From: Alexander Gordeev Date: Fri, 31 Oct 2014 14:55:05 +0000 (+0000) Subject: rcu: Remove redundant rcu_is_cpu_rrupt_from_idle() from tiny RCU X-Git-Tag: firefly_0821_release~176^2~2265^2^2~1^2~10 X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=ca9558a33f658155c3b69f92897c2e6a848684f5;p=firefly-linux-kernel-4.4.55.git 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: Alexander Gordeev Cc: Paul E. McKenney Signed-off-by: Paul E. McKenney --- diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c index 0db5649f8817..805b6d59c854 100644 --- a/kernel/rcu/tiny.c +++ b/kernel/rcu/tiny.c @@ -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();