[PATCH 1/2] rcu: Cleanup rcu_init_geometry() code and arithmetics

[PATCH 1/2] rcu: Cleanup rcu_init_geometry() code and arithmetics

[Alexander Gordeev]

This update simplifies rcu_init_geometry() code flow
and makes calculation of the total number of rcu_node
structures more easy to read.

The update relies on the fact num_rcu_lvl[] is never
accessed beyond rcu_num_lvls index by the rest of the
code. Therefore, there is no need initialize the whole
num_rcu_lvl[].

Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
---
 kernel/rcu/tree.c | 24 ++++++++++--------------
 1 file changed, 10 insertions(+), 14 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index d77b0fb..0e8c24d 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4038,7 +4038,6 @@ static void __init rcu_init_geometry(void)
 {
 	ulong d;
 	int i;
-	int j;
 	int rcu_capacity[MAX_RCU_LVLS + 1];
 
 	/*
@@ -4089,24 +4088,21 @@ static void __init rcu_init_geometry(void)
 		return;
 	}
 
+	/* Calculate the number of levels in the tree. */
+	for (i = 1; nr_cpu_ids > rcu_capacity[i]; i++) {
+	}
+	rcu_num_lvls = i;
+
 	/* Calculate the number of rcu_nodes at each level of the tree. */
-	for (i = 1; i <= MAX_RCU_LVLS; i++)
-		if (nr_cpu_ids <= rcu_capacity[i]) {
-			for (j = 0; j <= i; j++) {
-				int cap = rcu_capacity[i - j];
-				num_rcu_lvl[j] = DIV_ROUND_UP(nr_cpu_ids, cap);
-			}
-			rcu_num_lvls = i;
-			for (j = i + 1; j <= MAX_RCU_LVLS; j++)
-				num_rcu_lvl[j] = 0;
-			break;
-		}
+	for (i = 0; i < rcu_num_lvls; i++) {
+		int cap = rcu_capacity[rcu_num_lvls - i];
+		num_rcu_lvl[i] = DIV_ROUND_UP(nr_cpu_ids, cap);
+	}
 
 	/* Calculate the total number of rcu_node structures. */
 	rcu_num_nodes = 0;
-	for (i = 0; i <= MAX_RCU_LVLS; i++)
+	for (i = 0; i < rcu_num_lvls; i++)
 		rcu_num_nodes += num_rcu_lvl[i];
-	rcu_num_nodes -= nr_cpu_ids;
 }
 
 void __init rcu_init(void)
-- 
1.8.3.1

Re: [PATCH 1/2] rcu: Cleanup rcu_init_geometry() code and arithmetics

[Alexander Gordeev]

On Wed, Jun 03, 2015 at 08:18:32AM +0200, Alexander Gordeev wrote:
> This update simplifies rcu_init_geometry() code flow
> and makes calculation of the total number of rcu_node
> structures more easy to read.
> 
> The update relies on the fact num_rcu_lvl[] is never
> accessed beyond rcu_num_lvls index by the rest of the
> code. Therefore, there is no need initialize the whole
> num_rcu_lvl[].

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 | 24 ++++++++++--------------
>  1 file changed, 10 insertions(+), 14 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index d77b0fb..0e8c24d 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -4038,7 +4038,6 @@ static void __init rcu_init_geometry(void)
>  {
>  	ulong d;
>  	int i;
> -	int j;
>  	int rcu_capacity[MAX_RCU_LVLS + 1];
>  
>  	/*
> @@ -4089,24 +4088,21 @@ static void __init rcu_init_geometry(void)
>  		return;
>  	}
>  
> +	/* Calculate the number of levels in the tree. */
> +	for (i = 1; nr_cpu_ids > rcu_capacity[i]; i++) {
> +	}
> +	rcu_num_lvls = i;
> +
>  	/* Calculate the number of rcu_nodes at each level of the tree. */
> -	for (i = 1; i <= MAX_RCU_LVLS; i++)
> -		if (nr_cpu_ids <= rcu_capacity[i]) {
> -			for (j = 0; j <= i; j++) {
> -				int cap = rcu_capacity[i - j];
> -				num_rcu_lvl[j] = DIV_ROUND_UP(nr_cpu_ids, cap);
> -			}
> -			rcu_num_lvls = i;
> -			for (j = i + 1; j <= MAX_RCU_LVLS; j++)
> -				num_rcu_lvl[j] = 0;
> -			break;
> -		}
> +	for (i = 0; i < rcu_num_lvls; i++) {
> +		int cap = rcu_capacity[rcu_num_lvls - i];
> +		num_rcu_lvl[i] = DIV_ROUND_UP(nr_cpu_ids, cap);
> +	}
>  
>  	/* Calculate the total number of rcu_node structures. */
>  	rcu_num_nodes = 0;
> -	for (i = 0; i <= MAX_RCU_LVLS; i++)
> +	for (i = 0; i < rcu_num_lvls; i++)
>  		rcu_num_nodes += num_rcu_lvl[i];
> -	rcu_num_nodes -= nr_cpu_ids;
>  }
>  
>  void __init rcu_init(void)
> -- 
> 1.8.3.1
> 

-- 
Regards,
Alexander Gordeev
agordeev@redhat.com