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

[Wei Yang <richard.weiyang@gmail.com>]

On Tue, Nov 12, 2024 at 09:46:20AM -0500, Liam R. Howlett wrote:
>* 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.
>

Thanks, this looks much better :-)

>> 
>> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
>
>Fixes: ?
>Cc: stable ?
>

Will add this.

BTW, as this is a fix, do you think the test case in patch 2 of v1 is still
necessary?

>> 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
>> 

-- 
Wei Yang
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