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Subject[tip:core/urgent] rcu: Cure load woes
Commit-ID:  d72bce0e67e8afc6eb959f656013cbb577426f1e
Gitweb: http://git.kernel.org/tip/d72bce0e67e8afc6eb959f656013cbb577426f1e
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Mon, 30 May 2011 13:34:51 +0200
Committer: Ingo Molnar <mingo@elte.hu>
CommitDate: Tue, 31 May 2011 10:01:48 +0200

rcu: Cure load woes

Commit cc3ce5176d83 (rcu: Start RCU kthreads in TASK_INTERRUPTIBLE
state) fudges a sleeping task' state, resulting in the scheduler seeing
a TASK_UNINTERRUPTIBLE task going to sleep, but a TASK_INTERRUPTIBLE
task waking up. The result is unbalanced load calculation.

The problem that patch tried to address is that the RCU threads could
stay in UNINTERRUPTIBLE state for quite a while and triggering the hung
task detector due to on-demand wake-ups.

Cure the problem differently by always giving the tasks at least one
wake-up once the CPU is fully up and running, this will kick them out of
the initial UNINTERRUPTIBLE state and into the regular INTERRUPTIBLE
wait state.

[ The alternative would be teaching kthread_create() to start threads as
INTERRUPTIBLE but that needs a tad more thought. ]

Reported-by: Damien Wyart <damien.wyart@free.fr>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Paul E. McKenney <paul.mckenney@linaro.org>
Link: http://lkml.kernel.org/r/1306755291.1200.2872.camel@twins
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
kernel/rcutree.c | 54 ++++++++++++++++++++++++++++++++++++++++-------
kernel/rcutree_plugin.h | 11 ++++++++-
2 files changed, 56 insertions(+), 9 deletions(-)
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 77a7671..89419ff 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1648,7 +1648,6 @@ static int __cpuinit rcu_spawn_one_cpu_kthread(int cpu)
if (IS_ERR(t))
return PTR_ERR(t);
kthread_bind(t, cpu);
- set_task_state(t, TASK_INTERRUPTIBLE);
per_cpu(rcu_cpu_kthread_cpu, cpu) = cpu;
WARN_ON_ONCE(per_cpu(rcu_cpu_kthread_task, cpu) != NULL);
per_cpu(rcu_cpu_kthread_task, cpu) = t;
@@ -1756,7 +1755,6 @@ static int __cpuinit rcu_spawn_one_node_kthread(struct rcu_state *rsp,
if (IS_ERR(t))
return PTR_ERR(t);
raw_spin_lock_irqsave(&rnp->lock, flags);
- set_task_state(t, TASK_INTERRUPTIBLE);
rnp->node_kthread_task = t;
raw_spin_unlock_irqrestore(&rnp->lock, flags);
sp.sched_priority = 99;
@@ -1765,6 +1763,8 @@ static int __cpuinit rcu_spawn_one_node_kthread(struct rcu_state *rsp,
return rcu_spawn_one_boost_kthread(rsp, rnp, rnp_index);
}

+static void rcu_wake_one_boost_kthread(struct rcu_node *rnp);
+
/*
* Spawn all kthreads -- called as soon as the scheduler is running.
*/
@@ -1772,18 +1772,30 @@ static int __init rcu_spawn_kthreads(void)
{
int cpu;
struct rcu_node *rnp;
+ struct task_struct *t;

rcu_kthreads_spawnable = 1;
for_each_possible_cpu(cpu) {
per_cpu(rcu_cpu_has_work, cpu) = 0;
- if (cpu_online(cpu))
+ if (cpu_online(cpu)) {
(void)rcu_spawn_one_cpu_kthread(cpu);
+ t = per_cpu(rcu_cpu_kthread_task, cpu);
+ if (t)
+ wake_up_process(t);
+ }
}
rnp = rcu_get_root(rcu_state);
(void)rcu_spawn_one_node_kthread(rcu_state, rnp);
+ if (rnp->node_kthread_task)
+ wake_up_process(rnp->node_kthread_task);
if (NUM_RCU_NODES > 1) {
- rcu_for_each_leaf_node(rcu_state, rnp)
+ rcu_for_each_leaf_node(rcu_state, rnp) {
(void)rcu_spawn_one_node_kthread(rcu_state, rnp);
+ t = rnp->node_kthread_task;
+ if (t)
+ wake_up_process(t);
+ rcu_wake_one_boost_kthread(rnp);
+ }
}
return 0;
}
@@ -2188,14 +2200,14 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible)
raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
}

