drm/i915: Ratelimit i915_globals_park

When doing our global park, we like to be a good citizen and shrink our
slab caches (of which we have quite a few now), but each
kmem_cache_shrink() incurs a stop_machine() and so ends up being quite
expensive, causing machine-wide stalls. While ideally we would like to
throw away unused pages in our slab caches whenever it appears that we
are idling, doing so will require a much cheaper mechanism. In the
meantime use a delayed worked to impose a rate-limit that means we have
to have been idle for more than 2 seconds before we start shrinking.

References: https://gitlab.freedesktop.org/drm/intel/issues/848
Signed-off-by: Chris Wilson <[e-mail]>
Cc: Tvrtko Ursulin <[e-mail]>
Reviewed-by: Tvrtko Ursulin <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/i915/i915_globals.c

···
 static atomic_t active;
 static atomic_t epoch;
 static struct park_work {
-	struct rcu_work work;
+	struct delayed_work work;
+	struct rcu_head rcu;
+	unsigned long flags;
+#define PENDING 0
 	int epoch;
 } park;
 
···
 		global->shrink();
 }
 
+static void __i915_globals_grace(struct rcu_head *rcu)
+{
+	/* Ratelimit parking as shrinking is quite slow */
+	schedule_delayed_work(&park.work, round_jiffies_up_relative(2 * HZ));
+}
+
+static void __i915_globals_queue_rcu(void)
+{
+	park.epoch = atomic_inc_return(&epoch);
+	if (!atomic_read(&active)) {
+		init_rcu_head(&park.rcu);
+		call_rcu(&park.rcu, __i915_globals_grace);
+	}
+}
+
 static void __i915_globals_park(struct work_struct *work)
 {
+	destroy_rcu_head(&park.rcu);
+
 	/* Confirm nothing woke up in the last grace period */
-	if (park.epoch == atomic_read(&epoch))
-		i915_globals_shrink();
+	if (park.epoch != atomic_read(&epoch)) {
+		__i915_globals_queue_rcu();
+		return;
+	}
+
+	clear_bit(PENDING, &park.flags);
+	i915_globals_shrink();
 }
 
 void __init i915_global_register(struct i915_global *global)
···
 		}
 	}
 
-	INIT_RCU_WORK(&park.work, __i915_globals_park);
+	INIT_DELAYED_WORK(&park.work, __i915_globals_park);
 	return 0;
 }
 
···
 	if (!atomic_dec_and_test(&active))
 		return;
 
-	park.epoch = atomic_inc_return(&epoch);
-	queue_rcu_work(system_wq, &park.work);
+	/* Queue cleanup after the next RCU grace period has freed slabs */
+	if (!test_and_set_bit(PENDING, &park.flags))
+		__i915_globals_queue_rcu();
 }
 
 void i915_globals_unpark(void)
···
 	atomic_inc(&active);
 }
 
+static void __exit __i915_globals_flush(void)
+{
+	atomic_inc(&active); /* skip shrinking */
+
+	rcu_barrier(); /* wait for the work to be queued */
+	flush_delayed_work(&park.work);
+
+	atomic_dec(&active);
+}
+
 void __exit i915_globals_exit(void)
 {
-	/* Flush any residual park_work */
-	atomic_inc(&epoch);
-	flush_rcu_work(&park.work);
+	GEM_BUG_ON(atomic_read(&active));
 
+	__i915_globals_flush();
 	__i915_globals_cleanup();
 
 	/* And ensure that our DESTROY_BY_RCU slabs are truly destroyed */