drm/i915: Remove request retirement before each batch

This reimplements the denial-of-service protection against igt from
commit 227f782e4667 ("drm/i915: Retire requests before creating a new
one") and transfers the stall from before each batch into get_pages().
The issue is that the stall is increasing latency between batches which
is detrimental in some cases (especially coupled with execlists) to
keeping the GPU well fed. Also we have made the observation that retiring
requests can of itself free objects (and requests) and therefore makes
a good first step when shrinking.

v2: Recycle objects prior to i915_gem_object_get_pages()
v3: Remove the reference to the ring from i915_gem_requests_ring() as it
operates on an intel_engine_cs.
v4: Since commit 9b5f4e5ed6fd ("drm/i915: Retire oldest completed request
before allocating next") we no longer need the safeguard to retire
requests before get_pages(). We no longer see the huge latencies when
hitting the shrinker between allocations.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470324762-2545-4-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_drv.h

@@ -3169,7 +3169,6 @@ struct drm_i915_gem_request *
 i915_gem_find_active_request(struct intel_engine_cs *engine);
 
 void i915_gem_retire_requests(struct drm_i915_private *dev_priv);
-void i915_gem_retire_requests_ring(struct intel_engine_cs *engine);
 
 static inline u32 i915_reset_counter(struct i915_gpu_error *error)
 {

drivers/gpu/drm/i915/i915_gem_execbuffer.c

@@ -781,8 +781,6 @@ i915_gem_execbuffer_reserve(struct intel_engine_cs *engine,
 	bool has_fenced_gpu_access = INTEL_GEN(engine->i915) < 4;
 	int retry;
 
-	i915_gem_retire_requests_ring(engine);
-
 	vm = list_first_entry(vmas, struct i915_vma, exec_list)->vm;
 
 	INIT_LIST_HEAD(&ordered_vmas);

drivers/gpu/drm/i915/i915_gem_request.c

@@ -732,7 +732,7 @@ complete:
 	return ret;
 }
 
-void i915_gem_retire_requests_ring(struct intel_engine_cs *engine)
+static void engine_retire_requests(struct intel_engine_cs *engine)
 {
 	struct drm_i915_gem_request *request, *next;
 
@@ -756,7 +756,7 @@ void i915_gem_retire_requests(struct drm_i915_private *dev_priv)
 	GEM_BUG_ON(!dev_priv->gt.awake);
 
 	for_each_engine(engine, dev_priv) {
-		i915_gem_retire_requests_ring(engine);
+		engine_retire_requests(engine);
 		if (list_empty(&engine->request_list))
 			dev_priv->gt.active_engines &= ~intel_engine_flag(engine);
 	}