rcu: Allow for smp_call_function() running callbacks from idle

Current RCU hard relies on smp_call_function() callbacks running from
interrupt context. A pending optimization is going to break that, it
will allow idle CPUs to run the callbacks from the idle loop. This
avoids raising the IPI on the requesting CPU and avoids handling an
exception on the receiving CPU.

Change rcu_is_cpu_rrupt_from_idle() to also accept task context,
provided it is the idle task.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Link: https://lore.kernel.org/r/20200527171236.GC706495@hirez.programming.kicks-ass.net

kernel/rcu/tree.c

@@ -418,16 +418,23 @@ void rcu_momentary_dyntick_idle(void)
 EXPORT_SYMBOL_GPL(rcu_momentary_dyntick_idle);
 
 /**
- * rcu_is_cpu_rrupt_from_idle - see if interrupted from idle
+ * rcu_is_cpu_rrupt_from_idle - see if 'interrupted' from idle
  *
  * If the current CPU is idle and running at a first-level (not nested)
- * interrupt from idle, return true.  The caller must have at least
- * disabled preemption.
+ * interrupt, or directly, from idle, return true.
+ *
+ * The caller must have at least disabled IRQs.
  */
 static int rcu_is_cpu_rrupt_from_idle(void)
 {
-	/* Called only from within the scheduling-clock interrupt */
-	lockdep_assert_in_irq();
+	long nesting;
+
+	/*
+	 * Usually called from the tick; but also used from smp_function_call()
+	 * for expedited grace periods. This latter can result in running from
+	 * the idle task, instead of an actual IPI.
+	 */
+	lockdep_assert_irqs_disabled();
 
 	/* Check for counter underflows */
 	RCU_LOCKDEP_WARN(__this_cpu_read(rcu_data.dynticks_nesting) < 0,
@@ -436,9 +443,15 @@ static int rcu_is_cpu_rrupt_from_idle(void)
 			 "RCU dynticks_nmi_nesting counter underflow/zero!");
 
 	/* Are we at first interrupt nesting level? */
-	if (__this_cpu_read(rcu_data.dynticks_nmi_nesting) != 1)
+	nesting = __this_cpu_read(rcu_data.dynticks_nmi_nesting);
+	if (nesting > 1)
 		return false;
 
+	/*
+	 * If we're not in an interrupt, we must be in the idle task!
+	 */
+	WARN_ON_ONCE(!nesting && !is_idle_task(current));
+
 	/* Does CPU appear to be idle from an RCU standpoint? */
 	return __this_cpu_read(rcu_data.dynticks_nesting) == 0;
 }