linux-kernel - [PATCH RFC tip/core/rcu 16/20] rcu: fine-tune grace-period begin/end checks

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Date:	Fri, 17 Dec 2010 12:54:47 -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, 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,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH RFC tip/core/rcu 16/20] rcu: fine-tune grace-period begin/end checks

Use the CPU's bit in rnp->qsmask to determine whether or not the CPU
should try to report a quiescent state.  Handle overflow in the check
for rdp->gpnum having fallen behind.

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 kernel/rcutree.c |   27 ++++++++++++++++++---------
 1 files changed, 18 insertions(+), 9 deletions(-)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 8105271..c39ec5b 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -617,9 +617,17 @@ static void __init check_cpu_stall_init(void)
 static void __note_new_gpnum(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_data *rdp)
 {
 	if (rdp->gpnum != rnp->gpnum) {
-		rdp->qs_pending = 1;
-		rdp->passed_quiesc = 0;
+		/*
+		 * If the current grace period is waiting for this CPU,
+		 * set up to detect a quiescent state, otherwise don't
+		 * go looking for one.
+		 */
 		rdp->gpnum = rnp->gpnum;
+		if (rnp->qsmask & rdp->grpmask) {
+			rdp->qs_pending = 1;
+			rdp->passed_quiesc = 0;
+		} else
+			rdp->qs_pending = 0;
 	}
 }
 
@@ -681,19 +689,20 @@ __rcu_process_gp_end(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_dat
 
 		/*
 		 * If we were in an extended quiescent state, we may have
-		 * missed some grace periods that others CPUs took care on
+		 * missed some grace periods that others CPUs handled on
 		 * our behalf. Catch up with this state to avoid noting
-		 * spurious new grace periods.
+		 * spurious new grace periods.  If another grace period
+		 * has started, then rnp->gpnum will have advanced, so
+		 * we will detect this later on.
 		 */
-		if (rdp->completed > rdp->gpnum)
+		if (ULONG_CMP_LT(rdp->gpnum, rdp->completed))
 			rdp->gpnum = rdp->completed;
 
 		/*
-		 * If another CPU handled our extended quiescent states and
-		 * we have no more grace period to complete yet, then stop
-		 * chasing quiescent states.
+		 * If RCU does not need a quiescent state from this CPU,
+		 * then make sure that this CPU doesn't go looking for one.
 		 */
-		if (rdp->completed == rnp->gpnum)
+		if ((rnp->qsmask & rdp->grpmask) == 0)
 			rdp->qs_pending = 0;
 	}
 }
-- 
1.7.3.2

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