rcu: add support for dynamic ticks and preempt rcu

The PREEMPT-RCU can get stuck if a CPU goes idle and NO_HZ is set. The
idle CPU will not progress the RCU through its grace period and a
synchronize_rcu my get stuck. Without this patch I have a box that will
not boot when PREEMPT_RCU and NO_HZ are set. That same box boots fine
with this patch.

This patch comes from the -rt kernel where it has been tested for
several months.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 3 insertions, 0 deletions

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index fa9bb73dbdb4..2968298f8f36 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -282,6 +282,7 @@ void tick_nohz_stop_sched_tick(void)
                         ts->idle_tick = ts->sched_timer.expires;
                         ts->tick_stopped = 1;
                         ts->idle_jiffies = last_jiffies;
+                        rcu_enter_nohz();
                 }
 
                 /*
@@ -375,6 +376,8 @@ void tick_nohz_restart_sched_tick(void)
                 return;
         }
 
+        rcu_exit_nohz();
+
         /* Update jiffies first */
         select_nohz_load_balancer(0);
         now = ktime_get();