[PATCH tip/core/rcu 9/9] nohz_full: Force RCU's grace-period kthreads onto timekeeping CPU

Paul E. McKenney
2013-08-18T01:50:06Z

From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

Because RCU's quiescent-state-forcing mechanism is used to drive the
full-system-idle state machine, and because this mechanism is executed
by RCU's grace-period kthreads, this commit forces these kthreads to
run on the timekeeping CPU (tick_do_timer_cpu). To do otherwise would
mean that the RCU grace-period kthreads would force the system into
non-idle state every time they drove the state machine, which would
be just a bit on the futile side.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
---
kernel/rcutree.c | 1 +
kernel/rcutree.h | 1 +
kernel/rcutree_plugin.h | 21 ++++++++++++++++++++-
3 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 3d898e5..1c88e26 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1302,6 +1302,7 @@ static int rcu_gp_init(struct rcu_state *rsp)
struct rcu_data *rdp;
struct rcu_node *rnp = rcu_get_root(rsp);

+ rcu_bind_gp_kthread();
raw_spin_lock_irq(&rnp->lock);
rsp->gp_flags = 0; /* Clear all flags: New grace period. */

diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 6fd3659..5f97eab 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -560,6 +560,7 @@ static void rcu_sysidle_check_cpu(struct rcu_data *rdp, bool *isidle,
static bool is_sysidle_rcu_state(struct rcu_state *rsp);
static void rcu_sysidle_report_gp(struct rcu_state *rsp, int isidle,
unsigned long maxj);
+static void rcu_bind_gp_kthread(void);
static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp);

#endif /* #ifndef RCU_TREE_NONCORE */
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 58573c8..50a90bb 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -2542,7 +2542,8 @@ static void rcu_sysidle_check_cpu(struct rcu_data *rdp, bool *isidle,
if (!*isidle || rdp->rsp != rcu_sysidle_state ||
cpu_is_offline(rdp->cpu) || rdp->cpu == tick_do_timer_cpu)
return;
- /* WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu); */
+ if (rcu_gp_in_progress(rdp->rsp))
+ WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu);

/* Pick up current idle and NMI-nesting counter and check. */
cur = atomic_read(&rdtp->dynticks_idle);
@@ -2568,6 +2569,20 @@ static bool is_sysidle_rcu_state(struct rcu_state *rsp)
}

/*
+ * Bind the grace-period kthread for the sysidle flavor of RCU to the
+ * timekeeping CPU.
+ */
+static void rcu_bind_gp_kthread(void)
+{
+ int cpu = ACCESS_ONCE(tick_do_timer_cpu);
+
+ if (cpu < 0 || cpu >= nr_cpu_ids)
+ return;
+ if (raw_smp_processor_id() != cpu)
+ set_cpus_allowed_ptr(current, cpumask_of(cpu));
+}
+
+/*
* Return a delay in jiffies based on the number of CPUs, rcu_node
* leaf fanout, and jiffies tick rate. The idea is to allow larger
* systems more time to transition to full-idle state in order to
@@ -2765,6 +2780,10 @@ static bool is_sysidle_rcu_state(struct rcu_state *rsp)
return false;
}

+static void rcu_bind_gp_kthread(void)
+{
+}
+
static void rcu_sysidle_report_gp(struct rcu_state *rsp, int isidle,
unsigned long maxj)
{
--
1.8.1.5