Re: [PATCH v5 1/3] lib/plist: Provide plist_add_head() for nodes with the same prio

[Steven Rostedt]

On Thu,  9 Apr 2015 11:27:16 +0800
Xunlei Pang <xlpang@xxxxxxx> wrote:

> From: Xunlei Pang <pang.xunlei@xxxxxxxxxx>
> 
> If there're multiple nodes with the same prio as @node, currently
> plist_add() will add @node behind all of them. Now we need to add
> @node before all of these nodes for SMP RT scheduler.
> 
> This patch adds a common __plist_add() for adding @node before or
> after existing nodes with the same prio, then adds plist_add_head()
> and plist_add_tail() inline wrapper functions for convenient uses.
> 
> Finally, define plist_add() as plist_add_tail() which has the same
> behaviour as before.
> 
> Reviewed-by: Dan Streetman <ddstreet@xxxxxxxx>
> Signed-off-by: Xunlei Pang <pang.xunlei@xxxxxxxxxx>
> ---
>  include/linux/plist.h | 30 +++++++++++++++++++++++++++++-
>  lib/plist.c           | 22 +++++++++++++++++++---
>  2 files changed, 48 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/plist.h b/include/linux/plist.h
> index 9788360..10c834c 100644
> --- a/include/linux/plist.h
> +++ b/include/linux/plist.h
> @@ -138,7 +138,35 @@ static inline void plist_node_init(struct plist_node *node, int prio)
>  	INIT_LIST_HEAD(&node->node_list);
>  }
>  
> -extern void plist_add(struct plist_node *node, struct plist_head *head);
> +extern void __plist_add(struct plist_node *node,
> +				struct plist_head *head, bool is_head);
> +
> +/**
> + * plist_add_head - add @node to @head, before all existing same-prio nodes
> + *
> + * @node:	&struct plist_node pointer
> + * @head:	&struct plist_head pointer

I just noticed that the plist kerneldoc is really pathetic. Not your
fault, but lets not continue the trend. The above descriptions of the
@node and @head are really lame. It adds nothing to the prototype. It's
equivalent to:

	/* increment i */
	i++;

Something like this is better:

 * @node:  The plist_node to be added to @head
 * @head:  The plist_head that @node is being added to

The oneline description you have above is good. Ideally, we should have
a bit more detailed description here too. But for now we can leave it.
The rest of the @node and @head's should be fixed too.


> + */
> +static inline
> +void plist_add_head(struct plist_node *node, struct plist_head *head)
> +{
> +	__plist_add(node, head, true);
> +}
> +
> +/**
> + * plist_add_tail - add @node to @head, after all existing same-prio nodes
> + *
> + * @node:	&struct plist_node pointer
> + * @head:	&struct plist_head pointer
> + */
> +static inline
> +void plist_add_tail(struct plist_node *node, struct plist_head *head)
> +{
> +	__plist_add(node, head, false);
> +}
> +
> +#define plist_add plist_add_tail
> +
>  extern void plist_del(struct plist_node *node, struct plist_head *head);
>  
>  extern void plist_requeue(struct plist_node *node, struct plist_head *head);
> diff --git a/lib/plist.c b/lib/plist.c
> index 3a30c53..9f396f1 100644
> --- a/lib/plist.c
> +++ b/lib/plist.c
> @@ -66,12 +66,16 @@ static void plist_check_head(struct plist_head *head)
>  #endif
>  
>  /**
> - * plist_add - add @node to @head
> + * __plist_add - add @node to @head
>   *
>   * @node:	&struct plist_node pointer
>   * @head:	&struct plist_head pointer
> + * @is_head:	bool

As I said above. This horrible kerneldoc was there before and is not
your fault. But if you are going to touch it, lets fix it up first.

 * @is_head: True if adding to head of prio list, false otherwise


> + *
> + * If there're any nodes with the same prio, add @node
> + * behind or before all of them according to @is_head.

The above is very lacking if I want to use this, or know how to use it.
It does not tell me what @is_head must be to add before or behind them.

 * For nodes of the same prio, @node will be added at the head of
 * previously added nodes if @is_head is true, or it will be added
 * at the tail of previously added nodes if @is_head is false.

That will be much more descriptive for people looking at this code for
the first time.

The rest of the code looks fine.

-- Steve

>   */
> -void plist_add(struct plist_node *node, struct plist_head *head)
> +void __plist_add(struct plist_node *node, struct plist_head *head, bool is_head)
>  {
>  	struct plist_node *first, *iter, *prev = NULL;
>  	struct list_head *node_next = &head->node_list;
> @@ -96,8 +100,20 @@ void plist_add(struct plist_node *node, struct plist_head *head)
>  				struct plist_node, prio_list);
>  	} while (iter != first);
>  
> -	if (!prev || prev->prio != node->prio)
> +	if (!prev || prev->prio != node->prio) {
>  		list_add_tail(&node->prio_list, &iter->prio_list);
> +	} else if (is_head) {
> +		/*
> +		 * prev has the same priority as the node that is being
> +		 * added. It is also the first node for this priority,
> +		 * but the new node needs to be added ahead of it.
> +		 * To accomplish this, replace prev in the prio_list
> +		 * with node. Then set node_next to prev->node_list so
> +		 * that the new node gets added before prev and not iter.
> +		 */
> +		list_replace_init(&prev->prio_list, &node->prio_list);
> +		node_next = &prev->node_list;
> +	}
>  ins_node:
>  	list_add_tail(&node->node_list, node_next);
>  

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