[Intel-gfx] [PATCH v3] drm/i915: Only report a wakeup if the waiter was truly asleep

[Chris Wilson]

If we attempt to wake up a waiter, who is currently checking the seqno
it will be in the TASK_INTERRUPTIBLE state and ttwu will report success.
However, it is actually awake and functioning -- so delay reporting the
actual wake up until it sleeps.

v2: Defend against !CONFIG_SMP
v3: Don't filter out calls to wake_up_process

References: https://bugs.freedesktop.org/show_bug.cgi?id=100007
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
---
 drivers/gpu/drm/i915/intel_breadcrumbs.c | 18 ++++++++++++++++--
 drivers/gpu/drm/i915/intel_ringbuffer.h  |  4 ++++
 2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_breadcrumbs.c b/drivers/gpu/drm/i915/intel_breadcrumbs.c
index 9ccbf26124c6..808d3a3cda0a 100644
--- a/drivers/gpu/drm/i915/intel_breadcrumbs.c
+++ b/drivers/gpu/drm/i915/intel_breadcrumbs.c
@@ -27,6 +27,12 @@
 
 #include "i915_drv.h"
 
+#ifdef CONFIG_SMP
+#define task_asleep(tsk) (!(tsk)->on_cpu)
+#else
+#define task_asleep(tsk) ((tsk) != current)
+#endif
+
 static unsigned int __intel_breadcrumbs_wakeup(struct intel_breadcrumbs *b)
 {
 	struct intel_wait *wait;
@@ -37,8 +43,16 @@ static unsigned int __intel_breadcrumbs_wakeup(struct intel_breadcrumbs *b)
 	wait = b->irq_wait;
 	if (wait) {
 		result = ENGINE_WAKEUP_WAITER;
-		if (wake_up_process(wait->tsk))
+
+		/* Be careful not to report a successful wakeup if the waiter
+		 * is currently processing the seqno, where it will have
+		 * already called set_task_state(TASK_INTERRUPTIBLE).
+		 */
+		if (task_asleep(wait->tsk))
 			result |= ENGINE_WAKEUP_ASLEEP;
+
+		if (wake_up_process(wait->tsk))
+			result |= ENGINE_WAKEUP_SUCCESS;
 	}
 
 	return result;
@@ -98,7 +112,7 @@ static void intel_breadcrumbs_hangcheck(unsigned long data)
 	 * but we still have a waiter. Assuming all batches complete within
 	 * DRM_I915_HANGCHECK_JIFFIES [1.5s]!
 	 */
-	if (intel_engine_wakeup(engine) & ENGINE_WAKEUP_ASLEEP) {
+	if (intel_engine_wakeup(engine) == ENGINE_WAKEUP) {
 		missed_breadcrumb(engine);
 		mod_timer(&engine->breadcrumbs.fake_irq, jiffies + 1);
 	} else {
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index cbe61d3f31da..974a5928bec9 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -663,6 +663,10 @@ static inline bool intel_engine_has_waiter(const struct intel_engine_cs *engine)
 unsigned int intel_engine_wakeup(struct intel_engine_cs *engine);
 #define ENGINE_WAKEUP_WAITER BIT(0)
 #define ENGINE_WAKEUP_ASLEEP BIT(1)
+#define ENGINE_WAKEUP_SUCCESS BIT(2)
+#define ENGINE_WAKEUP (ENGINE_WAKEUP_WAITER | \
+		       ENGINE_WAKEUP_ASLEEP | \
+		       ENGINE_WAKEUP_SUCCESS)
 
 void __intel_engine_disarm_breadcrumbs(struct intel_engine_cs *engine);
 void intel_engine_disarm_breadcrumbs(struct intel_engine_cs *engine);
-- 
2.11.0