On Fri, Oct 31, 2014 at 02:55:05PM +0000, Alexander Gordeev wrote:
> 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: Alexander Gordeev <[email protected]>
> Cc: Paul E. McKenney <[email protected]>
Queued for 3.20, thank you!
Thanx, Paul
> ---
> kernel/rcu/tiny.c | 12 +-----------
> 1 file changed, 1 insertion(+), 11 deletions(-)
>
> diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c
> index c0623fc..0bfb3b9 100644
> --- a/kernel/rcu/tiny.c
> +++ b/kernel/rcu/tiny.c
> @@ -186,16 +186,6 @@ 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
> * invoking call_rcu().
> @@ -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();
> --
> 1.8.3.1
>
> --
> Regards,
> Alexander Gordeev
> [email protected]
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