[Intel-gfx] [PATCH 39/62] drm/i915: Replace priolist rbtree with a skiplist

From: Chris Wilson <chris@chris-wilson.co.uk>
To: intel-gfx@lists.freedesktop.org
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Subject: [Intel-gfx] [PATCH 39/62] drm/i915: Replace priolist rbtree with a skiplist
Date: Wed, 23 Dec 2020 11:11:03 +0000	[thread overview]
Message-ID: <20201223111126.3338-39-chris@chris-wilson.co.uk> (raw)
In-Reply-To: <20201223111126.3338-1-chris@chris-wilson.co.uk>

Replace the priolist rbtree with a skiplist. The crucial difference is
that walking and removing the first element of a skiplist is O(1), but
O(lgN) for an rbtree, as we need to rebalance on remove. This is a
hindrance for submission latency as it occurs between picking a request
for the priolist and submitting it to hardware, as well effectively
trippling the number of O(lgN) operations required under the irqoff lock.
This is critical to reducing the latency jitter with multiple clients.

The downsides to skiplists are that lookup/insertion is only
probablistically O(lgN) and there is a significant memory penalty to
as each skip node is larger than the rbtree equivalent. Furthermore, we
don't use dynamic arrays for the skiplist, so the allocation is fixed,
and imposes an upper bound on the scalability wrt to the number of
inflight requests.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
---
 .../drm/i915/gt/intel_execlists_submission.c  |  63 ++++---
 .../gpu/drm/i915/gt/uc/intel_guc_submission.c |  30 +--
 drivers/gpu/drm/i915/i915_priolist_types.h    |  29 ++-
 drivers/gpu/drm/i915/i915_scheduler.c         | 178 ++++++++++++------
 drivers/gpu/drm/i915/i915_scheduler.h         |  11 +-
 drivers/gpu/drm/i915/i915_scheduler_types.h   |   2 +-
 .../drm/i915/selftests/i915_mock_selftests.h  |   1 +
 .../gpu/drm/i915/selftests/i915_scheduler.c   |  42 ++++-
 8 files changed, 241 insertions(+), 115 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_execlists_submission.c b/drivers/gpu/drm/i915/gt/intel_execlists_submission.c
index 0a14b2d305d2..d99d682d8569 100644
--- a/drivers/gpu/drm/i915/gt/intel_execlists_submission.c
+++ b/drivers/gpu/drm/i915/gt/intel_execlists_submission.c
@@ -246,11 +246,6 @@ ring_set_paused(const struct intel_engine_cs *engine, int state)
 		wmb();
 }
 
-static inline struct i915_priolist *to_priolist(struct rb_node *rb)
-{
-	return rb_entry(rb, struct i915_priolist, node);
-}
-
 static inline int rq_prio(const struct i915_request *rq)
 {
 	return READ_ONCE(rq->sched.attr.priority);
@@ -274,15 +269,31 @@ static int effective_prio(const struct i915_request *rq)
 	return prio;
 }
 
-static int queue_prio(const struct i915_sched_engine *se)
+static struct i915_request *first_request(struct i915_sched_engine *se)
 {
-	struct rb_node *rb;
+	struct i915_priolist *pl;
+
+	for_each_priolist(pl, &se->queue) {
+		if (likely(!list_empty(&pl->requests)))
+			return list_first_entry(&pl->requests,
+						struct i915_request,
+						sched.link);
+
+		i915_priolist_advance(&se->queue, pl);
+	}
+
+	return NULL;
+}
 
-	rb = rb_first_cached(&se->queue);
-	if (!rb)
+static int queue_prio(struct i915_sched_engine *se)
+{
+	struct i915_request *rq;
+
+	rq = first_request(se);
+	if (!rq)
 		return INT_MIN;
 
-	return to_priolist(rb)->priority;
+	return rq_prio(rq);
 }
 
 static int virtual_prio(const struct intel_engine_execlists *el)
@@ -292,7 +303,7 @@ static int virtual_prio(const struct intel_engine_execlists *el)
 	return rb ? rb_entry(rb, struct ve_node, rb)->prio : INT_MIN;
 }
 
