linux-kernel - [PATCH 22/24] nohz: Don't turn off the tick if rcu needs it

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Message-Id: <1356028391-14427-23-git-send-email-fweisbec@gmail.com>
Date:	Thu, 20 Dec 2012 19:33:09 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Alessio Igor Bogani <abogani@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.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>,
	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH 22/24] nohz: Don't turn off the tick if rcu needs it

If RCU is waiting for the current CPU to complete a grace
period, don't turn off the tick. Unlike dynctik-idle, we
are not necessarily going to enter into rcu extended quiescent
state, so we may need to keep the tick to note current CPU's
quiescent states.

[added build fix from Zen Lin]

CHECKME: OTOH we don't want to handle a locally started
grace period, this should be offloaded for rcu_nocb CPUs.
What we want is to be kicked if we stay dynticks in the kernel
for too long (ie: to report a quiescent state).
rcu_pending() is perhaps an overkill just for that.

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: Ingo Molnar <mingo@...nel.org>
Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Paul Gortmaker <paul.gortmaker@...driver.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
---
 include/linux/rcupdate.h |    1 +
 kernel/rcutree.c         |    3 +--
 kernel/time/tick-sched.c |    3 +++
 3 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 829312e..2ebadac 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -211,6 +211,7 @@ static inline int rcu_preempt_depth(void)
 extern void rcu_sched_qs(int cpu);
 extern void rcu_bh_qs(int cpu);
 extern void rcu_check_callbacks(int cpu, int user);
+extern int rcu_pending(int cpu);
 struct notifier_block;
 extern void rcu_idle_enter(void);
 extern void rcu_idle_exit(void);
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index e9e0ffa..6ba3e02 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -232,7 +232,6 @@ module_param(jiffies_till_next_fqs, ulong, 0644);
 
 static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *));
 static void force_quiescent_state(struct rcu_state *rsp);
-static int rcu_pending(int cpu);
 
 /*
  * Return the number of RCU-sched batches processed thus far for debug & stats.
@@ -2521,7 +2520,7 @@ static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp)
  * by the current CPU, returning 1 if so.  This function is part of the
  * RCU implementation; it is -not- an exported member of the RCU API.
  */
-static int rcu_pending(int cpu)
+int rcu_pending(int cpu)
 {
 	struct rcu_state *rsp;
 
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 2808e02..0c9281a 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -594,6 +594,9 @@ static bool can_stop_full_tick(int cpu)
 	if (!rcu_is_nocb_cpu(cpu))
 		return false;
 
+	if (rcu_pending(cpu))
+		return false;
+
 	return true;
 }
 #endif
-- 
1.7.5.4

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