm_i915_private *i915, struct i915_priotree *pt)
 {
 	struct i915_dependency *dep, *next;
 
+	GEM_BUG_ON(!RB_EMPTY_NODE(&pt->node));
+
 	/* Everyone we depended upon (the fences we wait to be signaled)
 	 * should retire before us and remove themselves from our list.
 	 * However, retirement is run independently on each timeline and
@@ -182,6 +185,8 @@ i915_priotree_init(struct i915_priotree *pt)
 {
 	INIT_LIST_HEAD(&pt->signalers_list);
 	INIT_LIST_HEAD(&pt->waiters_list);
+	RB_CLEAR_NODE(&pt->node);
+	pt->priority = INT_MIN;
 }
 
 void i915_gem_retire_noop(struct i915_gem_active *active,

drivers/gpu/drm/i915/i915_gem_request.h

@@ -48,6 +48,7 @@ struct i915_dependency {
 	struct i915_priotree *signaler;
 	struct list_head signal_link;
 	struct list_head wait_link;
+	struct list_head dfs_link;
 	unsigned long flags;
 #define I915_DEPENDENCY_ALLOC BIT(0)
 };
@@ -64,6 +65,10 @@ struct i915_dependency {
 struct i915_priotree {
 	struct list_head signalers_list; /* those before us, we depend upon */
 	struct list_head waiters_list; /* those after us, they depend upon us */
+	struct rb_node node;
+	int priority;
+#define I915_PRIORITY_MAX 1024
+#define I915_PRIORITY_MIN (-I915_PRIORITY_MAX)
 };
 
 /**
@@ -194,9 +199,6 @@ struct drm_i915_gem_request {
 	struct drm_i915_file_private *file_priv;
 	/** file_priv list entry for this request */
 	struct list_head client_list;
-
-	/** Link in the execlist submission queue, guarded by execlist_lock. */
-	struct list_head execlist_link;
 };
 
 extern const struct dma_fence_ops i915_fence_ops;

drivers/gpu/drm/i915/i915_guc_submission.c

@@ -1532,6 +1532,7 @@ int i915_guc_submission_enable(struct drm_i915_private *dev_priv)
 	/* Take over from manual control of ELSP (execlists) */
 	for_each_engine(engine, dev_priv, id) {
 		engine->submit_request = i915_guc_submit;
+		engine->schedule = NULL;
 
 		/* Replay the current set of previously submitted requests */
 		list_for_each_entry(request,

drivers/gpu/drm/i915/intel_engine_cs.c

@@ -239,7 +239,8 @@ static void intel_engine_init_timeline(struct intel_engine_cs *engine)
  */
 void intel_engine_setup_common(struct intel_engine_cs *engine)
 {
-	INIT_LIST_HEAD(&engine->execlist_queue);
+	engine->execlist_queue = RB_ROOT;
+	engine->execlist_first = NULL;
 
 	intel_engine_init_timeline(engine);
 	intel_engine_init_hangcheck(engine);

drivers/gpu/drm/i915/intel_lrc.c

@@ -432,9 +432,10 @@ static bool can_merge_ctx(const struct i915_gem_context *prev,
 
 static void execlists_dequeue(struct intel_engine_cs *engine)
 {
-	struct drm_i915_gem_request *cursor, *last;
+	struct drm_i915_gem_request *last;
 	struct execlist_port *port = engine->execlist_port;
 	unsigned long flags;
+	struct rb_node *rb;
 	bool submit = false;
 
 	last = port->request;
@@ -471,7 +472,11 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 	 */
 
 	spin_lock_irqsave(&engine->timeline->lock, flags);
-	list_for_each_entry(cursor, &engine->execlist_queue, execlist_link) {
+	rb = engine->execlist_first;
+	while (rb) {
+		struct drm_i915_gem_request *cursor =
+			rb_entry(rb, typeof(*cursor), priotree.node);
+
 		/* Can we combine this request with the current port? It has to
 		 * be the same context/ringbuffer and not have any exceptions
 		 * (e.g. GVT saying never to combine contexts).
@@ -503,6 +508,11 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 			port++;
 		}
 
