cpuidle: menu governor: reduce latency on exit

czoccolo · torvalds · commit 672917dcc781 · 2009-09-22T07:17:45.000-07:00
Move the state residency accounting and statistics computation off the hot
exit path.

On exit, the need to recompute statistics is recorded, and new statistics
will be computed when menu_select is called again.

The expected effect is to reduce processor wakeup latency from sleep
(C-states).  We are speaking of few hundreds of cycles reduction out of a
several microseconds latency (determined by the hardware transition), so
it is difficult to measure.

Signed-off-by: Corrado Zoccolo &lt;czoccolo@gmail.com&gt;
Cc: Venkatesh Pallipadi &lt;venkatesh.pallipadi@intel.com&gt;
Cc: Len Brown &lt;len.brown@intel.com&gt;
Cc: Adam Belay &lt;abelay@novell.com
Acked-by: Arjan van de Ven &lt;arjan@linux.intel.com&gt;
Signed-off-by: Andrew Morton &lt;akpm@linux-foundation.org&gt;
Signed-off-by: Linus Torvalds &lt;torvalds@linux-foundation.org&gt;
diff --git a/drivers/cpuidle/governors/menu.c b/drivers/cpuidle/governors/menu.c
@@ -96,6 +96,7 @@
 
 struct menu_device {
 	int		last_state_idx;
+	int             needs_update;
 
 	unsigned int	expected_us;
 	u64		predicted_us;
@@ -166,6 +167,8 @@ static inline int performance_multiplier(void)
 
 static DEFINE_PER_CPU(struct menu_device, menu_devices);
 
+static void menu_update(struct cpuidle_device *dev);
+
 /**
  * menu_select - selects the next idle state to enter
  * @dev: the CPU
@@ -180,6 +183,11 @@ static int menu_select(struct cpuidle_device *dev)
 	data->last_state_idx = 0;
 	data->exit_us = 0;
 
+	if (data->needs_update) {
+		menu_update(dev);
+		data->needs_update = 0;
+	}
+
 	/* Special case when user has set very strict latency requirement */
 	if (unlikely(latency_req == 0))
 		return 0;
@@ -231,13 +239,23 @@ static int menu_select(struct cpuidle_device *dev)
 }
 
 /**
- * menu_reflect - attempts to guess what happened after entry
+ * menu_reflect - records that data structures need update
  * @dev: the CPU
  *
  * NOTE: it's important to be fast here because this operation will add to
  *       the overall exit latency.
  */
 static void menu_reflect(struct cpuidle_device *dev)
+{
+	struct menu_device *data = &__get_cpu_var(menu_devices);
+	data->needs_update = 1;
+}
+
+/**
+ * menu_update - attempts to guess what happened after entry
+ * @dev: the CPU
+ */
+static void menu_update(struct cpuidle_device *dev)
 {
 	struct menu_device *data = &__get_cpu_var(menu_devices);
 	int last_idx = data->last_state_idx;
