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Commit ca9558a3 authored by Alexander Gordeev's avatar Alexander Gordeev Committed by Paul E. McKenney
Browse files

rcu: Remove redundant rcu_is_cpu_rrupt_from_idle() from tiny RCU 



Let's start assuming that something in the idle loop posts a callback,
and scheduling-clock interrupt occurs:

1. The system is idle and stays that way, no runnable tasks.

2. Scheduling-clock interrupt occurs, rcu_check_callbacks() is called
   as result, which in turn calls rcu_is_cpu_rrupt_from_idle().

3. rcu_is_cpu_rrupt_from_idle() reports the CPU was interrupted from
   idle, which results in rcu_sched_qs() call, which does a
   raise_softirq(RCU_SOFTIRQ).

4. Upon return from interrupt, rcu_irq_exit() is invoked, which calls
   rcu_idle_enter_common(), which in turn calls rcu_sched_qs() again,
   which does another raise_softirq(RCU_SOFTIRQ).

5. The softirq happens shortly and invokes rcu_process_callbacks(),
   which invokes __rcu_process_callbacks().

6. So now callbacks can be invoked. At least they can be if
   ->donetail has been updated. Which it will have been because
   rcu_sched_qs() invokes rcu_qsctr_help().

In the described scenario rcu_sched_qs() and raise_softirq(RCU_SOFTIRQ)
get called twice in steps 3 and 4. This redundancy could be eliminated
by removing rcu_is_cpu_rrupt_from_idle() function.

Signed-off-by: default avatarAlexander Gordeev <agordeev@redhat.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent 734d1680

kernel/rcu/tiny.c

+1 −11
Original line number Diff line number Diff line
@@ -185,16 +185,6 @@ EXPORT_SYMBOL(__rcu_is_watching); 


#endif /* defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_RCU_TRACE) */



/*

 * Test whether the current CPU was interrupted from idle.  Nested

 * interrupts don't count, we must be running at the first interrupt

 * level.

 */

static int rcu_is_cpu_rrupt_from_idle(void)

{

	return rcu_dynticks_nesting <= 1;

}



/*

 * Helper function for rcu_sched_qs() and rcu_bh_qs().

 * Also irqs are disabled to avoid confusion due to interrupt handlers
@@ -250,7 +240,7 @@ void rcu_bh_qs(void) 
void rcu_check_callbacks(int user)

{

	RCU_TRACE(check_cpu_stalls());

	if (user || rcu_is_cpu_rrupt_from_idle())

	if (user)

		rcu_sched_qs();

	else if (!in_softirq())

		rcu_bh_qs();