+		rb = rb_next(rb);
+		rb_erase(&cursor->priotree.node, &engine->execlist_queue);
+		RB_CLEAR_NODE(&cursor->priotree.node);
+		cursor->priotree.priority = INT_MAX;
+
 		/* We keep the previous context alive until we retire the
 		 * following request. This ensures that any the context object
 		 * is still pinned for any residual writes the HW makes into it
@@ -517,11 +527,8 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 		submit = true;
 	}
 	if (submit) {
-		/* Decouple all the requests submitted from the queue */
-		engine->execlist_queue.next = &cursor->execlist_link;
-		cursor->execlist_link.prev = &engine->execlist_queue;
-
 		i915_gem_request_assign(&port->request, last);
+		engine->execlist_first = rb;
 	}
 	spin_unlock_irqrestore(&engine->timeline->lock, flags);
 
@@ -626,6 +633,32 @@ static void intel_lrc_irq_handler(unsigned long data)
 	intel_uncore_forcewake_put(dev_priv, engine->fw_domains);
 }
 
+static bool insert_request(struct i915_priotree *pt, struct rb_root *root)
+{
+	struct rb_node **p, *rb;
+	bool first = true;
+
+	/* most positive priority is scheduled first, equal priorities fifo */
+	rb = NULL;
+	p = &root->rb_node;
+	while (*p) {
+		struct i915_priotree *pos;
+
+		rb = *p;
+		pos = rb_entry(rb, typeof(*pos), node);
+		if (pt->priority > pos->priority) {
+			p = &rb->rb_left;
+		} else {
+			p = &rb->rb_right;
+			first = false;
+		}
+	}
+	rb_link_node(&pt->node, rb, p);
+	rb_insert_color(&pt->node, root);
+
+	return first;
+}
+
 static void execlists_submit_request(struct drm_i915_gem_request *request)
 {
 	struct intel_engine_cs *engine = request->engine;
@@ -634,13 +667,112 @@ static void execlists_submit_request(struct drm_i915_gem_request *request)
 	/* Will be called from irq-context when using foreign fences. */
 	spin_lock_irqsave(&engine->timeline->lock, flags);
 
-	list_add_tail(&request->execlist_link, &engine->execlist_queue);
+	if (insert_request(&request->priotree, &engine->execlist_queue))
+		engine->execlist_first = &request->priotree.node;
 	if (execlists_elsp_idle(engine))
 		tasklet_hi_schedule(&engine->irq_tasklet);
 
