[PATCH 2/2] rcu: Simplify rcu_init_geometry() capacity arithmetics

[PATCH 2/2] rcu: Simplify rcu_init_geometry() capacity arithmetics

[Alexander Gordeev]

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@linux.vnet.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
---
 kernel/rcu/tree.c | 16 +++++++---------
 1 file changed, 7 insertions(+), 9 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 0e8c24d..e6ddd20 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4038,7 +4038,7 @@ 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.
@@ -4062,12 +4062,10 @@ 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] * CONFIG_RCU_FANOUT;
 
 	/*
@@ -4080,7 +4078,7 @@ static void __init rcu_init_geometry(void)
 	 * the rcu_node masks.  Complain and fall back to the compile-
 	 * time values if these limits are exceeded.
 	 */
-	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");
 	else if (rcu_fanout_leaf < CONFIG_RCU_FANOUT_LEAF ||
 		 rcu_fanout_leaf > sizeof(unsigned long) * 8) {
@@ -4091,11 +4089,11 @@ static void __init rcu_init_geometry(void)
 	/* Calculate the number of levels in the tree. */
 	for (i = 1; 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);
 	}
 
-- 
1.8.3.1

Re: [PATCH 2/2] rcu: Simplify rcu_init_geometry() capacity arithmetics

[Alexander Gordeev]

On Wed, Jun 03, 2015 at 08:18:33AM +0200, Alexander Gordeev wrote:
> 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.

Please, ignore this one.

> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
> ---
>  kernel/rcu/tree.c | 16 +++++++---------
>  1 file changed, 7 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 0e8c24d..e6ddd20 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -4038,7 +4038,7 @@ 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.
> @@ -4062,12 +4062,10 @@ 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] * CONFIG_RCU_FANOUT;
>  
>  	/*
> @@ -4080,7 +4078,7 @@ static void __init rcu_init_geometry(void)
>  	 * the rcu_node masks.  Complain and fall back to the compile-
>  	 * time values if these limits are exceeded.
>  	 */
> -	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");
>  	else if (rcu_fanout_leaf < CONFIG_RCU_FANOUT_LEAF ||
>  		 rcu_fanout_leaf > sizeof(unsigned long) * 8) {
> @@ -4091,11 +4089,11 @@ static void __init rcu_init_geometry(void)
>  	/* Calculate the number of levels in the tree. */
>  	for (i = 1; 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);
>  	}
>  
> -- 
> 1.8.3.1
> 

-- 
Regards,
Alexander Gordeev
agordeev@redhat.com