rcu: Allow expedited grace periods to be disabled at init

Expedited grace periods can speed up boot, but are undesirable in
aggressive real-time systems.  This commit therefore introduces a
kernel parameter rcupdate.rcu_normal_after_boot that disables
expedited grace periods just before init is spawned.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 7673943..197305b 100644 (file)
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -3318,6 +3318,11 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
                        grace-period latency.  This parameter overrides
                        rcupdate.rcu_expedited.
 
+       rcupdate.rcu_normal_after_boot= [KNL]
+                       Once boot has completed (that is, after
+                       rcu_end_inkernel_boot() has been invoked), use
+                       only normal grace-period primitives.
+
        rcupdate.rcu_task_stall_timeout= [KNL]
                        Set timeout in jiffies for RCU task stall warning
                        messages.  Disable with a value less than or equal

diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 8fccda3..12b91f5 100644 (file)
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -63,6 +63,9 @@ MODULE_ALIAS("rcupdate");
 module_param(rcu_expedited, int, 0);
 module_param(rcu_normal, int, 0);
 
+static int rcu_normal_after_boot;
+module_param(rcu_normal_after_boot, int, 0);
+
 #if defined(CONFIG_DEBUG_LOCK_ALLOC) && defined(CONFIG_PREEMPT_COUNT)
 /**
  * rcu_read_lock_sched_held() - might we be in RCU-sched read-side critical section?
@@ -178,6 +181,8 @@ void rcu_end_inkernel_boot(void)
 {
        if (IS_ENABLED(CONFIG_RCU_EXPEDITE_BOOT))
                rcu_unexpedite_gp();
+       if (rcu_normal_after_boot)
+               WRITE_ONCE(rcu_normal, 1);
 }
 
 #ifdef CONFIG_PREEMPT_RCU