[PATCH 14/32] nohz/cpuset: Don't turn off the tick if rcu needs it

Frederic Weisbecker
2011-08-15T15:53:30Z

If RCU is waiting for the current CPU to complete a grace
period, don't turn off the tick. Unlike dynctik-idle, we
are not necessarily going to enter into rcu extended quiescent
state, so we may need to keep the tick to note current CPU's
quiescent states.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Anton Blanchard <anton@au1.ibm.com>
Cc: Avi Kivity <avi@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: Paul E . McKenney <paulmck@linux.vnet.ibm.com>
Cc: Paul Menage <menage@google.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephen Hemminger <shemminger@vyatta.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tim Pepper <lnxninja@linux.vnet.ibm.com>
---
include/linux/rcupdate.h | 1 +
kernel/rcutree.c | 3 +--
kernel/sched.c | 14 ++++++++++++++
3 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 99f9aa7..55a482a 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -133,6 +133,7 @@ static inline int rcu_preempt_depth(void)
extern void rcu_sched_qs(int cpu);
extern void rcu_bh_qs(int cpu);
extern void rcu_check_callbacks(int cpu, int user);
+extern int rcu_pending(int cpu);
struct notifier_block;

#ifdef CONFIG_NO_HZ
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index ba06207..0009bfc 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -205,7 +205,6 @@ int rcu_cpu_stall_suppress __read_mostly;
module_param(rcu_cpu_stall_suppress, int, 0644);

static void force_quiescent_state(struct rcu_state *rsp, int relaxed);
-static int rcu_pending(int cpu);

/*
* Return the number of RCU-sched batches processed thus far for debug & stats.
@@ -1729,7 +1728,7 @@ static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp)
* by the current CPU, returning 1 if so. This function is part of the
* RCU implementation; it is -not- an exported member of the RCU API.
*/
-static int rcu_pending(int cpu)
+int rcu_pending(int cpu)
{
return __rcu_pending(&rcu_sched_state, &per_cpu(rcu_sched_data, cpu)) ||
__rcu_pending(&rcu_bh_state, &per_cpu(rcu_bh_data, cpu)) ||
diff --git a/kernel/sched.c b/kernel/sched.c
index 0e1aa4e..353a66f 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2439,6 +2439,7 @@ DEFINE_PER_CPU(int, task_nohz_mode);
bool cpuset_nohz_can_stop_tick(void)
{
struct rq *rq;
+ int cpu;

rq = this_rq();

@@ -2446,6 +2447,19 @@ bool cpuset_nohz_can_stop_tick(void)
if (rq->nr_running > 1)
return false;

+ cpu = smp_processor_id();
+
+ /*
+ * FIXME: will probably be removed soon as it's
+ * already checked from tick_nohz_stop_sched_tick()
+ */
+ if (rcu_needs_cpu(cpu))
+ return false;
+
+ /* Is there a grace period to complete ? */
+ if (rcu_pending(cpu))
+ return false;
+
return true;
}

--
1.7.5.4