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Subject[PATCH RFC tip/core/rcu 07/13] rcu: Avoid needlessly IPIing CPUs at GP end
Date
From: Paul E. McKenney <paul.mckenney@linaro.org>

If a CPU enters dyntick-idle mode with callbacks pending, it will need
an IPI at the end of the grace period. However, if it exits dyntick-idle
mode before the grace period ends, it will be needlessly IPIed at the
end of the grace period.

Therefore, this commit clears the per-CPU rcu_awake_at_gp_end flag
when a CPU determines that it does not need it. This in turn requires
disabling interrupts across much of rcu_prepare_for_idle() in order to
avoid having nested interrupts clearing this state out from under us.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
kernel/rcutree_plugin.h | 16 ++++++++++++++--
1 files changed, 14 insertions(+), 2 deletions(-)
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 45790bf..c4daf1e 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -2027,6 +2027,9 @@ int rcu_needs_cpu(int cpu)
static void rcu_prepare_for_idle(int cpu)
{
int c = 0;
+ unsigned long flags;
+
+ local_irq_save(flags);

/*
* If there are no callbacks on this CPU or if RCU has no further
@@ -2036,14 +2039,17 @@ static void rcu_prepare_for_idle(int cpu)
if (!rcu_cpu_has_callbacks(cpu)) {
per_cpu(rcu_dyntick_holdoff, cpu) = jiffies - 1;
per_cpu(rcu_dyntick_drain, cpu) = 0;
+ per_cpu(rcu_awake_at_gp_end, cpu) = 0;
+ local_irq_restore(flags);
trace_rcu_prep_idle("No callbacks");
return;
}
if (!rcu_pending(cpu)) {
- trace_rcu_prep_idle("Dyntick with callbacks");
per_cpu(rcu_dyntick_holdoff, cpu) = jiffies - 1;
per_cpu(rcu_dyntick_drain, cpu) = 0;
per_cpu(rcu_awake_at_gp_end, cpu) = 1;
+ local_irq_restore(flags);
+ trace_rcu_prep_idle("Dyntick with callbacks");
return; /* Nothing to do immediately. */
}

@@ -2052,6 +2058,7 @@ static void rcu_prepare_for_idle(int cpu)
* refrained from disabling the scheduling-clock tick.
*/
if (per_cpu(rcu_dyntick_holdoff, cpu) == jiffies) {
+ local_irq_restore(flags);
trace_rcu_prep_idle("In holdoff");
return;
}
@@ -2060,9 +2067,11 @@ static void rcu_prepare_for_idle(int cpu)
if (per_cpu(rcu_dyntick_drain, cpu) <= 0) {
/* First time through, initialize the counter. */
per_cpu(rcu_dyntick_drain, cpu) = RCU_NEEDS_CPU_FLUSHES;
+ per_cpu(rcu_awake_at_gp_end, cpu) = 0;
} else if (--per_cpu(rcu_dyntick_drain, cpu) <= 0) {
/* We have hit the limit, so time to give up. */
per_cpu(rcu_dyntick_holdoff, cpu) = jiffies;
+ local_irq_restore(flags);
trace_rcu_prep_idle("Begin holdoff");
invoke_rcu_core(); /* Force the CPU out of dyntick-idle. */
return;
@@ -2095,10 +2104,13 @@ static void rcu_prepare_for_idle(int cpu)
* So try forcing the callbacks through the grace period.
*/
if (c) {
+ local_irq_restore(flags);
trace_rcu_prep_idle("More callbacks");
invoke_rcu_core();
- } else
+ } else {
+ local_irq_restore(flags);
trace_rcu_prep_idle("Callbacks drained");
+ }
}

/*
--
1.7.3.2


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