RCU: make the period of the idle timer configurable

Make it possible for the user to specify, with the boot
time parameter rcu-idle-timer-period-ms, how frequently
a CPU that went idle with pending RCU callbacks should be
woken up to check if the grace period ended.

Typical values (i.e., some of the values used by Linux as
the tick frequency) are 10, 4 or 1 ms. Default valus (used
when this parameter is not specified) is 10ms. Maximum is
100ms.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

docs/misc/xen-command-line.markdown

@@ -1423,6 +1423,16 @@ The following resources are available:
     sum of CBMs is fixed, that means actual `cos_max` in use will automatically
     reduce to half when CDP is enabled.
 
+### rcu-idle-timer-period-ms
+> `= <integer>`
+
+> Default: `10`
+
+How frequently a CPU which has gone idle, but with pending RCU callbacks,
+should be woken up to check if the grace period has completed, and the
+callbacks are safe to be executed. Expressed in milliseconds; maximum is
+100, and it can't be 0.
+
 ### reboot
 > `= t[riple] | k[bd] | a[cpi] | p[ci] | P[ower] | e[fi] | n[o] [, [w]arm | [c]old]`
 

xen/common/rcupdate.c

@@ -110,10 +110,16 @@ 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.
+ *
+ * By default, we use 10ms, to enable at least some power saving on the
+ * CPU that is going idle. The user can change this, via a boot time
+ * parameter, but only up to 100ms.
  */
-#define RCU_IDLE_TIMER_PERIOD MILLISECS(10)
+#define IDLE_TIMER_PERIOD_MAX     MILLISECS(100)
+#define IDLE_TIMER_PERIOD_DEFAULT MILLISECS(10)
+
+static s_time_t __read_mostly idle_timer_period;
 
 static DEFINE_PER_CPU(struct rcu_data, rcu_data);
 
@@ -453,7 +459,7 @@ void rcu_idle_timer_start()
     if (likely(!rdp->curlist))
         return;
 
-    set_timer(&rdp->idle_timer, NOW() + RCU_IDLE_TIMER_PERIOD);
+    set_timer(&rdp->idle_timer, NOW() + idle_timer_period);
     rdp->idle_timer_active = true;
 }
 
@@ -571,6 +577,20 @@ static struct notifier_block cpu_nfb = {
 void __init rcu_init(void)
 {
     void *cpu = (void *)(long)smp_processor_id();
+    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);
+
+    /* We don't allow 0, or anything higher than IDLE_TIMER_PERIOD_MAX */
+    if ( idle_timer_period_ms == 0 ||
+         idle_timer_period_ms > IDLE_TIMER_PERIOD_MAX / MILLISECS(1) )
+    {
+        idle_timer_period_ms = IDLE_TIMER_PERIOD_DEFAULT / MILLISECS(1);
+        printk("WARNING: rcu-idle-timer-period-ms outside of "
+               "(0,%"PRI_stime"]. Resetting it to %u.\n",
+               IDLE_TIMER_PERIOD_MAX / MILLISECS(1), idle_timer_period_ms);
+    }
+    idle_timer_period = MILLISECS(idle_timer_period_ms);
 
     cpumask_clear(&rcu_ctrlblk.idle_cpumask);
     cpu_callback(&cpu_nfb, CPU_UP_PREPARE, cpu);