Re: [patch 10/15] sched: allow for positional tg_tree walks

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Tue, 2011-05-03 at 02:28 -0700, Paul Turner wrote:
> plain text document attachment (sched-bwc-refactor-walk_tg_tree.patch)
> Extend walk_tg_tree to accept a positional argument
> 
> static int walk_tg_tree_from(struct task_group *from,
> 			     tg_visitor down, tg_visitor up, void *data)
> 
> Existing semantics are preserved, caller must hold rcu_lock() or sufficient
> analogue.
> 
> Signed-off-by: Paul Turner <pjt@google.com>
> ---
>  kernel/sched.c |   34 +++++++++++++++++++++++-----------
>  1 file changed, 23 insertions(+), 11 deletions(-)
> 
> Index: tip/kernel/sched.c
> ===================================================================
> --- tip.orig/kernel/sched.c
> +++ tip/kernel/sched.c
> @@ -1430,21 +1430,19 @@ static inline void dec_cpu_load(struct r
>  #if (defined(CONFIG_SMP) && defined(CONFIG_FAIR_GROUP_SCHED)) || defined(CONFIG_RT_GROUP_SCHED)
>  typedef int (*tg_visitor)(struct task_group *, void *);
>  
> -/*
> - * Iterate the full tree, calling @down when first entering a node and @up when
> - * leaving it for the final time.
> - */
> -static int walk_tg_tree(tg_visitor down, tg_visitor up, void *data)
> +/* Iterate task_group tree rooted at *from */
> +static int walk_tg_tree_from(struct task_group *from,
> +			     tg_visitor down, tg_visitor up, void *data)
>  {
>  	struct task_group *parent, *child;
>  	int ret;
>  
> -	rcu_read_lock();
> -	parent = &root_task_group;
> +	parent = from;
> +
>  down:
>  	ret = (*down)(parent, data);
>  	if (ret)
> -		goto out_unlock;
> +		goto out;
>  	list_for_each_entry_rcu(child, &parent->children, siblings) {
>  		parent = child;
>  		goto down;
> @@ -1453,14 +1451,28 @@ up:
>  		continue;
>  	}
>  	ret = (*up)(parent, data);
> -	if (ret)
> -		goto out_unlock;
> +	if (ret || parent == from)
> +		goto out;
>  
>  	child = parent;
>  	parent = parent->parent;
>  	if (parent)
>  		goto up;
> -out_unlock:
> +out:
> +	return ret;
> +}
> +
> +/*
> + * Iterate the full tree, calling @down when first entering a node and @up when
> + * leaving it for the final time.
> + */
> +
> +static inline int walk_tg_tree(tg_visitor down, tg_visitor up, void *data)
> +{
> +	int ret;
> +
> +	rcu_read_lock();
> +	ret = walk_tg_tree_from(&root_task_group, down, up, data);
>  	rcu_read_unlock();
>  
>  	return ret;

I don't much like the different locking rules for these two functions. I
don't much care which you pick, but please make them consistent.