rcu: Ignore userspace extended quiescent state by default

By default we don't want to enter into RCU extended quiescent
state while in userspace because doing this produces some overhead
(eg: use of syscall slowpath). Set it off by default and ready to
run when some feature like adaptive tickless need it.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Alessio Igor Bogani <abogani@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Avi Kivity <avi@redhat.com>
Cc: Chris Metcalf <cmetcalf@tilera.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Geoff Levand <geoff@infradead.org>
Cc: Gilad Ben Yossef <gilad@benyossef.com>
Cc: Hakan Akkan <hakanakkan@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Kevin Hilman <khilman@ti.com>
Cc: Max Krasnyansky <maxk@qualcomm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephen Hemminger <shemminger@vyatta.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Sven-Thorsten Dietrich <thebigcorporation@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

1 files changed, 4 insertions, 1 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index d62c04482228..6b82a9565149 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -206,6 +206,9 @@ EXPORT_SYMBOL_GPL(rcu_note_context_switch);
 DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
         .dynticks_nesting = DYNTICK_TASK_EXIT_IDLE,
         .dynticks = ATOMIC_INIT(1),
+#ifdef CONFIG_RCU_USER_QS
+        .ignore_user_qs = true,
+#endif
 };
 
 static int blimit = 10;         /* Maximum callbacks per rcu_do_batch. */
@@ -430,7 +433,7 @@ void rcu_user_enter(void)
 
         local_irq_save(flags);
         rdtp = &__get_cpu_var(rcu_dynticks);
-        if (!rdtp->in_user) {
+        if (!rdtp->ignore_user_qs && !rdtp->in_user) {
                 rdtp->in_user = true;
                 rcu_eqs_enter(1);
         }