[Xen-devel] [PATCH v2 6/6] xen: try to prevent idle timer from firing too often.

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

Idea is: the more CPUs are still active in a grace period,
the more we can wait to check whether it's time to invoke
the callbacks (on those CPUs that have already quiesced,
are idle, and have callbacks queued).

What we're trying to avoid is one of those idle CPUs to
wake up, only to discover that the grace period is still
running, and that it hence could have be slept longer
(saving more power).

This patch implements an heuristic aimed at achieving
that, at the price of having to call cpumask_weight() on
the 'entering idle' path, on CPUs with queued callbacks.

Of course, we, at the same time, don't want to delay
recognising that we can invoke the callbacks for too
much, so we also set a maximum.

Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
---
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Cc: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Cc: Tim Deegan <tim@xxxxxxx>
Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
Cc: Julien Grall <julien.grall@xxxxxxx>
---
 xen/common/rcupdate.c |   18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/xen/common/rcupdate.c b/xen/common/rcupdate.c
index e27bfed..b9ae6cc 100644
--- a/xen/common/rcupdate.c
+++ b/xen/common/rcupdate.c
@@ -110,10 +110,17 @@ struct rcu_data {
  * About how far in the future the timer should be programmed each time,
  * it's hard to tell (guess!!). Since this mimics Linux's periodic timer
  * tick, take values used there as an indication. In Linux 2.6.21, tick
- * period can be 10ms, 4ms, 3.33ms or 1ms. Let's use 10ms, to enable
- * at least some power saving on the CPU that is going idle.
+ * period can be 10ms, 4ms, 3.33ms or 1ms.
+ *
+ * That being said, we can assume that, the more CPUs are still active in
+ * the current grace period, the longer it will take for it to come to its
+ * end. We wait 10ms for each active CPU, as minimizing the wakeups enables
+ * more effective power saving, on the CPU that has gone idle. But we also
+ * never wait more than 100ms, to avoid delaying recognising the end of a
+ * grace period (and the invocation of the callbacks) by too much.
  */
-#define RCU_IDLE_TIMER_PERIOD MILLISECS(10)
+#define RCU_IDLE_TIMER_CPU_DELAY  MILLISECS(10)
+#define RCU_IDLE_TIMER_PERIOD_MAX MILLISECS(100)
 
 static DEFINE_PER_CPU(struct rcu_data, rcu_data);
 
@@ -444,6 +451,7 @@ int rcu_needs_cpu(int cpu)
 void rcu_idle_timer_start()
 {
     struct rcu_data *rdp = &this_cpu(rcu_data);
+    s_time_t next;
 
     /*
      * Note that we don't check rcu_pending() here. In fact, we don't want
@@ -453,7 +461,9 @@ void rcu_idle_timer_start()
     if (likely(!rdp->curlist))
         return;
 
-    set_timer(&rdp->idle_timer, NOW() + RCU_IDLE_TIMER_PERIOD);
+    next = min_t(s_time_t, RCU_IDLE_TIMER_PERIOD_MAX,
+                 cpumask_weight(&rcu_ctrlblk.cpumask) * 
RCU_IDLE_TIMER_CPU_DELAY);
+    set_timer(&rdp->idle_timer, NOW() + next);
     rdp->idle_timer_active = true;
 }
 


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