kern/rcu: make the window check interval configurable

2 files changed, 17 insertions, 3 deletions

diff --git a/kern/Kconfig b/kern/Kconfig
index 8125f94..fced67c 100644
--- a/kern/Kconfig
+++ b/kern/Kconfig
@@ -63,6 +63,20 @@ config MUTEX_PLAIN
 
 endchoice
 
+config RCU_WINDOW_CHECK_INTERVAL
+	int "Interval between RCU window checks"
+	range 1 100
+	default 10
+	---help---
+	  Time (in milliseconds) between two RCU window checks.
+
+	  The RCU system keeps memory used by read-side critical sections
+	  until it is safe to release it, which can only be determined when
+	  checking windows. As a result, checking windows more frequently
+	  may help lower latencies on synchronous RCU waits, and in turn,
+	  the amount of memory pending release, at the cost of increased
+	  CPU overhead.
+
 config SHELL
 	bool "Embedded shell"
 	default n
diff --git a/kern/rcu.c b/kern/rcu.c
index 1903dcf..834a99b 100644
--- a/kern/rcu.c
+++ b/kern/rcu.c
@@ -105,12 +105,12 @@
 #define RCU_WINDOW_ID_INIT_VALUE ((unsigned int)-500)
 
 /*
- * Interval between window checking.
+ * Interval (in milliseconds) between window checking.
  *
  * When windows are checked, a flip occurs if the previous window isn't
  * active any more.
  */
-#define RCU_WINDOW_CHECK_INTERVAL_MS 10
+#define RCU_WINDOW_CHECK_INTERVAL CONFIG_RCU_WINDOW_CHECK_INTERVAL
 
 /*
  * Grace period states.
@@ -376,7 +376,7 @@ rcu_data_schedule_timer(struct rcu_data *data, uint64_t now)
 {
     uint64_t ticks;
 
-    ticks = clock_ticks_from_ms(RCU_WINDOW_CHECK_INTERVAL_MS);
+    ticks = clock_ticks_from_ms(RCU_WINDOW_CHECK_INTERVAL);
     timer_schedule(&data->timer, now + ticks);
 }