On unwinding the active request we give it a small (limited to internal
priority levels) boost to prevent it from being gazumped a second time.
However, this means that it can be promoted to above the request that
triggered the preemption request, causing a preempt-to-idle cycle for no
change. We can avoid this if we take the boost into account when
checking if the preemption request is valid.
v2: After preemption the active request will be after the preemptee if
they end up with equal priority.
Signed-off-by: Chris Wilson <ch...@chris-wilson.co.uk>
---
drivers/gpu/drm/i915/intel_lrc.c | 39 ++++++++++++++++++++++++++++----
1 file changed, 35 insertions(+), 4 deletions(-)
diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index 37defd14c198..6cef458f6b43 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -163,6 +163,8 @@
#define WA_TAIL_DWORDS 2
#define WA_TAIL_BYTES (sizeof(u32) * WA_TAIL_DWORDS)
+#define ACTIVE_PRIORITY (I915_PRIORITY_NEWCLIENT)
+
static int execlists_context_deferred_alloc(struct i915_gem_context *ctx,
struct intel_engine_cs *engine,
struct intel_context *ce);
@@ -181,6 +183,34 @@ static inline int rq_prio(const struct i915_request *rq)
return rq->sched.attr.priority;
}
+static inline int active_prio(const struct i915_request *rq)
+{
+ int prio = rq_prio(rq);
+
+ /*
+ * On unwinding the active request, we give it a priority bump
+ * equivalent to a freshly submitted request. This protects it from
+ * being gazumped again, but it would be preferable if we didn't
+ * let it be gazumped in the first place!
+ *
+ * See __unwind_incomplete_requests()
+ */
+ if ((prio & ACTIVE_PRIORITY) != ACTIVE_PRIORITY &&
+ i915_request_started(rq)) {
+ /*
+ * After preemption, we insert the active request at the
+ * end of the new priority level. This means that we will be
+ * _lower_ priority than the preemptee all things equal (and
+ * so the preemption is valid), so adjust our comparison
+ * accordingly.
+ */
+ prio |= ACTIVE_PRIORITY;
+ prio--;
+ }
+
+ return prio;
+}
+
static int queue_prio(const struct intel_engine_execlists *execlists)
{
struct i915_priolist *p;
@@ -203,7 +233,7 @@ static inline bool need_preempt(const struct
intel_engine_cs *engine,
int q_prio)
{
const struct intel_context *ctx = rq->hw_context;
- const int last_prio = rq_prio(rq);
+ int last_prio;
if (!intel_engine_has_preemption(engine))
return false;
@@ -218,6 +248,7 @@ 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 = active_prio(rq);
if (!__execlists_need_preempt(q_prio, last_prio))
return false;
@@ -344,7 +375,7 @@ static void __unwind_incomplete_requests(struct
intel_engine_cs *engine)
{
struct i915_request *rq, *rn, *active = NULL;
struct list_head *uninitialized_var(pl);
- int prio = I915_PRIORITY_INVALID | I915_PRIORITY_NEWCLIENT;
+ int prio = I915_PRIORITY_INVALID | ACTIVE_PRIORITY;
lockdep_assert_held(&engine->timeline.lock);
@@ -376,8 +407,8 @@ static void __unwind_incomplete_requests(struct
intel_engine_cs *engine)
* stream, so give it the equivalent small priority bump to prevent
* it being gazumped a second time by another peer.
*/
- if (!(prio & I915_PRIORITY_NEWCLIENT)) {
- prio |= I915_PRIORITY_NEWCLIENT;
+ if ((prio & ACTIVE_PRIORITY) != ACTIVE_PRIORITY) {
+ prio |= ACTIVE_PRIORITY;
active->sched.attr.priority = prio;
list_move_tail(&active->sched.link,
i915_sched_lookup_priolist(engine, prio));
--
2.20.1
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