1 files changed, 43 insertions, 11 deletions

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index adb6e666c6f4..8cd9efe7e81f 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -1994,8 +1994,40 @@ static void rcu_prepare_for_idle(int cpu)
 
 #else /* #if !defined(CONFIG_RCU_FAST_NO_HZ) */
 
-#define RCU_NEEDS_CPU_FLUSHES 5         /* Allow for callback self-repost. */
+/*
+ * This code is invoked when a CPU goes idle, at which point we want
+ * to have the CPU do everything required for RCU so that it can enter
+ * the energy-efficient dyntick-idle mode.  This is handled by a
+ * state machine implemented by rcu_prepare_for_idle() below.
+ *
+ * The following three proprocessor symbols control this state machine:
+ *
+ * RCU_IDLE_FLUSHES gives the maximum number of times that we will attempt
+ *      to satisfy RCU.  Beyond this point, it is better to incur a periodic
+ *      scheduling-clock interrupt than to loop through the state machine
+ *      at full power.
+ * RCU_IDLE_OPT_FLUSHES gives the number of RCU_IDLE_FLUSHES that are
+ *      optional if RCU does not need anything immediately from this
+ *      CPU, even if this CPU still has RCU callbacks queued.  The first
+ *      times through the state machine are mandatory: we need to give
+ *      the state machine a chance to communicate a quiescent state
+ *      to the RCU core.
+ * RCU_IDLE_GP_DELAY gives the number of jiffies that a CPU is permitted
+ *      to sleep in dyntick-idle mode with RCU callbacks pending.  This
+ *      is sized to be roughly one RCU grace period.  Those energy-efficiency
+ *      benchmarkers who might otherwise be tempted to set this to a large
+ *      number, be warned: Setting RCU_IDLE_GP_DELAY too high can hang your
+ *      system.  And if you are -that- concerned about energy efficiency,
+ *      just power the system down and be done with it!
+ *
+ * The values below work well in practice.  If future workloads require
+ * adjustment, they can be converted into kernel config parameters, though
+ * making the state machine smarter might be a better option.
+ */
+#define RCU_IDLE_FLUSHES 5              /* Number of dyntick-idle tries. */
+#define RCU_IDLE_OPT_FLUSHES 3          /* Optional dyntick-idle tries. */
 #define RCU_IDLE_GP_DELAY 6             /* Roughly one grace period. */
+
 static DEFINE_PER_CPU(int, rcu_dyntick_drain);
 static DEFINE_PER_CPU(unsigned long, rcu_dyntick_holdoff);
 static DEFINE_PER_CPU(struct hrtimer, rcu_idle_gp_timer);
@@ -2110,17 +2142,17 @@ static void rcu_prepare_for_idle(int cpu)
         /* Check and update the rcu_dyntick_drain sequencing. */
         if (per_cpu(rcu_dyntick_drain, cpu) <= 0) {
                 /* First time through, initialize the counter. */
-                per_cpu(rcu_dyntick_drain, cpu) = RCU_NEEDS_CPU_FLUSHES;
-        } else if (--per_cpu(rcu_dyntick_drain, cpu) <= 0) {
+                per_cpu(rcu_dyntick_drain, cpu) = RCU_IDLE_FLUSHES;
+        } else if (per_cpu(rcu_dyntick_drain, cpu) <= RCU_IDLE_OPT_FLUSHES &&
+                   !rcu_pending(cpu)) {
                 /* Can we go dyntick-idle despite still having callbacks? */
-                if (!rcu_pending(cpu)) {
-                        trace_rcu_prep_idle("Dyntick with callbacks");
-                        per_cpu(rcu_dyntick_holdoff, cpu) = jiffies - 1;
-                        hrtimer_start(&per_cpu(rcu_idle_gp_timer, cpu),
-                                      rcu_idle_gp_wait, HRTIMER_MODE_REL);
-                        return; /* Nothing more to do immediately. */
-                }
-
+                trace_rcu_prep_idle("Dyntick with callbacks");
+                per_cpu(rcu_dyntick_drain, cpu) = 0;
+                per_cpu(rcu_dyntick_holdoff, cpu) = jiffies - 1;
+                hrtimer_start(&per_cpu(rcu_idle_gp_timer, cpu),
+                              rcu_idle_gp_wait, HRTIMER_MODE_REL);
+                return; /* Nothing more to do immediately. */
+        } else if (--per_cpu(rcu_dyntick_drain, cpu) <= 0) {
                 /* We have hit the limit, so time to give up. */
                 per_cpu(rcu_dyntick_holdoff, cpu) = jiffies;
                 local_irq_restore(flags);