linux-kernel - [PATCH tip/core/rcu 05/12] rcu: Simplify rcu_init

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Message-Id: <1437172263-15466-5-git-send-email-paulmck@linux.vnet.ibm.com>
Date:	Fri, 17 Jul 2015 15:30:56 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	mingo@...nel.org, jiangshanlai@...il.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
	bobby.prani@...il.com, Alexander Gordeev <agordeev@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 05/12] rcu: Simplify rcu_init_geometry() capacity arithmetics

From: Alexander Gordeev <agordeev@...hat.com>

Current code suggests that introducing the extra level to
rcu_capacity[] array makes some of the arithmetic easier.
Well, in fact it appears rather confusing and unnecessary.

Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Signed-off-by: Alexander Gordeev <agordeev@...hat.com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 kernel/rcu/tree.c | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 37ca8a867a1c..2103beedb49f 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4082,7 +4082,7 @@ static void __init rcu_init_geometry(void)
 {
 	ulong d;
 	int i;
-	int rcu_capacity[MAX_RCU_LVLS + 1];
+	int rcu_capacity[MAX_RCU_LVLS];
 
 	/*
 	 * Initialize any unspecified boot parameters.
@@ -4119,29 +4119,27 @@ static void __init rcu_init_geometry(void)
 
 	/*
 	 * Compute number of nodes that can be handled an rcu_node tree
-	 * with the given number of levels.  Setting rcu_capacity[0] makes
-	 * some of the arithmetic easier.
+	 * with the given number of levels.
 	 */
-	rcu_capacity[0] = 1;
-	rcu_capacity[1] = rcu_fanout_leaf;
-	for (i = 2; i <= MAX_RCU_LVLS; i++)
+	rcu_capacity[0] = rcu_fanout_leaf;
+	for (i = 1; i < MAX_RCU_LVLS; i++)
 		rcu_capacity[i] = rcu_capacity[i - 1] * RCU_FANOUT;
 
 	/*
 	 * The tree must be able to accommodate the configured number of CPUs.
 	 * If this limit is exceeded than we have a serious problem elsewhere.
 	 */
-	if (nr_cpu_ids > rcu_capacity[MAX_RCU_LVLS])
+	if (nr_cpu_ids > rcu_capacity[MAX_RCU_LVLS - 1])
 		panic("rcu_init_geometry: rcu_capacity[] is too small");
 
 	/* Calculate the number of levels in the tree. */
-	for (i = 1; nr_cpu_ids > rcu_capacity[i]; i++) {
+	for (i = 0; nr_cpu_ids > rcu_capacity[i]; i++) {
 	}
-	rcu_num_lvls = i;
+	rcu_num_lvls = i + 1;
 
 	/* Calculate the number of rcu_nodes at each level of the tree. */
 	for (i = 0; i < rcu_num_lvls; i++) {
-		int cap = rcu_capacity[rcu_num_lvls - i];
+		int cap = rcu_capacity[(rcu_num_lvls - 1) - i];
 		num_rcu_lvl[i] = DIV_ROUND_UP(nr_cpu_ids, cap);
 	}
 
-- 
1.8.1.5

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