-static inline bool need_preempt(const struct intel_engine_cs *engine,
+static inline bool need_preempt(struct intel_engine_cs *engine,
 				const struct i915_request *rq)
 {
 	int last_prio;
@@ -1112,6 +1123,7 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 	struct i915_request ** const last_port = port + execlists->port_mask;
 	struct i915_request *last = *execlists->active;
 	struct virtual_engine *ve;
+	struct i915_priolist *pl;
 	struct rb_node *rb;
 	bool submit = false;
 
@@ -1317,11 +1329,10 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 			break;
 	}
 
-	while ((rb = rb_first_cached(&engine->active.queue))) {
-		struct i915_priolist *p = to_priolist(rb);
+	for_each_priolist(pl, &engine->active.queue) {
 		struct i915_request *rq, *rn;
 
-		priolist_for_each_request_consume(rq, rn, p) {
+		priolist_for_each_request_safe(rq, rn, pl) {
 			bool merge = true;
 
 			/*
@@ -1396,8 +1407,7 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 			}
 		}
 
-		rb_erase_cached(&p->node, &engine->active.queue);
-		i915_priolist_free(p);
+		i915_priolist_advance(&engine->active.queue, pl);
 	}
 done:
 	*port++ = i915_request_get(last);
@@ -2621,6 +2631,7 @@ static void execlists_reset_cancel(struct intel_engine_cs *engine)
 {
 	struct intel_engine_execlists * const execlists = &engine->execlists;
 	struct i915_request *rq, *rn;
+	struct i915_priolist *pl;
 	struct rb_node *rb;
 	unsigned long flags;
 
@@ -2651,16 +2662,12 @@ static void execlists_reset_cancel(struct intel_engine_cs *engine)
 	intel_engine_signal_breadcrumbs(engine);
 
 	/* Flush the queued requests to the timeline list (for retiring). */
-	while ((rb = rb_first_cached(&engine->active.queue))) {
-		struct i915_priolist *p = to_priolist(rb);
-
-		priolist_for_each_request_consume(rq, rn, p) {
+	for_each_priolist(pl, &engine->active.queue) {
+		priolist_for_each_request_safe(rq, rn, pl) {
 			mark_eio(rq);
 			__i915_request_submit(rq);
 		}
-
-		rb_erase_cached(&p->node, &engine->active.queue);
-		i915_priolist_free(p);
+		i915_priolist_advance(&engine->active.queue, pl);
 	}
 	GEM_BUG_ON(!i915_sched_is_idle(&engine->active));
 
@@ -2693,7 +2700,6 @@ static void execlists_reset_cancel(struct intel_engine_cs *engine)
 	/* Remaining _unready_ requests will be nop'ed when submitted */
 
 	execlists->queue_priority_hint = INT_MIN;
-	engine->active.queue = RB_ROOT_CACHED;
 
 	GEM_BUG_ON(__tasklet_is_enabled(&engine->active.tasklet));
 	engine->active.tasklet.func = nop_submission_tasklet;
@@ -3076,6 +3082,8 @@ static void virtual_context_exit(struct intel_context *ce)
 
 	for (n = 0; n < ve->num_siblings; n++)
 		intel_engine_pm_put(ve->siblings[n]);
+
+	i915_sched_park_engine(&ve->base.active);
 }
 
 static const struct intel_context_ops virtual_context_ops = {
@@ -3486,6 +3494,7 @@ void intel_execlists_show_requests(struct intel_engine_cs *engine,
 {
 	const struct intel_engine_execlists *execlists = &engine->execlists;
 	struct i915_request *rq, *last;
+	struct i915_priolist *pl;
 	unsigned long flags;
 	unsigned int count;
 	struct rb_node *rb;
@@ -3515,10 +3524,8 @@ void intel_execlists_show_requests(struct intel_engine_cs *engine,
 
 	last = NULL;
 	count = 0;
-	for (rb = rb_first_cached(&engine->active.queue); rb; rb = rb_next(rb)) {
-		struct i915_priolist *p = rb_entry(rb, typeof(*p), node);
-
-		priolist_for_each_request(rq, p) {
+	for_each_priolist(pl, &engine->active.queue) {
+		priolist_for_each_request(rq, pl) {
 			if (count++ < max - 1)
 				show_request(m, rq, "\t\t", 0);
 			else
diff --git a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
index b9cb6807d101..34be8dc8949e 100644
--- a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
+++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
@@ -55,11 +55,6 @@
  *
  */
 
