Re: [PATCH v2 1/2] maple_tree: simplify split calculation

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

* Wei Yang <richard.weiyang@gmail.com> [241109 08:45]:
> We have been too smart to calculate split value.
> 
> The purpose of current calculation is to avoid having a range less than
> the slot count. But this seems to push too hard to suffer from jitter
> problem.
> 
> Considering this only matters if the range is less than the slot count,
> so the real world implications of the calculation will be negligible. So
> we decide to simplify the calculation of split.
> 
> Also current code may lead to deficient node, the condition to check
> should be (b_end - split - 1 > slot_min). After this change, this one is
> gone together.

This comment is difficult to understand.

Maybe something like:
The current calculation for splitting nodes tries to enforce a minimum
span on the leaf nodes.  This code is complex and never worked correctly
to begin with, due to the min value being passed as 0 for all leaves.

The calculation should just split the data as equally as possible
between the new nodes.  Note that b_end will be one more than the data,
so the left side is still favoured in the calculation.

The current code may also lead to a deficient node by not leaving enough
data for the right side of the split. This issue is also addressed with
the split calculation change.

> 
> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>

Fixes: ?
Cc: stable ?

> CC: Liam R. Howlett <Liam.Howlett@Oracle.com>
> CC: Sidhartha Kumar <sidhartha.kumar@oracle.com>
> CC: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> 
> ---
> v2:
>   instead of fixing deficient split, simplify the calculation
> ---
>  lib/maple_tree.c | 23 ++++++-----------------
>  1 file changed, 6 insertions(+), 17 deletions(-)
> 
> diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> index d0ae808f3a14..4f2950a1c38d 100644
> --- a/lib/maple_tree.c
> +++ b/lib/maple_tree.c
> @@ -1863,11 +1863,11 @@ static inline int mab_no_null_split(struct maple_big_node *b_node,
>   * Return: The first split location.  The middle split is set in @mid_split.
>   */
>  static inline int mab_calc_split(struct ma_state *mas,
> -	 struct maple_big_node *bn, unsigned char *mid_split, unsigned long min)
> +	 struct maple_big_node *bn, unsigned char *mid_split)
>  {
>  	unsigned char b_end = bn->b_end;
>  	int split = b_end / 2; /* Assume equal split. */
> -	unsigned char slot_min, slot_count = mt_slots[bn->type];
> +	unsigned char slot_count = mt_slots[bn->type];
>  
>  	/*
>  	 * To support gap tracking, all NULL entries are kept together and a node cannot
> @@ -1900,18 +1900,7 @@ static inline int mab_calc_split(struct ma_state *mas,
>  		split = b_end / 3;
>  		*mid_split = split * 2;
>  	} else {
> -		slot_min = mt_min_slots[bn->type];
> -
>  		*mid_split = 0;
> -		/*
> -		 * Avoid having a range less than the slot count unless it
> -		 * causes one node to be deficient.
> -		 * NOTE: mt_min_slots is 1 based, b_end and split are zero.
> -		 */
> -		while ((split < slot_count - 1) &&
> -		       ((bn->pivot[split] - min) < slot_count - 1) &&
> -		       (b_end - split > slot_min))
> -			split++;
>  	}
>  
>  	/* Avoid ending a node on a NULL entry */
> @@ -2377,7 +2366,7 @@ static inline struct maple_enode
>  static inline unsigned char mas_mab_to_node(struct ma_state *mas,
>  	struct maple_big_node *b_node, struct maple_enode **left,
>  	struct maple_enode **right, struct maple_enode **middle,
> -	unsigned char *mid_split, unsigned long min)
> +	unsigned char *mid_split)
>  {
>  	unsigned char split = 0;
>  	unsigned char slot_count = mt_slots[b_node->type];
> @@ -2390,7 +2379,7 @@ static inline unsigned char mas_mab_to_node(struct ma_state *mas,
>  	if (b_node->b_end < slot_count) {
>  		split = b_node->b_end;
>  	} else {
> -		split = mab_calc_split(mas, b_node, mid_split, min);
> +		split = mab_calc_split(mas, b_node, mid_split);
>  		*right = mas_new_ma_node(mas, b_node);
>  	}
>  
> @@ -2877,7 +2866,7 @@ static void mas_spanning_rebalance(struct ma_state *mas,
>  		mast->bn->b_end--;
>  		mast->bn->type = mte_node_type(mast->orig_l->node);
>  		split = mas_mab_to_node(mas, mast->bn, &left, &right, &middle,
> -					&mid_split, mast->orig_l->min);
> +					&mid_split);
>  		mast_set_split_parents(mast, left, middle, right, split,
>  				       mid_split);
>  		mast_cp_to_nodes(mast, left, middle, right, split, mid_split);
> @@ -3365,7 +3354,7 @@ static void mas_split(struct ma_state *mas, struct maple_big_node *b_node)
>  		if (mas_push_data(mas, height, &mast, false))
>  			break;
>  
> -		split = mab_calc_split(mas, b_node, &mid_split, prev_l_mas.min);
> +		split = mab_calc_split(mas, b_node, &mid_split);
>  		mast_split_data(&mast, mas, split);
>  		/*
>  		 * Usually correct, mab_mas_cp in the above call overwrites
> -- 
> 2.34.1
>