-static void __cpuinit rcu_online_cpu(int cpu)
+static void __cpuinit rcu_prepare_cpu(int cpu)
{
rcu_init_percpu_data(cpu, &rcu_sched_state, 0);
rcu_init_percpu_data(cpu, &rcu_bh_state, 0);
rcu_preempt_init_percpu_data(cpu);
}

-static void __cpuinit rcu_online_kthreads(int cpu)
+static void __cpuinit rcu_prepare_kthreads(int cpu)
{
struct rcu_data *rdp = per_cpu_ptr(rcu_state->rda, cpu);
struct rcu_node *rnp = rdp->mynode;
@@ -2209,6 +2221,31 @@ static void __cpuinit rcu_online_kthreads(int cpu)
}

/*
+ * kthread_create() creates threads in TASK_UNINTERRUPTIBLE state,
+ * but the RCU threads are woken on demand, and if demand is low this
+ * could be a while triggering the hung task watchdog.
+ *
+ * In order to avoid this, poke all tasks once the CPU is fully
+ * up and running.
+ */
+static void __cpuinit rcu_online_kthreads(int cpu)
+{
+ struct rcu_data *rdp = per_cpu_ptr(rcu_state->rda, cpu);
+ struct rcu_node *rnp = rdp->mynode;
+ struct task_struct *t;
+
+ t = per_cpu(rcu_cpu_kthread_task, cpu);
+ if (t)
+ wake_up_process(t);
+
+ t = rnp->node_kthread_task;
+ if (t)
+ wake_up_process(t);
+
+ rcu_wake_one_boost_kthread(rnp);
+}
+
+/*
* Handle CPU online/offline notification events.
*/
static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
@@ -2221,10 +2258,11 @@ static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
switch (action) {
case CPU_UP_PREPARE:
case CPU_UP_PREPARE_FROZEN:
- rcu_online_cpu(cpu);
- rcu_online_kthreads(cpu);
+ rcu_prepare_cpu(cpu);
+ rcu_prepare_kthreads(cpu);
break;
case CPU_ONLINE:
+ rcu_online_kthreads(cpu);
case CPU_DOWN_FAILED:
rcu_node_kthread_setaffinity(rnp, -1);
rcu_cpu_kthread_setrt(cpu, 1);
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index a767b7d..c8bff30 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -1295,7 +1295,6 @@ static int __cpuinit rcu_spawn_one_boost_kthread(struct rcu_state *rsp,
if (IS_ERR(t))
return PTR_ERR(t);
raw_spin_lock_irqsave(&rnp->lock, flags);
- set_task_state(t, TASK_INTERRUPTIBLE);
rnp->boost_kthread_task = t;
raw_spin_unlock_irqrestore(&rnp->lock, flags);
sp.sched_priority = RCU_KTHREAD_PRIO;
@@ -1303,6 +1302,12 @@ static int __cpuinit rcu_spawn_one_boost_kthread(struct rcu_state *rsp,
return 0;
}

+static void __cpuinit rcu_wake_one_boost_kthread(struct rcu_node *rnp)
+{
+ if (rnp->boost_kthread_task)
+ wake_up_process(rnp->boost_kthread_task);
+}
+
#else /* #ifdef CONFIG_RCU_BOOST */

static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
@@ -1326,6 +1331,10 @@ static int __cpuinit rcu_spawn_one_boost_kthread(struct rcu_state *rsp,
return 0;
}

+static void __cpuinit rcu_wake_one_boost_kthread(struct rcu_node *rnp)
+{
+}
+
#endif /* #else #ifdef CONFIG_RCU_BOOST */

#ifndef CONFIG_SMP

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