Commit-ID: ca9558a33f658155c3b69f92897c2e6a848684f5
Gitweb: http://git.kernel.org/tip/ca9558a33f658155c3b69f92897c2e6a848684f5
Author: Alexander Gordeev <agordeev@xxxxxxxxxx>
AuthorDate: Fri, 31 Oct 2014 14:55:05 +0000
Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
CommitDate: Tue, 30 Dec 2014 17:40:18 -0800
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 <agordeev@xxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---
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 0db5649..805b6d5 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();
--
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