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>
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 28fd92a..51eef41 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -95,7 +95,7 @@
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 @@
kthread_stop(t);
rcu_stop_boost_kthread(rnp);
} else
- rcu_node_kthread_setaffinity(rnp);
+ rcu_node_kthread_setaffinity(rnp, -1);
}
/*
@@ -1644,8 +1644,12 @@
* 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 @@
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 @@
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: