linux-kernel - [PATCH tip/core/rcu 06/26] rcu: Ignore userspace extended quiescent state by default

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Message-Id: <1346360743-3628-6-git-send-email-paulmck@linux.vnet.ibm.com>
Date:	Thu, 30 Aug 2012 14:05:23 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	mingo@...e.hu, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	fweisbec@...il.com, sbw@....edu, patches@...aro.org,
	Alessio Igor Bogani <abogani@...nel.org>,
	Avi Kivity <avi@...hat.com>,
	Chris Metcalf <cmetcalf@...era.com>,
	Christoph Lameter <cl@...ux.com>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	Kevin Hilman <khilman@...com>,
	Max Krasnyansky <maxk@...lcomm.com>,
	Stephen Hemminger <shemminger@...tta.com>,
	Sven-Thorsten Dietrich <thebigcorporation@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 06/26] rcu: Ignore userspace extended quiescent state by default

From: Frederic Weisbecker <fweisbec@...il.com>

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@...il.com>
Cc: Alessio Igor Bogani <abogani@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Avi Kivity <avi@...hat.com>
Cc: Chris Metcalf <cmetcalf@...era.com>
Cc: Christoph Lameter <cl@...ux.com>
Cc: Geoff Levand <geoff@...radead.org>
Cc: Gilad Ben Yossef <gilad@...yossef.com>
Cc: Hakan Akkan <hakanakkan@...il.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Josh Triplett <josh@...htriplett.org>
Cc: Kevin Hilman <khilman@...com>
Cc: Max Krasnyansky <maxk@...lcomm.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Stephen Hemminger <shemminger@...tta.com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Sven-Thorsten Dietrich <thebigcorporation@...il.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 kernel/rcutree.c |    5 ++++-
 kernel/rcutree.h |    1 +
 2 files changed, 5 insertions(+), 1 deletions(-)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 8bbc7fb..e2fd370 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 0dd5fd6..c190582 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
 };
-- 
1.7.8

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