linux-kernel - [PATCH RFC tip/core/rcu 10/18] rcu: make force_quiescent

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Date:	Tue, 15 Dec 2009 15:02:33 -0800
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, dvhltc@...ibm.com, niv@...ibm.com,
	tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
	Valdis.Kletnieks@...edu, dhowells@...hat.com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH RFC tip/core/rcu 10/18] rcu: make force_quiescent_state() start grace period if needed

From: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

Proposed for 2.6.34, not for inclusion.

Grace periods cannot be started while force_quiescent_state() is
active.  This is OK in that the affected CPUs will try again later,
but it does induce needless grace-period delays.  This patch causes
rcu_start_gp() to record a failed attempt to start a grace period.
When force_quiescent_state() prepares to return, it then starts the
grace period if there was such a failed attempt.

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 kernel/rcutree.c |    8 ++++++++
 kernel/rcutree.h |    5 +++++
 2 files changed, 13 insertions(+), 0 deletions(-)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index d920285..55e8f6e 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -660,6 +660,8 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
 	struct rcu_node *rnp = rcu_get_root(rsp);
 
 	if (!cpu_needs_another_gp(rsp, rdp) || rsp->fqs_active) {
+		if (cpu_needs_another_gp(rsp, rdp))
+			rsp->fqs_need_gp = 1;
 		if (rnp->completed == rsp->completed) {
 			spin_unlock_irqrestore(&rnp->lock, flags);
 			return;
@@ -1239,6 +1241,12 @@ static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
 		break;
 	}
 	rsp->fqs_active = 0;
+	if (rsp->fqs_need_gp) {
+		spin_unlock(&rsp->fqslock); /* irqs remain disabled */
+		rsp->fqs_need_gp = 0;
+		rcu_start_gp(rsp, flags); /* releases rnp->lock */
+		return;
+	}
 	spin_unlock(&rnp->lock);  /* irqs remain disabled */
 unlock_fqs_ret:
 	spin_unlock_irqrestore(&rsp->fqslock, flags);
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index edb6fae..bd5d78a 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -278,6 +278,11 @@ struct rcu_state {
 						/* Force QS state. */
 	u8	fqs_active;			/* force_quiescent_state() */
 						/*  is running. */
+	u8	fqs_need_gp;			/* A CPU was prevented from */
+						/*  starting a new grace */
+						/*  period because */
+						/*  force_quiescent_state() */
+						/*  was running. */
 	long	gpnum;				/* Current gp number. */
 	long	completed;			/* # of last completed gp. */
 
-- 
1.5.2.5

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