rcu: move TREE_RCU from softirq to kthread

If RCU priority boosting is to be meaningful, callback invocation must
be boosted in addition to preempted RCU readers.  Otherwise, in presence
of CPU real-time threads, the grace period ends, but the callbacks don't
get invoked.  If the callbacks don't get invoked, the associated memory
doesn't get freed, so the system is still subject to OOM.

But it is not reasonable to priority-boost RCU_SOFTIRQ, so this commit
moves the callback invocations to a kthread, which can be boosted easily.

Also add comments and properly synchronized all accesses to
rcu_cpu_kthread_task, as suggested by Lai Jiangshan.

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>

2 files changed, 1 insertions, 3 deletions

diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
index bea0ac750712..6c12989839d9 100644
--- a/include/linux/interrupt.h
+++ b/include/linux/interrupt.h
@@ -414,7 +414,6 @@ enum
         TASKLET_SOFTIRQ,
         SCHED_SOFTIRQ,
         HRTIMER_SOFTIRQ,
-        RCU_SOFTIRQ,    /* Preferable RCU should always be the last softirq */
 
         NR_SOFTIRQS
 };
diff --git a/include/trace/events/irq.h b/include/trace/events/irq.h
index 1c09820df585..ae045ca7d356 100644
--- a/include/trace/events/irq.h
+++ b/include/trace/events/irq.h
@@ -20,8 +20,7 @@ struct softirq_action;
                          softirq_name(BLOCK_IOPOLL),    \
                          softirq_name(TASKLET),         \
                          softirq_name(SCHED),           \
-                         softirq_name(HRTIMER),         \
-                         softirq_name(RCU))
+                         softirq_name(HRTIMER))
 
 /**
  * irq_handler_entry - called immediately before the irq action handler