 	spin_unlock_irqrestore(&engine->timeline->lock, flags);
 }
 
+static struct intel_engine_cs *
+pt_lock_engine(struct i915_priotree *pt, struct intel_engine_cs *locked)
+{
+	struct intel_engine_cs *engine;
+
+	engine = container_of(pt,
+			      struct drm_i915_gem_request,
+			      priotree)->engine;
+	if (engine != locked) {
+		if (locked)
+			spin_unlock_irq(&locked->timeline->lock);
+		spin_lock_irq(&engine->timeline->lock);
+	}
+
+	return engine;
+}
+
+static void execlists_schedule(struct drm_i915_gem_request *request, int prio)
+{
+	struct intel_engine_cs *engine = NULL;
+	struct i915_dependency *dep, *p;
+	struct i915_dependency stack;
+	LIST_HEAD(dfs);
+
+	if (prio <= READ_ONCE(request->priotree.priority))
+		return;
+
+	/* Need BKL in order to use the temporary link inside i915_dependency */
+	lockdep_assert_held(&request->i915->drm.struct_mutex);
+
+	stack.signaler = &request->priotree;
+	list_add(&stack.dfs_link, &dfs);
+
+	/* Recursively bump all dependent priorities to match the new request.
+	 *
+	 * A naive approach would be to use recursion:
+	 * static void update_priorities(struct i915_priotree *pt, prio) {
+	 *	list_for_each_entry(dep, &pt->signalers_list, signal_link)
+	 *		update_priorities(dep->signal, prio)
+	 *	insert_request(pt);
+	 * }
+	 * but that may have unlimited recursion depth and so runs a very
+	 * real risk of overunning the kernel stack. Instead, we build
+	 * a flat list of all dependencies starting with the current request.
+	 * As we walk the list of dependencies, we add all of its dependencies
+	 * to the end of the list (this may include an already visited
+	 * request) and continue to walk onwards onto the new dependencies. The
+	 * end result is a topological list of requests in reverse order, the
+	 * last element in the list is the request we must execute first.
+	 */
+	list_for_each_entry_safe(dep, p, &dfs, dfs_link) {
+		struct i915_priotree *pt = dep->signaler;
+
+		list_for_each_entry(p, &pt->signalers_list, signal_link)
+			if (prio > READ_ONCE(p->signaler->priority))
+				list_move_tail(&p->dfs_link, &dfs);
+
+		p = list_next_entry(dep, dfs_link);
+		if (!RB_EMPTY_NODE(&pt->node))
+			continue;
+
+		engine = pt_lock_engine(pt, engine);
+
+		/* If it is not already in the rbtree, we can update the
+		 * priority inplace and skip over it (and its dependencies)
+		 * if it is referenced *again* as we descend the dfs.
+		 */
+		if (prio > pt->priority && RB_EMPTY_NODE(&pt->node)) {
+			pt->priority = prio;
+			list_del_init(&dep->dfs_link);
+		}
+	}
+
+	/* Fifo and depth-first replacement ensure our deps execute before us */
+	list_for_each_entry_safe_reverse(dep, p, &dfs, dfs_link) {
+		struct i915_priotree *pt = dep->signaler;
+
+		INIT_LIST_HEAD(&dep->dfs_link);
+
+		engine = pt_lock_engine(pt, engine);
+
+		if (prio <= pt->priority)
+			continue;
+
+		GEM_BUG_ON(RB_EMPTY_NODE(&pt->node));
+
+		pt->priority = prio;
+		rb_erase(&pt->node, &engine->execlist_queue);
+		if (insert_request(pt, &engine->execlist_queue))
+			engine->execlist_first = &pt->node;
+	}
+
+	if (engine)
+		spin_unlock_irq(&engine->timeline->lock);
+
+	/* XXX Do we need to preempt to make room for us and our deps? */
+}
+
 int intel_logical_ring_alloc_request_extras(struct drm_i915_gem_request *request)
 {
 	struct intel_engine_cs *engine = request->engine;
@@ -1677,8 +1809,10 @@ void intel_execlists_enable_submission(struct drm_i915_private *dev_priv)
 	struct intel_engine_cs *engine;
 	enum intel_engine_id id;
 
-	for_each_engine(engine, dev_priv, id)
+	for_each_engine(engine, dev_priv, id) {
 		engine->submit_request = execlists_submit_request;
+		engine->schedule = execlists_schedule;
+	}
 }
 
 static void
@@ -1691,6 +1825,7 @@ logical_ring_default_vfuncs(struct intel_engine_cs *engine)
 	engine->emit_breadcrumb = gen8_emit_breadcrumb;
 	engine->emit_breadcrumb_sz = gen8_emit_breadcrumb_sz;
 	engine->submit_request = execlists_submit_request;
+	engine->schedule = execlists_schedule;
 
 	engine->irq_enable = gen8_logical_ring_enable_irq;
 	engine->irq_disable = gen8_logical_ring_disable_irq;

drivers/gpu/drm/i915/intel_ringbuffer.h

@@ -348,7 +348,8 @@ struct intel_engine_cs {
 		struct drm_i915_gem_request *request;
 		unsigned int count;
 	} execlist_port[2];
-	struct list_head execlist_queue;
+	struct rb_root execlist_queue;
+	struct rb_node *execlist_first;
 	unsigned int fw_domains;
 	bool disable_lite_restore_wa;
 	bool preempt_wa;