[PATCH 2/6] drm/i915: Always run hangcheck while the GPU is busy

[Chris Wilson]

Previously, we relied on only running the hangcheck while somebody was
waiting on the GPU, in order to minimise the amount of time hangcheck
had to run. (If nobody was watching the GPU, nobody would notice if the
GPU wasn't responding -- eventually somebody would care and so kick
hangcheck into action.) However, this falls apart from around commit
4680816be336 ("drm/i915: Wait first for submission, before waiting for
request completion"), as not all waiters declare themselves to hangcheck
and so we could switch off hangcheck and miss GPU hangs even when
waiting under the struct_mutex.

If we enable hangcheck from the first request submission, and let it run
until the GPU is idle again, we forgo all the complexity involved with
only enabling around waiters. Instead we have to be careful that we do
not declare a GPU hang when idly waiting for the next request to be come
ready.

Fixes: 4680816be336 ("drm/i915: Wait first for submission, before waiting for request completion"
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala at linux.intel.com>
---
 drivers/gpu/drm/i915/i915_gem.c          |  7 +++----
 drivers/gpu/drm/i915/i915_gem_request.c  |  2 ++
 drivers/gpu/drm/i915/intel_breadcrumbs.c | 11 -----------
 drivers/gpu/drm/i915/intel_hangcheck.c   |  7 +------
 4 files changed, 6 insertions(+), 21 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 062b21408698..63308ec016a3 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -3322,16 +3322,15 @@ i915_gem_retire_work_handler(struct work_struct *work)
 		mutex_unlock(&dev->struct_mutex);
 	}
 
-	/* Keep the retire handler running until we are finally idle.
+	/*
+	 * Keep the retire handler running until we are finally idle.
 	 * We do not need to do this test under locking as in the worst-case
 	 * we queue the retire worker once too often.
 	 */
-	if (READ_ONCE(dev_priv->gt.awake)) {
-		i915_queue_hangcheck(dev_priv);
+	if (READ_ONCE(dev_priv->gt.awake))
 		queue_delayed_work(dev_priv->wq,
 				   &dev_priv->gt.retire_work,
 				   round_jiffies_up_relative(HZ));
-	}
 }
 
 static void shrink_caches(struct drm_i915_private *i915)
diff --git a/drivers/gpu/drm/i915/i915_gem_request.c b/drivers/gpu/drm/i915/i915_gem_request.c
index 7192521e3d10..164a1bb8d655 100644
--- a/drivers/gpu/drm/i915/i915_gem_request.c
+++ b/drivers/gpu/drm/i915/i915_gem_request.c
@@ -393,6 +393,8 @@ static void mark_busy(struct drm_i915_private *i915)
 
 	intel_engines_unpark(i915);
 
+	i915_queue_hangcheck(i915);
+
 	queue_delayed_work(i915->wq,
 			   &i915->gt.retire_work,
 			   round_jiffies_up_relative(HZ));
diff --git a/drivers/gpu/drm/i915/intel_breadcrumbs.c b/drivers/gpu/drm/i915/intel_breadcrumbs.c
index 86acac010bb8..62b2a20bc24e 100644
--- a/drivers/gpu/drm/i915/intel_breadcrumbs.c
+++ b/drivers/gpu/drm/i915/intel_breadcrumbs.c
@@ -149,17 +149,6 @@ static void intel_breadcrumbs_fake_irq(struct timer_list *t)
 		return;
 
 	mod_timer(&b->fake_irq, jiffies + 1);
-
-	/* Ensure that even if the GPU hangs, we get woken up.
-	 *
-	 * However, note that if no one is waiting, we never notice
-	 * a gpu hang. Eventually, we will have to wait for a resource
-	 * held by the GPU and so trigger a hangcheck. In the most
-	 * pathological case, this will be upon memory starvation! To
-	 * prevent this, we also queue the hangcheck from the retire
-	 * worker.
-	 */
-	i915_queue_hangcheck(engine->i915);
 }
 
 static void irq_enable(struct intel_engine_cs *engine)
diff --git a/drivers/gpu/drm/i915/intel_hangcheck.c b/drivers/gpu/drm/i915/intel_hangcheck.c
index 31f01d64c021..348a4f7ffb67 100644
--- a/drivers/gpu/drm/i915/intel_hangcheck.c
+++ b/drivers/gpu/drm/i915/intel_hangcheck.c
@@ -411,7 +411,6 @@ static void i915_hangcheck_elapsed(struct work_struct *work)
 	struct intel_engine_cs *engine;
 	enum intel_engine_id id;
 	unsigned int hung = 0, stuck = 0;
-	int busy_count = 0;
 
 	if (!i915_modparams.enable_hangcheck)
 		return;
@@ -429,7 +428,6 @@ static void i915_hangcheck_elapsed(struct work_struct *work)
 	intel_uncore_arm_unclaimed_mmio_detection(dev_priv);
 
 	for_each_engine(engine, dev_priv, id) {
-		const bool busy = intel_engine_has_waiter(engine);
 		struct intel_engine_hangcheck hc;
 
 		semaphore_clear_deadlocks(dev_priv);
@@ -443,16 +441,13 @@ static void i915_hangcheck_elapsed(struct work_struct *work)
 			if (hc.action != ENGINE_DEAD)
 				stuck |= intel_engine_flag(engine);
 		}
-
-		busy_count += busy;
 	}
 
 	if (hung)
 		hangcheck_declare_hang(dev_priv, hung, stuck);
 
 	/* Reset timer in case GPU hangs without another request being added */
-	if (busy_count)
-		i915_queue_hangcheck(dev_priv);
+	i915_queue_hangcheck(dev_priv);
 }
 
 void intel_engine_init_hangcheck(struct intel_engine_cs *engine)
-- 
2.15.1