The core of the scheduling algorithm is that we compute the topological
order of the fence DAG. Knowing that we have a DAG, we should be able to
use a DFS to compute the topological sort in linear time. However,
during the conversion of the recursive algorithm into an iterative one,
the memoization of how far we had progressed down a branch was
forgotten. The result was that instead of running in linear time, it was
running in geometric time and could easily run for a few hundred
milliseconds given a wide enough graph, not the microseconds as required.
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Reviewed-by: Andi Shyti <andi.shyti@xxxxxxxxx>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
---
drivers/gpu/drm/i915/i915_scheduler.c | 58 ++++++++++++---------
drivers/gpu/drm/i915/i915_scheduler_types.h | 6 ++-
2 files changed, 39 insertions(+), 25 deletions(-)
diff --git a/drivers/gpu/drm/i915/i915_scheduler.c b/drivers/gpu/drm/i915/i915_scheduler.c
index 27bda7617b29..9e88417bf451 100644
--- a/drivers/gpu/drm/i915/i915_scheduler.c
+++ b/drivers/gpu/drm/i915/i915_scheduler.c
@@ -242,6 +242,26 @@ void __i915_priolist_free(struct i915_priolist *p)
kmem_cache_free(global.slab_priorities, p);
}
+static struct i915_request *
+stack_push(struct i915_request *rq,
+ struct i915_request *prev,
+ struct list_head *pos)
+{
+ prev->sched.dfs.pos = pos;
+ rq->sched.dfs.prev = prev;
+ return rq;
+}
+
+static struct i915_request *
+stack_pop(struct i915_request *rq,
+ struct list_head **pos)
+{
+ rq = rq->sched.dfs.prev;
+ if (rq)
+ *pos = rq->sched.dfs.pos;
+ return rq;
+}
+
static inline bool need_preempt(int prio, int active)
{
/*
@@ -306,11 +326,10 @@ static void ipi_priority(struct i915_request *rq, int prio)
static void __i915_request_set_priority(struct i915_request *rq, int prio)
{
struct intel_engine_cs *engine = rq->engine;
- struct i915_request *rn;
+ struct list_head *pos = &rq->sched.signalers_list;
struct list_head *plist;
- LIST_HEAD(dfs);
- list_add(&rq->sched.dfs, &dfs);
+ plist = i915_sched_lookup_priolist(engine, prio);
/*
* Recursively bump all dependent priorities to match the new request.
@@ -330,40 +349,31 @@ static void __i915_request_set_priority(struct i915_request *rq, int prio)
* 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(rq, &dfs, sched.dfs) {
- struct i915_dependency *p;
-
- /* Also release any children on this engine that are ready */
- GEM_BUG_ON(rq->engine != engine);
-
- for_each_signaler(p, rq) {
+ rq->sched.dfs.prev = NULL;
+ do {
+ list_for_each_continue(pos, &rq->sched.signalers_list) {
+ struct i915_dependency *p =
+ list_entry(pos, typeof(*p), signal_link);
struct i915_request *s =
container_of(p->signaler, typeof(*s), sched);
- GEM_BUG_ON(s == rq);
-
if (rq_prio(s) >= prio)
continue;
if (__i915_request_is_complete(s))
continue;
- if (s->engine != rq->engine) {
+ if (s->engine != engine) {
ipi_priority(s, prio);
continue;
}
- list_move_tail(&s->sched.dfs, &dfs);
+ /* Remember our position along this branch */
+ rq = stack_push(s, rq, pos);
+ pos = &rq->sched.signalers_list;
}
- }
- plist = i915_sched_lookup_priolist(engine, prio);
-
- /* Fifo and depth-first replacement ensure our deps execute first */
- list_for_each_entry_safe_reverse(rq, rn, &dfs, sched.dfs) {
- GEM_BUG_ON(rq->engine != engine);
-
- INIT_LIST_HEAD(&rq->sched.dfs);
+ RQ_TRACE(rq, "set-priority:%d\n", prio);
WRITE_ONCE(rq->sched.attr.priority, prio);
/*
@@ -377,12 +387,13 @@ static void __i915_request_set_priority(struct i915_request *rq, int prio)
if (!i915_request_is_ready(rq))
continue;
+ GEM_BUG_ON(rq->engine != engine);
if (i915_request_in_priority_queue(rq))
list_move_tail(&rq->sched.link, plist);
/* Defer (tasklet) submission until after all updates. */
kick_submission(engine, rq, prio);
- }
+ } while ((rq = stack_pop(rq, &pos)));
}
#define all_signalers_checked(p, rq) \
@@ -456,7 +467,6 @@ void i915_sched_node_init(struct i915_sched_node *node)
INIT_LIST_HEAD(&node->signalers_list);
INIT_LIST_HEAD(&node->waiters_list);
INIT_LIST_HEAD(&node->link);
- INIT_LIST_HEAD(&node->dfs);
node->ipi_link = NULL;
diff --git a/drivers/gpu/drm/i915/i915_scheduler_types.h b/drivers/gpu/drm/i915/i915_scheduler_types.h
index 2a5265d9aff1..28138c3fcc81 100644
--- a/drivers/gpu/drm/i915/i915_scheduler_types.h
+++ b/drivers/gpu/drm/i915/i915_scheduler_types.h
@@ -69,7 +69,11 @@ struct i915_sched_node {
struct list_head signalers_list; /* those before us, we depend upon */
struct list_head waiters_list; /* those after us, they depend upon us */
struct list_head link; /* guarded by engine->active.lock */
- struct list_head dfs; /* guarded by engine->active.lock */
+ struct i915_sched_stack {
+ /* Branch memoization used during depth-first search */
+ struct i915_request *prev;
+ struct list_head *pos;
+ } dfs; /* guarded by engine->active.lock */
struct i915_sched_attr attr;
unsigned long flags;
#define I915_SCHED_HAS_EXTERNAL_CHAIN BIT(0)
--
2.20.1
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