rcu: Eliminate softirq-mediated RCU_FAST_NO_HZ idle-entry loop

If a softirq is pending, the current CPU has RCU callbacks pending,
and RCU does not immediately need anything from this CPU, then the
current code resets the RCU_FAST_NO_HZ state machine.  This means that
upon exit from the subsequent softirq handler, RCU_FAST_NO_HZ will
try really hard to force RCU into dyntick-idle mode.  And if the same
conditions hold after a few tries (determined by RCU_IDLE_OPT_FLUSHES),
the same situation can repeat, possibly endlessly.  This scenario is
not particularly good for battery lifetime.

This commit therefore suppresses the early exit from the RCU_FAST_NO_HZ
state machine in the case where there is a softirq pending.  This change
forces the state machine to retain its memory, and to enter holdoff if
this condition persists.

Reported-by: "Abou Gazala, Neven M" <neven.m.abou.gazala@intel.com>
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcutree_plugin.h

@@ -2133,7 +2133,8 @@ static void rcu_prepare_for_idle(int cpu)
 		/* First time through, initialize the counter. */
 		per_cpu(rcu_dyntick_drain, cpu) = RCU_IDLE_FLUSHES;
 	} else if (per_cpu(rcu_dyntick_drain, cpu) <= RCU_IDLE_OPT_FLUSHES &&
-		   !rcu_pending(cpu)) {
+		   !rcu_pending(cpu) &&
+		   !local_softirq_pending()) {
 		/* Can we go dyntick-idle despite still having callbacks? */
 		trace_rcu_prep_idle("Dyntick with callbacks");
 		per_cpu(rcu_dyntick_drain, cpu) = 0;