drm/i915/breadcrumbs: Reduce signaler rbtree to a sorted list

The goal here is to try and reduce the latency of signaling additional
requests following the wakeup from interrupt by reducing the list of
to-be-signaled requests from an rbtree to a sorted linked list. The
original choice of using an rbtree was to facilitate random insertions
of request into the signaler while maintaining a sorted list. However,
if we assume that most new requests are added when they are submitted,
we see those new requests in execution order making a insertion sort
fast, and the reduction in overhead of each signaler iteration
significant.

Since commit 56299fb7 ("drm/i915: Signal first fence from irq handler
if complete"), we signal most fences directly from notify_ring() in the
interrupt handler greatly reducing the amount of work that actually
needs to be done by the signaler kthread. All the thread is then
required to do is operate as the bottom-half, cleaning up after the
interrupt handler and preparing the next waiter. This includes signaling
all later completed fences in a saturated system, but on a mostly idle
system we only have to rebuild the wait rbtree in time for the next
interrupt. With this de-emphasis of the signaler's role, we want to
rejig it's datastructures to reduce the amount of work we require to
both setup the signal tree and maintain it on every interrupt.

References: 56299fb7 ("drm/i915: Signal first fence from irq handler if complete")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180222092545.17216-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_request.h

@@ -44,8 +44,8 @@ struct intel_wait {
 };
 
 struct intel_signal_node {
-	struct rb_node node;
 	struct intel_wait wait;
+	struct list_head link;
 };
 
 struct i915_dependency {

drivers/gpu/drm/i915/intel_ringbuffer.h

@@ -356,9 +356,9 @@ struct intel_engine_cs {
 
 		spinlock_t rb_lock; /* protects the rb and wraps irq_lock */
 		struct rb_root waiters; /* sorted by retirement, priority */
-		struct rb_root signals; /* sorted by retirement */
+		struct list_head signals; /* sorted by retirement */
 		struct task_struct *signaler; /* used for fence signalling */
-		struct i915_request __rcu *first_signal;
+
 		struct timer_list fake_irq; /* used after a missed interrupt */
 		struct timer_list hangcheck; /* detect missed interrupts */