[Xen-devel] [PATCH v2 3/3] xen: RCU: make the period of the idle timer adaptive.

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

Basically, if the RCU idle timer, when (if!) it fires,
finds that the grace period isn't over, we increase the
timer's period (i.e., it will fire later, next time).
If, OTOH, it finds the grace period is already finished,
we decrease the timer's period (i.e., it will fire a bit
earlier next time).

The goal is to let the period timer sefl-adjust to a
number of 'misses', of the order of 1%.

Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
Suggested-by: George Dunlap <george.dunlap@xxxxxxxxxx>
---
Cc: Jan Beulich <JBeulich@xxxxxxxx>
Cc: Julien Grall <julien.grall@xxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Cc: George Dunlap <george.dunlap@xxxxxxxxxx>
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>,
Cc: Tim Deegan <tim@xxxxxxx>
---
Changes from v1:
- removed pointless braches;
- switched from min_t() to min(), and from max_t() to max().
---
 xen/common/rcupdate.c |   20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/xen/common/rcupdate.c b/xen/common/rcupdate.c
index 2381df1..b2d0e2e 100644
--- a/xen/common/rcupdate.c
+++ b/xen/common/rcupdate.c
@@ -118,6 +118,7 @@ struct rcu_data {
  */
 #define IDLE_TIMER_PERIOD_MAX     MILLISECS(100)
 #define IDLE_TIMER_PERIOD_DEFAULT MILLISECS(10)
+#define IDLE_TIMER_PERIOD_MIN     MICROSECS(100)
 
 static s_time_t __read_mostly idle_timer_period;
 
@@ -125,6 +126,17 @@ static unsigned int __initdata idle_timer_period_ms =
                                     IDLE_TIMER_PERIOD_DEFAULT / MILLISECS(1);
 integer_param("rcu-idle-timer-period-ms", idle_timer_period_ms);
 
+/*
+ * Increment and decrement values for the idle timer handler. The algorithm
+ * works as follows:
+ * - if the timer actually fires, and it finds out that the grace period isn't
+ *   over yet, we add IDLE_TIMER_PERIOD_INCR to the timer's period;
+ * - if the timer actually fires and it finds the grace period over, we
+ *   subtract IDLE_TIMER_PERIOD_DECR from the timer's period.
+ */
+#define IDLE_TIMER_PERIOD_INCR    MILLISECS(10)
+#define IDLE_TIMER_PERIOD_DECR    MICROSECS(100)
+
 static DEFINE_PER_CPU(struct rcu_data, rcu_data);
 
 static int blimit = 10;
@@ -494,8 +506,14 @@ void rcu_idle_timer_stop()
 
 static void rcu_idle_timer_handler(void* data)
 {
-    /* Nothing, really... Just count the number of times we fire */
     perfc_incr(rcu_idle_timer);
+
+    if ( !cpumask_empty(&rcu_ctrlblk.cpumask) )
+        idle_timer_period = min(idle_timer_period + IDLE_TIMER_PERIOD_INCR,
+                                IDLE_TIMER_PERIOD_MAX);
+    else
+        idle_timer_period = max(idle_timer_period - IDLE_TIMER_PERIOD_DECR,
+                                IDLE_TIMER_PERIOD_MIN);
 }
 
 void rcu_check_callbacks(int cpu)


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