rcu: put per-CPU kthread at non-RT priority during CPU hotplug operations

If you are doing CPU hotplug operations, it is best not to have
CPU-bound realtime tasks running CPU-bound on the outgoing CPU.
So this commit makes per-CPU kthreads run at non-realtime priority
during that time.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

1 files changed, 26 insertions, 0 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 51eef4193e7c..198e4df7d83e 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1464,6 +1464,30 @@ static void invoke_rcu_node_kthread(struct rcu_node *rnp)
 }
 
 /*
+ * Set the specified CPU's kthread to run RT or not, as specified by
+ * the to_rt argument.  The CPU-hotplug locks are held, so the task
+ * is not going away.
+ */
+static void rcu_cpu_kthread_setrt(int cpu, int to_rt)
+{
+        int policy;
+        struct sched_param sp;
+        struct task_struct *t;
+
+        t = per_cpu(rcu_cpu_kthread_task, cpu);
+        if (t == NULL)
+                return;
+        if (to_rt) {
+                policy = SCHED_FIFO;
+                sp.sched_priority = RCU_KTHREAD_PRIO;
+        } else {
+                policy = SCHED_NORMAL;
+                sp.sched_priority = 0;
+        }
+        sched_setscheduler_nocheck(t, policy, &sp);
+}
+
+/*
  * Timer handler to initiate the waking up of per-CPU kthreads that
  * have yielded the CPU due to excess numbers of RCU callbacks.
  * We wake up the per-rcu_node kthread, which in turn will wake up
@@ -2166,9 +2190,11 @@ static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
         case CPU_ONLINE:
         case CPU_DOWN_FAILED:
                 rcu_node_kthread_setaffinity(rnp, -1);
+                rcu_cpu_kthread_setrt(cpu, 1);
                 break;
         case CPU_DOWN_PREPARE:
                 rcu_node_kthread_setaffinity(rnp, cpu);
+                rcu_cpu_kthread_setrt(cpu, 0);
                 break;
         case CPU_DYING:
         case CPU_DYING_FROZEN: