2015-06-03 06:19:51

by Alexander Gordeev

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

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" <[email protected]>
Signed-off-by: Alexander Gordeev <[email protected]>
---
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

2015-06-03 06:23:27

by Alexander Gordeev

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

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" <[email protected]>
> Signed-off-by: Alexander Gordeev <[email protected]>
> ---
> 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
[email protected]