drm/i915: Apply a cond_resched() to the saturated signaler

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.

v2: Tvrtko dug into cond_resched() on x86 and found that it only
depended upon preempt_count and not tif_need_resched() - which means
that we would always call schedule() at that point.

Fixes: c81d46138d ("drm/i915: Convert trace-irq to the breadcrumb waiter")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170404120531.10737-1-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

drivers/gpu/drm/i915/intel_breadcrumbs.c

@@ -580,6 +580,8 @@ static int intel_breadcrumbs_signaler(void *arg)
 	signaler_set_rtpriority();
 
 	do {
+		bool do_schedule = true;
+
 		set_current_state(TASK_INTERRUPTIBLE);
 
 		/* We are either woken up by the interrupt bottom-half,
@@ -626,7 +628,18 @@ static int intel_breadcrumbs_signaler(void *arg)
 			spin_unlock_irq(&b->rb_lock);
 
 			i915_gem_request_put(request);
-		} else {
+
+			/* 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
+			 * and relinquish the CPU if we burn through the
+			 * entire RT timeslice!
+			 */
+			do_schedule = need_resched();
+		}
+
+		if (unlikely(do_schedule)) {
 			DEFINE_WAIT(exec);
 
 			if (kthread_should_park())