rcu: Keep invoking callbacks if CPU otherwise idle

The rcu_do_batch() function that invokes callbacks for TREE_RCU and
TREE_PREEMPT_RCU normally throttles callback invocation to avoid degrading
scheduling latency.  However, as long as the CPU would otherwise be idle,
there is no downside to continuing to invoke any callbacks that have passed
through their grace periods.  In fact, processing such callbacks in a
timely manner has the benefit of increasing the probability that the
CPU can enter the power-saving dyntick-idle mode.

Therefore, this commit allows callback invocation to continue beyond the
preset limit as long as the scheduler does not have some other task to
run and as long as context is that of the idle task or the relevant
RCU kthread.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index dbcea6b93aea..adb6e666c6f4 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -1337,6 +1337,15 @@ static void invoke_rcu_callbacks_kthread(void)
 }
 
 /*
+ * Is the current CPU running the RCU-callbacks kthread?
+ * Caller must have preemption disabled.
+ */
+static bool rcu_is_callbacks_kthread(void)
+{
+	return __get_cpu_var(rcu_cpu_kthread_task) == current;
+}
+
+/*
  * Set the affinity of the boost kthread.  The CPU-hotplug locks are
  * held, so no one should be messing with the existence of the boost
  * kthread.
@@ -1780,6 +1789,11 @@ static void invoke_rcu_callbacks_kthread(void)
 	WARN_ON_ONCE(1);
 }
 
+static bool rcu_is_callbacks_kthread(void)
+{
+	return false;
+}
+
 static void rcu_preempt_boost_start_gp(struct rcu_node *rnp)
 {
 }