drm/i915: Improve execute_cb struct packing

Reduce the irq_work llist for attaching the callbacks to the signal for
both smaller structs (two fewer pointers!) and simpler [debug] code:

Function                                     old     new   delta
irq_execute_cb                                35      34      -1
__igt_breadcrumbs_smoketest                 1684    1682      -2
i915_request_retire                         2003    1996      -7
__i915_request_create                       1047    1040      -7
__notify_execute_cb                          135     126      -9
__i915_request_ctor                          188     178     -10
__await_execution.part.constprop             451     440     -11
igt_wait_request                             924     714    -210

One minor artifact is that the order of cb exection is reversed. No
current use cases are affected by that change.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200526112051.10229-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_request.c

@@ -42,7 +42,6 @@
 #include "intel_pm.h"
 
 struct execute_cb {
-	struct list_head link;
 	struct irq_work work;
 	struct i915_sw_fence *fence;
 	void (*hook)(struct i915_request *rq, struct dma_fence *signal);
@@ -189,14 +188,14 @@ static void irq_execute_cb_hook(struct irq_work *wrk)
 
 static void __notify_execute_cb(struct i915_request *rq)
 {
-	struct execute_cb *cb;
+	struct execute_cb *cb, *cn;
 
 	lockdep_assert_held(&rq->lock);
 
-	if (list_empty(&rq->execute_cb))
+	if (llist_empty(&rq->execute_cb))
 		return;
 
-	list_for_each_entry(cb, &rq->execute_cb, link)
+	llist_for_each_entry_safe(cb, cn, rq->execute_cb.first, work.llnode)
 		irq_work_queue(&cb->work);
 
 	/*
@@ -209,7 +208,7 @@ static void __notify_execute_cb(struct i915_request *rq)
 	 * preempt-to-idle cycle on the target engine, all the while the
 	 * master execute_cb may refire.
 	 */
-	INIT_LIST_HEAD(&rq->execute_cb);
+	init_llist_head(&rq->execute_cb);
 }
 
 static inline void
@@ -327,7 +326,7 @@ bool i915_request_retire(struct i915_request *rq)
 		set_bit(I915_FENCE_FLAG_ACTIVE, &rq->fence.flags);
 		__notify_execute_cb(rq);
 	}
-	GEM_BUG_ON(!list_empty(&rq->execute_cb));
+	GEM_BUG_ON(!llist_empty(&rq->execute_cb));
 	spin_unlock_irq(&rq->lock);
 
 	remove_from_client(rq);
@@ -357,6 +356,12 @@ void i915_request_retire_upto(struct i915_request *rq)
 	} while (i915_request_retire(tmp) && tmp != rq);
 }
 
+static void __llist_add(struct llist_node *node, struct llist_head *head)
+{
+	node->next = head->first;
+	head->first = node;
+}
+
 static int
 __await_execution(struct i915_request *rq,
 		  struct i915_request *signal,
@@ -395,7 +400,7 @@ __await_execution(struct i915_request *rq,
 		i915_sw_fence_complete(cb->fence);
 		kmem_cache_free(global.slab_execute_cbs, cb);
 	} else {
-		list_add_tail(&cb->link, &signal->execute_cb);
+		__llist_add(&cb->work.llnode, &signal->execute_cb);
 	}
 	spin_unlock_irq(&signal->lock);
 
@@ -704,7 +709,7 @@ static void __i915_request_ctor(void *arg)
 	rq->file_priv = NULL;
 	rq->capture_list = NULL;
 
-	INIT_LIST_HEAD(&rq->execute_cb);
+	init_llist_head(&rq->execute_cb);
 }
 
 struct i915_request *
@@ -794,7 +799,7 @@ __i915_request_create(struct intel_context *ce, gfp_t gfp)
 	rq->batch = NULL;
 	GEM_BUG_ON(rq->file_priv);
 	GEM_BUG_ON(rq->capture_list);
-	GEM_BUG_ON(!list_empty(&rq->execute_cb));
+	GEM_BUG_ON(!llist_empty(&rq->execute_cb));
 
 	/*
 	 * Reserve space in the ring buffer for all the commands required to

drivers/gpu/drm/i915/i915_request.h

@@ -214,7 +214,7 @@ struct i915_request {
 			ktime_t emitted;
 		} duration;
 	};
-	struct list_head execute_cb;
+	struct llist_head execute_cb;
 	struct i915_sw_fence semaphore;
 
 	/*