rcu: Avoid waking up CPUs having only kfree_rcu() callbacks

When CONFIG_RCU_FAST_NO_HZ is enabled, RCU will allow a given CPU to
enter dyntick-idle mode even if it still has RCU callbacks queued.
RCU avoids system hangs in this case by scheduling a timer for several
jiffies in the future.  However, if all of the callbacks on that CPU
are from kfree_rcu(), there is no reason to wake the CPU up, as it is
not a problem to defer freeing of memory.

This commit therefore tracks the number of callbacks on a given CPU
that are from kfree_rcu(), and avoids scheduling the timer if all of
a given CPU's callbacks are from kfree_rcu().

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

3 files changed, 9 insertions, 1 deletions

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 81c04f4348ec..a67d5f1072ea 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -841,7 +841,7 @@ void __kfree_rcu(struct rcu_head *head, unsigned long offset)
 	/* See the kfree_rcu() header comment. */
 	BUILD_BUG_ON(!__is_kfree_rcu_offset(offset));
 
-	call_rcu(head, (rcu_callback)offset);
+	kfree_call_rcu(head, (rcu_callback)offset);
 }
 
 /**
diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index 00b7a5e493d2..51bf29c81485 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -83,6 +83,12 @@ static inline void synchronize_sched_expedited(void)
 	synchronize_sched();
 }
 
+static inline void kfree_call_rcu(struct rcu_head *head,
+				  void (*func)(struct rcu_head *rcu))
+{
+	call_rcu(head, func);
+}
+
 #ifdef CONFIG_TINY_RCU
 
 static inline void rcu_preempt_note_context_switch(void)
diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index 73e7195f9997..73892483fd05 100644
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -61,6 +61,8 @@ extern void synchronize_rcu_bh(void);
 extern void synchronize_sched_expedited(void);
 extern void synchronize_rcu_expedited(void);
 
+void kfree_call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu));
+
 static inline void synchronize_rcu_bh_expedited(void)
 {
 	synchronize_sched_expedited();