rcu: Force per-rcu_node kthreads off of the outgoing CPU

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>

2 files changed, 24 insertions, 8 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 28fd92a9e0d0..51eef4193e7c 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 5964f82e2d96..4e486255df63 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)