rcu: Allow dyntick-idle mode for CPUs with callbacks

Currently, RCU does not permit a CPU to enter dyntick-idle mode if that
CPU has any RCU callbacks queued.  This means that workloads for which
each CPU wakes up and does some RCU updates every few ticks will never
enter dyntick-idle mode.  This can result in significant unnecessary power
consumption, so this patch permits a given to enter dyntick-idle mode if
it has callbacks, but only if that same CPU has completed all current
work for the RCU core.  We determine use rcu_pending() to determine
whether a given CPU has completed all current work for the RCU core.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index fd2f87db2ab1..ea32405177c9 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -88,6 +88,7 @@ struct rcu_dynticks {
                                     /* Process level is worth LLONG_MAX/2. */
         int dynticks_nmi_nesting;   /* Track NMI nesting level. */
         atomic_t dynticks;          /* Even value for idle, else odd. */
+        int wake_gp_end;            /* A GP ended, need to wake up CPUs. */
 };
 
 /* RCU's kthread states for tracing. */
@@ -467,5 +468,8 @@ static void rcu_yield(void (*f)(unsigned long), unsigned long arg);
 #endif /* #ifdef CONFIG_RCU_BOOST */
 static void rcu_cpu_kthread_setrt(int cpu, int to_rt);
 static void __cpuinit rcu_prepare_kthreads(int cpu);
+static void rcu_prepare_for_idle(int cpu);
+static void rcu_wake_cpus_for_gp_end(void);
+static void rcu_schedule_wake_gp_end(void);
 
 #endif /* #ifndef RCU_TREE_NONCORE */