linux-kernel - [RFC v2 4/5] rcu: Use for_each_leaf_node_cpu() in force_qs

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Date:   Thu, 15 Dec 2016 10:42:03 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     "Paul E . McKenney " <paulmck@...ux.vnet.ibm.com>,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Colin King <colin.king@...onical.com>,
        Mark Rutland <mark.rutland@....com>,
        Boqun Feng <boqun.feng@...il.com>
Subject: [RFC v2 4/5] rcu: Use for_each_leaf_node_cpu() in force_qs_rnp()

->qsmask of an RCU leaf node is usually more sparse than the
corresponding cpu_possible_mask. So replace the
for_each_leaf_node_possible_cpu() in force_qs_rnp() with
for_each_leaf_node_cpu() to save several checks.

[Note we need to use "1UL << bit" instead of "1 << bit" to generate the
corresponding mask for a bit because @mask is unsigned long, this was
spotted by Colin Ian King <colin.king@...onical.com> and CoverityScan in
a previous version of this patch.]

Signed-off-by: Boqun Feng <boqun.feng@...il.com>
---
 kernel/rcu/tree.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 4e5b81c843de..1ef13e63bc95 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3046,13 +3046,11 @@ static void force_qs_rnp(struct rcu_state *rsp,
 				continue;
 			}
 		}
-		for_each_leaf_node_possible_cpu(rnp, cpu) {
-			unsigned long bit = leaf_node_cpu_bit(rnp, cpu);
-			if ((rnp->qsmask & bit) != 0) {
-				if (f(per_cpu_ptr(rsp->rda, cpu), isidle, maxj))
-					mask |= bit;
-			}
-		}
+
+		for_each_leaf_node_cpu(rnp, rnp->qsmask, cpu)
+			if (f(per_cpu_ptr(rsp->rda, cpu), isidle, maxj))
+				mask |= leaf_node_cpu_bit(rnp, cpu);
+
 		if (mask != 0) {
 			/* Idle/offline CPUs, report (releases rnp->lock. */
 			rcu_report_qs_rnp(mask, rsp, rnp, rnp->gpnum, flags);
-- 
2.10.2