Messages in this thread Patch in this message | | | Date | Fri, 31 Oct 2014 14:55:05 +0000 | From | Alexander Gordeev <> | Subject | [PATCH v2] rcu: Remove redundant rcu_is_cpu_rrupt_from_idle() from tiny RCU |
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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 <agordeev@redhat.com> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> --- 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 cpu, 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 agordeev@redhat.com
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