-static inline struct i915_priolist *to_priolist(struct rb_node *rb)
-{
-	return rb_entry(rb, struct i915_priolist, node);
-}
-
 static struct guc_stage_desc *__get_stage_desc(struct intel_guc *guc, u32 id)
 {
 	struct guc_stage_desc *base = guc->stage_desc_pool_vaddr;
@@ -292,8 +287,8 @@ static void __guc_dequeue(struct intel_engine_cs *engine)
 	struct i915_request ** const last_port = first + execlists->port_mask;
 	struct i915_request *last = first[0];
 	struct i915_request **port;
+	struct i915_priolist *pl;
 	bool submit = false;
-	struct rb_node *rb;
 
 	lockdep_assert_held(&engine->active.lock);
 
@@ -310,11 +305,10 @@ static void __guc_dequeue(struct intel_engine_cs *engine)
 	 * event.
 	 */
 	port = first;
-	while ((rb = rb_first_cached(&engine->active.queue))) {
-		struct i915_priolist *p = to_priolist(rb);
+	for_each_priolist(pl, &engine->active.queue) {
 		struct i915_request *rq, *rn;
 
-		priolist_for_each_request_consume(rq, rn, p) {
+		priolist_for_each_request_safe(rq, rn, pl) {
 			if (last && rq->context != last->context) {
 				if (port == last_port)
 					goto done;
@@ -330,12 +324,11 @@ static void __guc_dequeue(struct intel_engine_cs *engine)
 			last = rq;
 		}
 
-		rb_erase_cached(&p->node, &engine->active.queue);
-		i915_priolist_free(p);
+		i915_priolist_advance(&engine->active.queue, pl);
 	}
 done:
 	execlists->queue_priority_hint =
-		rb ? to_priolist(rb)->priority : INT_MIN;
+		pl != &engine->active.queue.sentinel ? pl->priority : INT_MIN;
 	if (submit) {
 		*port = schedule_in(last, port - execlists->inflight);
 		*++port = NULL;
@@ -450,7 +443,7 @@ static void guc_reset_cancel(struct intel_engine_cs *engine)
 {
 	struct intel_engine_execlists * const execlists = &engine->execlists;
 	struct i915_request *rq, *rn;
-	struct rb_node *rb;
+	struct i915_priolist *p;
 	unsigned long flags;
 
 	ENGINE_TRACE(engine, "\n");
@@ -481,25 +474,20 @@ static void guc_reset_cancel(struct intel_engine_cs *engine)
 	}
 
 	/* Flush the queued requests to the timeline list (for retiring). */
-	while ((rb = rb_first_cached(&engine->active.queue))) {
-		struct i915_priolist *p = to_priolist(rb);
-
-		priolist_for_each_request_consume(rq, rn, p) {
+	for_each_priolist(p, &engine->active.queue) {
+		priolist_for_each_request_safe(rq, rn, p) {
 			list_del_init(&rq->sched.link);
 			__i915_request_submit(rq);
 			dma_fence_set_error(&rq->fence, -EIO);
 			i915_request_mark_complete(rq);
 		}
-
-		rb_erase_cached(&p->node, &engine->active.queue);
-		i915_priolist_free(p);
+		i915_priolist_advance(&engine->active.queue, p);
 	}
 	GEM_BUG_ON(!i915_sched_is_idle(&engine->active));
 
 	/* Remaining _unready_ requests will be nop'ed when submitted */
 
 	execlists->queue_priority_hint = INT_MIN;
-	engine->active.queue = RB_ROOT_CACHED;
 
 	spin_unlock_irqrestore(&engine->active.lock, flags);
 }
diff --git a/drivers/gpu/drm/i915/i915_priolist_types.h b/drivers/gpu/drm/i915/i915_priolist_types.h
index bc2fa84f98a8..807cd8f65481 100644
--- a/drivers/gpu/drm/i915/i915_priolist_types.h
+++ b/drivers/gpu/drm/i915/i915_priolist_types.h
@@ -38,10 +38,37 @@ enum {
 #define I915_PRIORITY_UNPREEMPTABLE INT_MAX
 #define I915_PRIORITY_BARRIER (I915_PRIORITY_UNPREEMPTABLE - 1)
 
