[PATCH tip/core/rcu 07/86] rcu: Force per-rcu_node kthreads off of the outgoing CPU

Paul E. McKenney
2011-05-01T13:35:44Z

From: Paul E. McKenney <paul.mckenney@linaro.org>

The scheduler has had some heartburn in the past when too many real-time
kthreads were affinitied to the outgoing CPU. So, this commit lightens
the load by forcing the per-rcu_node and the boost kthreads off of the
outgoing CPU. Note that RCU's per-CPU kthread remains on the outgoing
CPU until the bitter end, as it must in order to preserve correctness.

Also avoid disabling hardirqs across calls to set_cpus_allowed_ptr(),
given that this function can block.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
kernel/rcutree.c | 24 +++++++++++++++++++-----
kernel/rcutree_plugin.h | 8 +++++---
2 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 1ce51b8..85b06c5 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -95,7 +95,7 @@ static DEFINE_PER_CPU(wait_queue_head_t, rcu_cpu_wq);
static DEFINE_PER_CPU(char, rcu_cpu_has_work);
static char rcu_kthreads_spawnable;

-static void rcu_node_kthread_setaffinity(struct rcu_node *rnp);
+static void rcu_node_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu);
static void invoke_rcu_cpu_kthread(void);

#define RCU_KTHREAD_PRIO 1 /* RT priority for per-CPU kthreads. */
@@ -1099,7 +1099,7 @@ static void __rcu_offline_cpu(int cpu, struct rcu_state *rsp)
kthread_stop(t);
rcu_stop_boost_kthread(rnp);
} else
- rcu_node_kthread_setaffinity(rnp);
+ rcu_node_kthread_setaffinity(rnp, -1);
}

/*
@@ -1644,8 +1644,12 @@ static int rcu_node_kthread(void *arg)
* Set the per-rcu_node kthread's affinity to cover all CPUs that are
* served by the rcu_node in question. The CPU hotplug lock is still
* held, so the value of rnp->qsmaskinit will be stable.
+ *
+ * We don't include outgoingcpu in the affinity set, use -1 if there is
+ * no outgoing CPU. If there are no CPUs left in the affinity set,
+ * this function allows the kthread to execute on any CPU.
*/
-static void rcu_node_kthread_setaffinity(struct rcu_node *rnp)
+static void rcu_node_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
{
cpumask_var_t cm;
int cpu;
@@ -1657,8 +1661,14 @@ static void rcu_node_kthread_setaffinity(struct rcu_node *rnp)
return;
cpumask_clear(cm);
for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++, mask >>= 1)
- if (mask & 0x1)
+ if ((mask & 0x1) && cpu != outgoingcpu)
cpumask_set_cpu(cpu, cm);
+ if (cpumask_weight(cm) == 0) {
+ cpumask_setall(cm);
+ for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++)
+ cpumask_clear_cpu(cpu, cm);
+ WARN_ON_ONCE(cpumask_weight(cm) == 0);
+ }
set_cpus_allowed_ptr(rnp->node_kthread_task, cm);
rcu_boost_kthread_setaffinity(rnp, cm);
free_cpumask_var(cm);
@@ -2154,7 +2164,11 @@ static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
rcu_online_kthreads(cpu);
break;
case CPU_ONLINE:
- rcu_node_kthread_setaffinity(rnp);
+ case CPU_DOWN_FAILED:
+ rcu_node_kthread_setaffinity(rnp, -1);
+ break;
+ case CPU_DOWN_PREPARE:
+ rcu_node_kthread_setaffinity(rnp, cpu);
break;
case CPU_DYING:
case CPU_DYING_FROZEN:
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 5964f82..4e48625 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -1212,17 +1212,19 @@ static void rcu_initiate_boost(struct rcu_node *rnp)
}
}

+/*
+ * 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.
+ */
static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp,
cpumask_var_t cm)
{
- unsigned long flags;
struct task_struct *t;

- raw_spin_lock_irqsave(&rnp->lock, flags);
t = rnp->boost_kthread_task;
if (t != NULL)
set_cpus_allowed_ptr(rnp->boost_kthread_task, cm);
- raw_spin_unlock_irqrestore(&rnp->lock, flags);
}

#define RCU_BOOST_DELAY_JIFFIES DIV_ROUND_UP(CONFIG_RCU_BOOST_DELAY * HZ, 1000)
--
1.7.3.2