cpuidle: Allow cpuidle drivers to take over RCU-idle

Some drivers have to do significant work, some of which relies on RCU
still being active. Instead of using RCU_NONIDLE in the drivers and
flipping RCU back on, allow drivers to take over RCU-idle duty.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/cpuidle/cpuidle.c

@@ -138,6 +138,7 @@ static void enter_s2idle_proper(struct cpuidle_driver *drv,
 				struct cpuidle_device *dev, int index)
 {
 	ktime_t time_start, time_end;
+	struct cpuidle_state *target_state = &drv->states[index];
 
 	time_start = ns_to_ktime(local_clock());
 
@@ -153,16 +154,18 @@ static void enter_s2idle_proper(struct cpuidle_driver *drv,
 	 * suspended is generally unsafe.
 	 */
 	stop_critical_timings();
-	rcu_idle_enter();
-	drv->states[index].enter_s2idle(dev, drv, index);
+	if (!(target_state->flags & CPUIDLE_FLAG_RCU_IDLE))
+		rcu_idle_enter();
+	target_state->enter_s2idle(dev, drv, index);
 	if (WARN_ON_ONCE(!irqs_disabled()))
 		local_irq_disable();
 	/*
 	 * timekeeping_resume() that will be called by tick_unfreeze() for the
 	 * first CPU executing it calls functions containing RCU read-side
 	 * critical sections, so tell RCU about that.
 	 */
-	rcu_idle_exit();
+	if (!(target_state->flags & CPUIDLE_FLAG_RCU_IDLE))
+		rcu_idle_exit();
 	tick_unfreeze();
 	start_critical_timings();
 
@@ -239,9 +242,11 @@ int cpuidle_enter_state(struct cpuidle_device *dev, struct cpuidle_driver *drv,
 	time_start = ns_to_ktime(local_clock());
 
 	stop_critical_timings();
-	rcu_idle_enter();
+	if (!(target_state->flags & CPUIDLE_FLAG_RCU_IDLE))
+		rcu_idle_enter();
 	entered_state = target_state->enter(dev, drv, index);
-	rcu_idle_exit();
+	if (!(target_state->flags & CPUIDLE_FLAG_RCU_IDLE))
+		rcu_idle_exit();
 	start_critical_timings();
 
 	sched_clock_idle_wakeup_event();

include/linux/cpuidle.h

@@ -82,6 +82,7 @@ struct cpuidle_state {
 #define CPUIDLE_FLAG_UNUSABLE		BIT(3) /* avoid using this state */
 #define CPUIDLE_FLAG_OFF		BIT(4) /* disable this state by default */
 #define CPUIDLE_FLAG_TLB_FLUSHED	BIT(5) /* idle-state flushes TLBs */
+#define CPUIDLE_FLAG_RCU_IDLE		BIT(6) /* idle-state takes care of RCU */
 
 struct cpuidle_device_kobj;
 struct cpuidle_state_kobj;