+#ifdef CONFIG_64BIT
+#define I915_PRIOLIST_HEIGHT 12
+#else
+#define I915_PRIOLIST_HEIGHT 11
+#endif
+
 struct i915_priolist {
 	struct list_head requests;
-	struct rb_node node;
 	int priority;
+
+	int level;
+	struct i915_priolist *next[I915_PRIOLIST_HEIGHT];
+};
+
+struct i915_priolist_root {
+	struct i915_priolist sentinel;
+	u32 prng;
 };
 
+#define priolist_first(root) ((root)->sentinel.next[0])
+#define i915_priolist_is_empty(root) (priolist_first(root) == &(root)->sentinel)
+
+#define for_each_priolist(p, root) \
+	for ((p) = priolist_first(root); \
+	     (p) != &(root)->sentinel; \
+	     (p) = (p)->next[0])
+
+#define priolist_for_each_request(it, plist) \
+	list_for_each_entry(it, &(plist)->requests, sched.link)
+
+#define priolist_for_each_request_safe(it, n, plist) \
+	list_for_each_entry_safe(it, n, &(plist)->requests, sched.link)
+
 #endif /* _I915_PRIOLIST_TYPES_H_ */
diff --git a/drivers/gpu/drm/i915/i915_scheduler.c b/drivers/gpu/drm/i915/i915_scheduler.c
index 99f1dd6a9719..a0e8ecd06d8d 100644
--- a/drivers/gpu/drm/i915/i915_scheduler.c
+++ b/drivers/gpu/drm/i915/i915_scheduler.c
@@ -4,7 +4,9 @@
  * Copyright © 2018 Intel Corporation
  */
 
+#include <linux/bitops.h>
 #include <linux/mutex.h>
+#include <linux/prandom.h>
 
 #include "gt/intel_ring.h"
 #include "gt/intel_lrc_reg.h"
@@ -91,15 +93,25 @@ static void i915_sched_init_ipi(struct i915_sched_ipi *ipi)
 	ipi->list = NULL;
 }
 
+static void init_priolist(struct i915_priolist_root *const root)
+{
+	struct i915_priolist *pl = &root->sentinel;
+
+	memset_p((void **)pl->next, pl, ARRAY_SIZE(pl->next));
+	pl->priority = INT_MIN;
+	INIT_LIST_HEAD(&pl->requests);
+	pl->level = 0;
+}
+
 void i915_sched_init_engine(struct i915_sched_engine *se,
 			    unsigned int subclass)
 {
 	spin_lock_init(&se->lock);
 	lockdep_set_subclass(&se->lock, subclass);
 
+	init_priolist(&se->queue);
 	INIT_LIST_HEAD(&se->requests);
 	INIT_LIST_HEAD(&se->hold);
-	se->queue = RB_ROOT_CACHED;
 
 	i915_sched_init_ipi(&se->ipi);
 
@@ -116,8 +128,37 @@ void i915_sched_init_engine(struct i915_sched_engine *se,
 #endif
 }
 
+__maybe_unused static bool priolist_idle(struct i915_priolist_root *root)
+{
+	struct i915_priolist *pl = &root->sentinel;
+	int lvl;
+
+	for (lvl = 0; lvl < ARRAY_SIZE(pl->next); lvl++) {
+		if (pl->next[lvl] != pl) {
+			GEM_TRACE_ERR("root[%d] is not empty\n", lvl);
+			return false;
+		}
+	}
+
+	if (pl->level) {
+		GEM_TRACE_ERR("root is not clear: %d\n", pl->level);
+		return false;
+	}
+
+	return true;
+}
+
 void i915_sched_park_engine(struct i915_sched_engine *se)
 {
+	struct i915_priolist_root *root = &se->queue;
+	struct i915_priolist *pl, *n;
+
+	GEM_BUG_ON(!priolist_idle(root));
+
+	list_for_each_entry_safe(pl, n, &root->sentinel.requests, requests)
+		kmem_cache_free(global.slab_priorities, pl);
+	INIT_LIST_HEAD(&root->sentinel.requests);
+
 	GEM_BUG_ON(!i915_sched_is_idle(se));
 	se->no_priolist = false;
 }
@@ -183,68 +224,54 @@ static inline bool node_signaled(const struct i915_sched_node *node)
 	return i915_request_completed(node_to_request(node));
 }
 
