Re: [PATCH 3/9] rcu: Cleanup rcu_init_geometry() code and arithmetics

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Sat, Mar 07, 2015 at 06:03:38PM +0100, 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[].
> 
> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> Signed-off-by: Alexander Gordeev <agordeev@redhat.com>

The rest of this series looks promising, but I do have to ask...  How have
you tested these?  The most straightforward approach would be to find
a KVM-capable system with at least 16 CPUs and type the following from
the top-level directory:

sh tools/testing/selftests/rcutorture/bin/kvm.sh --cpus 34 --duration 5

This will do a series of 16 build-boot-test cycles with various configs
(including various rcu_node tree shapes), and print a summary of the
outcome at the end.

For these sorts of changes, I usually also do some user-level testing.

							Thanx, Paul

> ---
>  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 fb89630..f9ef1e0 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3854,7 +3854,6 @@ static void __init rcu_init_geometry(void)
>  {
>  	ulong d;
>  	int i;
> -	int j;
>  	int rcu_capacity[MAX_RCU_LVLS + 1];
> 
>  	/*
> @@ -3905,24 +3904,21 @@ static void __init rcu_init_geometry(void)
>  		return;
>  	}
> 
> +	/* Calculate the number of levels in the tree. */
> +	for (i = 0; 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
>