[Intel-gfx] [PATCH] drm/i915: Apply a cond_resched() to the saturated signaler

[Chris Wilson]

If the engine is continually completing nops, we can saturate the
signaler and keep it working indefinitely. This angers the NMI watchdog!

A good example is to disable semaphores on snb and run igt/gem_exec_nop -
the parallel, multi-engine workloads are more than sufficient to hog the
CPU, preventing the system from even processing ICMP echo replies.

Fixes: c81d46138da6 ("drm/i915: Convert trace-irq to the breadcrumb waiter")
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala at intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
---
 drivers/gpu/drm/i915/intel_breadcrumbs.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/gpu/drm/i915/intel_breadcrumbs.c b/drivers/gpu/drm/i915/intel_breadcrumbs.c
index 1b14c3be6046..763ed497961f 100644
--- a/drivers/gpu/drm/i915/intel_breadcrumbs.c
+++ b/drivers/gpu/drm/i915/intel_breadcrumbs.c
@@ -622,6 +622,13 @@ static int intel_breadcrumbs_signaler(void *arg)
 			spin_unlock_irq(&b->rb_lock);
 
 			i915_gem_request_put(request);
+
+			/* If the engine is saturated we may be continually
+			 * processing completed requests. This angers the
+			 * NMI watchdog if we never let anything else
+			 * have access to the CPU. Let's pretend to be nice.
+			 */
+			cond_resched();
 		} else {
 			DEFINE_WAIT(exec);
 
-- 
2.11.0