-static inline struct i915_priolist *to_priolist(struct rb_node *rb)
+static inline unsigned int random_level(struct i915_priolist_root *root)
 {
-	return rb_entry(rb, struct i915_priolist, node);
-}
-
-static void assert_priolists(struct i915_sched_engine * const se)
-{
-	struct rb_node *rb;
-	long last_prio;
-
-	if (!IS_ENABLED(CONFIG_DRM_I915_DEBUG_GEM))
-		return;
-
-	GEM_BUG_ON(rb_first_cached(&se->queue) !=
-		   rb_first(&se->queue.rb_root));
-
-	last_prio = INT_MAX;
-	for (rb = rb_first_cached(&se->queue); rb; rb = rb_next(rb)) {
-		const struct i915_priolist *p = to_priolist(rb);
-
-		GEM_BUG_ON(p->priority > last_prio);
-		last_prio = p->priority;
-	}
+	root->prng = next_pseudo_random32(root->prng);
+	return min_t(int, __ffs(root->prng) / 2, I915_PRIOLIST_HEIGHT - 1);
 }
 
 static struct list_head *
 lookup_priolist(struct intel_engine_cs *engine, int prio)
 {
+	struct i915_priolist *update[I915_PRIOLIST_HEIGHT];
 	struct i915_sched_engine * const se = &engine->active;
-	struct i915_priolist *p;
-	struct rb_node **parent, *rb;
-	bool first = true;
-
-	lockdep_assert_held(&engine->active.lock);
-	assert_priolists(se);
+	struct i915_priolist_root *root = &se->queue;
+	struct i915_priolist *pl, *tmp;
+	int lvl;
 
+	lockdep_assert_held(&se->lock);
 	if (unlikely(se->no_priolist))
 		prio = I915_PRIORITY_NORMAL;
 
-find_priolist:
-	/* most positive priority is scheduled first, equal priorities fifo */
-	rb = NULL;
-	parent = &se->queue.rb_root.rb_node;
-	while (*parent) {
-		rb = *parent;
-		p = to_priolist(rb);
-		if (prio > p->priority) {
-			parent = &rb->rb_left;
-		} else if (prio < p->priority) {
-			parent = &rb->rb_right;
-			first = false;
-		} else {
-			return &p->requests;
-		}
+	for_each_priolist(pl, root) { /* recycle any empty elements before us */
+		if (pl->priority >= prio || !list_empty(&pl->requests))
+			break;
+
+		i915_priolist_advance(root, pl);
 	}
 
+find_priolist:
+	pl = &root->sentinel;
+	lvl = pl->level;
+	do {
+		while (tmp = pl->next[lvl], tmp->priority >= prio)
+			pl = tmp;
+		if (pl->priority == prio)
+			goto out;
+		update[lvl] = pl;
+	} while (--lvl >= 0);
+
 	if (prio == I915_PRIORITY_NORMAL) {
-		p = &se->default_priolist;
+		pl = &se->default_priolist;
+	} else if (!list_empty(&root->sentinel.requests)) {
+		pl = list_first_entry(&root->sentinel.requests,
+				      struct i915_priolist,
+				      requests);
+		__list_del_entry(&pl->requests);
 	} else {
-		p = kmem_cache_alloc(global.slab_priorities, GFP_ATOMIC);
+		pl = kmem_cache_alloc(global.slab_priorities, GFP_ATOMIC);
 		/* Convert an allocation failure to a priority bump */
-		if (unlikely(!p)) {
+		if (unlikely(!pl)) {
 			prio = I915_PRIORITY_NORMAL; /* recurses just once */
 
