From 5a6db1867ba2ede72c533f9879af57a18580ce92 Mon Sep 17 00:00:00 2001
From: Frederic Weisbecker <fweisbec@gmail.com>
Date: Wed, 11 Jul 2012 20:26:32 +0200
Subject: [PATCH] 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>
---
 kernel/rcutree.c | 5 ++++-
 kernel/rcutree.h | 1 +
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 95747d08eca0..440574b2f424 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -210,6 +210,9 @@ 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. */
@@ -443,7 +446,7 @@ 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);
 	}
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 0dd5fd6e794c..c190582e8f2f 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -103,6 +103,7 @@ struct rcu_dynticks {
 	int tick_nohz_enabled_snap; /* Previously seen value from sysfs. */
 #endif /* #ifdef CONFIG_RCU_FAST_NO_HZ */
 #ifdef CONFIG_RCU_USER_QS
+	bool ignore_user_qs;	    /* Treat userspace as extended QS or not */
 	bool in_user;		    /* Is the CPU in userland from RCU POV? */
 #endif
 };
-- 
2.39.5