Re: [PATCH 1/5] rbtree: Cache leftmost node internally

[Peter Zijlstra <peterz@infradead.org>]

On Mon, May 29, 2017 at 07:09:36PM -0700, Davidlohr Bueso wrote:
> diff --git a/include/linux/rbtree_augmented.h b/include/linux/rbtree_augmented.h
> index 9702b6e183bc..b84ee26c19d9 100644
> --- a/include/linux/rbtree_augmented.h
> +++ b/include/linux/rbtree_augmented.h
> @@ -41,7 +41,9 @@ struct rb_augment_callbacks {
>  	void (*rotate)(struct rb_node *old, struct rb_node *new);
>  };
>  
> -extern void __rb_insert_augmented(struct rb_node *node, struct rb_root *root,
> +extern void __rb_insert_augmented(struct rb_node *node,
> +				  struct rb_root *root,
> +				  bool newleft, struct rb_node **leftmost,
>  	void (*augment_rotate)(struct rb_node *old, struct rb_node *new));
>  /*
>   * Fixup the rbtree and update the augmented information when rebalancing.

> @@ -150,6 +161,7 @@ extern void __rb_erase_color(struct rb_node *parent, struct rb_root *root,
>  
>  static __always_inline struct rb_node *
>  __rb_erase_augmented(struct rb_node *node, struct rb_root *root,
> +		     bool cached, struct rb_node **leftmost,
>  		     const struct rb_augment_callbacks *augment)
>  {
>  	struct rb_node *child = node->rb_right;
> @@ -157,6 +169,9 @@ __rb_erase_augmented(struct rb_node *node, struct rb_root *root,
>  	struct rb_node *parent, *rebalance;
>  	unsigned long pc;
>  
> +	if (cached && node == *leftmost)
> +		*leftmost = rb_next(node);
> +
>  	if (!tmp) {
>  		/*
>  		 * Case 1: node to erase has no more than 1 child (easy!)

Why do we have @cached ? Wouldn't simply testing @leftmost for NULL not
do the same?