 			/* To maintain ordering with all rendering, after an
@@ -260,18 +287,61 @@ lookup_priolist(struct intel_engine_cs *engine, int prio)
 		}
 	}
 
-	p->priority = prio;
-	INIT_LIST_HEAD(&p->requests);
+	pl->priority = prio;
+	INIT_LIST_HEAD(&pl->requests);
+
+	lvl = random_level(root);
+	if (lvl > root->sentinel.level) {
+		lvl = ++root->sentinel.level;
+		update[lvl] = &root->sentinel;
+	}
 
-	rb_link_node(&p->node, rb, parent);
-	rb_insert_color_cached(&p->node, &se->queue, first);
+	pl->level = lvl;
+	do {
+		tmp = update[lvl];
+		pl->next[lvl] = update[lvl]->next[lvl];
+		tmp->next[lvl] = pl;
+	} while (--lvl >= 0);
+
+	if (IS_ENABLED(CONFIG_DRM_I915_DEBUG_GEM)) {
+		struct i915_priolist *chk;
+
+		chk = &root->sentinel;
+		lvl = chk->level;
+		do {
+			while (tmp = chk->next[lvl], tmp->priority >= prio)
+				chk = tmp;
+		} while (--lvl >= 0);
+
+		GEM_BUG_ON(chk != pl);
+	}
 
-	return &p->requests;
+out:
+	GEM_BUG_ON(pl == &root->sentinel);
+	return &pl->requests;
 }
 
-void __i915_priolist_free(struct i915_priolist *p)
+void i915_priolist_advance(struct i915_priolist_root *root,
+			   struct i915_priolist *pl)
 {
-	kmem_cache_free(global.slab_priorities, p);
+	int lvl;
+
+	GEM_BUG_ON(!list_empty(&pl->requests));
+	GEM_BUG_ON(pl == &root->sentinel);
+
+	if (pl->priority != I915_PRIORITY_NORMAL)
+		list_add(&pl->requests, &root->sentinel.requests);
+
+	for (lvl = 0; lvl <= pl->level; lvl++)
+		root->sentinel.next[lvl] = pl->next[lvl];
+
+	if (pl->level == root->sentinel.level) {
+		pl = &root->sentinel;
+		lvl = pl->level;
+		while (lvl > -0 && pl->next[lvl] == pl)
+			lvl--;
+		pl->level = lvl;
+	}
 }
 
 static struct i915_request *
diff --git a/drivers/gpu/drm/i915/i915_scheduler.h b/drivers/gpu/drm/i915/i915_scheduler.h
index a68747d682d2..4432aab230b6 100644
--- a/drivers/gpu/drm/i915/i915_scheduler.h
+++ b/drivers/gpu/drm/i915/i915_scheduler.h
@@ -16,12 +16,6 @@
 
 struct drm_printer;
 
-#define priolist_for_each_request(it, plist) \
-	list_for_each_entry(it, &(plist)->requests, sched.link)
-
-#define priolist_for_each_request_consume(it, n, plist) \
-	list_for_each_entry_safe(it, n, &(plist)->requests, sched.link)
-
 void i915_sched_node_init(struct i915_sched_node *node);
 void i915_sched_node_reinit(struct i915_sched_node *node);
 
@@ -64,7 +58,7 @@ static inline void i915_priolist_free(struct i915_priolist *p)
 
 static inline bool i915_sched_is_idle(const struct i915_sched_engine *se)
 {
-	return RB_EMPTY_ROOT(&se->queue.rb_root);
+	return i915_priolist_is_empty(&se->queue);
 }
 
 static inline bool
@@ -94,6 +88,9 @@ static inline void i915_sched_kick(struct i915_sched_engine *se)
 	tasklet_hi_schedule(&se->tasklet);
 }
 
+void i915_priolist_advance(struct i915_priolist_root *root,
+			   struct i915_priolist *old);
+
 void i915_request_show_with_schedule(struct drm_printer *m,
 				     const struct i915_request *rq,
 				     const char *prefix,
diff --git a/drivers/gpu/drm/i915/i915_scheduler_types.h b/drivers/gpu/drm/i915/i915_scheduler_types.h
index 92332fb71b14..c155e4faa30b 100644
--- a/drivers/gpu/drm/i915/i915_scheduler_types.h
+++ b/drivers/gpu/drm/i915/i915_scheduler_types.h
@@ -89,7 +89,7 @@ struct i915_sched_engine {
 	/**
 	 * @queue: queue of requests, in priority lists
 	 */
-	struct rb_root_cached queue;
+	struct i915_priolist_root queue;
 
 	struct i915_sched_ipi ipi;
 
diff --git a/drivers/gpu/drm/i915/selftests/i915_mock_selftests.h b/drivers/gpu/drm/i915/selftests/i915_mock_selftests.h
index 3db34d3eea58..946c93441c1f 100644
--- a/drivers/gpu/drm/i915/selftests/i915_mock_selftests.h
+++ b/drivers/gpu/drm/i915/selftests/i915_mock_selftests.h
@@ -25,6 +25,7 @@ selftest(ring, intel_ring_mock_selftests)
 selftest(engine, intel_engine_cs_mock_selftests)
 selftest(timelines, intel_timeline_mock_selftests)
 selftest(requests, i915_request_mock_selftests)
+selftest(scheduler, i915_scheduler_mock_selftests)
 selftest(objects, i915_gem_object_mock_selftests)
 selftest(phys, i915_gem_phys_mock_selftests)
 selftest(dmabuf, i915_gem_dmabuf_mock_selftests)
diff --git a/drivers/gpu/drm/i915/selftests/i915_scheduler.c b/drivers/gpu/drm/i915/selftests/i915_scheduler.c
index 0756ba8e68bb..085a00998e95 100644
--- a/drivers/gpu/drm/i915/selftests/i915_scheduler.c
+++ b/drivers/gpu/drm/i915/selftests/i915_scheduler.c
@@ -12,6 +12,43 @@
 #include "selftests/igt_spinner.h"
 #include "selftests/i915_random.h"
 
+static int mock_skiplist_levels(void *dummy)
+{
+	struct i915_priolist_root root = {};
+	struct i915_priolist *pl = &root.sentinel;
+	int count, lvl;
+
+	for (count = 0; count < 16384; count++) {
+		lvl = random_level(&root);
+		if (lvl > pl->level)
+			lvl = ++pl->level;
+
+		pl->next[lvl] = ptr_inc(pl->next[lvl]);
+	}
+
+	for (lvl = 0; lvl <= pl->level; lvl++) {
+		int x = ilog2((unsigned long)pl->next[lvl]);
+		char row[80];
+
+		memset(row, '*', x);
+		row[x] = '\0';
+
+		pr_info("[%2d] %5lu %s\n",
+			lvl, (unsigned long)pl->next[lvl], row);
+	}
+
+	return 0;
+}
+
+int i915_scheduler_mock_selftests(void)
+{
+	static const struct i915_subtest tests[] = {
+		SUBTEST(mock_skiplist_levels),
+	};
+
+	return i915_subtests(tests, NULL);
+}
+
 static void scheduling_disable(struct intel_engine_cs *engine)
 {
 	engine->props.preempt_timeout_ms = 0;
@@ -80,9 +117,9 @@ static int all_engines(struct drm_i915_private *i915,
 static bool check_context_order(struct intel_engine_cs *engine)
 {
 	u64 last_seqno, last_context;
+	struct i915_priolist *p;
 	unsigned long count;
 	bool result = false;
-	struct rb_node *rb;
 	int last_prio;
 
 	/* We expect the execution order to follow ascending fence-context */
@@ -92,8 +129,7 @@ static bool check_context_order(struct intel_engine_cs *engine)
 	last_context = 0;
 	last_seqno = 0;
 	last_prio = 0;
-	for (rb = rb_first_cached(&engine->active.queue); rb; rb = rb_next(rb)) {
-		struct i915_priolist *p = rb_entry(rb, typeof(*p), node);
+	for_each_priolist(p, &engine->active.queue) {
 		struct i915_request *rq;
 
 		priolist_for_each_request(rq, p) {
-- 
2.20.1

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