[ltt-dev] [PATCH 01/10 round10] introduce bucket_at() and improve readability

[mathieu.desnoyers@efficios.com (Mathieu Desnoyers)]

* Lai Jiangshan (laijs at cn.fujitsu.com) wrote:
> Fast path is not changed.
> It will slow down very little for slow path.

it cleans up the code, so it's generally fine with me. There is just one
part that I don't understand, see below,

> 
> Signed-off-by: Lai Jiangshan <laijs at cn.fujitsu.com>
> ---
>  rculfhash.c |  100 +++++++++++++++++++++++++++-------------------------------
>  1 files changed, 47 insertions(+), 53 deletions(-)
> 
> diff --git a/rculfhash.c b/rculfhash.c
> index bda3bd6..b0c7de5 100644
> --- a/rculfhash.c
> +++ b/rculfhash.c
> @@ -755,18 +755,14 @@ unsigned long _uatomic_xchg_monotonic_increase(unsigned long *ptr,
>  	return old2;
>  }
>  
> -static
> -struct cds_lfht_node *lookup_bucket(struct cds_lfht *ht, unsigned long size,
> -		unsigned long hash)
> +static inline
> +struct cds_lfht_node *bucket_at(struct cds_lfht *ht, unsigned long index)
>  {
> -	unsigned long index, order;
> -
> -	assert(size > 0);
> -	index = hash & (size - 1);
> +	unsigned long order;
>  
> -	if (index < ht->min_alloc_size) {
> -		dbg_printf("lookup hash %lu index %lu order 0 aridx 0\n",
> -			   hash, index);
> +	if ((__builtin_constant_p(index) && index == 0)
> +			|| index < ht->min_alloc_size) {
> +		dbg_printf("bucket index %lu order 0 aridx 0\n", index);
>  		return &ht->t.tbl[0]->nodes[index];
>  	}
>  	/*
> @@ -775,11 +771,19 @@ struct cds_lfht_node *lookup_bucket(struct cds_lfht *ht, unsigned long size,
>  	 * get_count_order_ulong.
>  	 */
>  	order = fls_ulong(index);
> -	dbg_printf("lookup hash %lu index %lu order %lu aridx %lu\n",
> -		   hash, index, order, index & ((1UL << (order - 1)) - 1));
> +	dbg_printf("bucket index %lu order %lu aridx %lu\n",
> +		   index, order, index & ((1UL << (order - 1)) - 1));
>  	return &ht->t.tbl[order]->nodes[index & ((1UL << (order - 1)) - 1)];
>  }
>  
> +static inline
> +struct cds_lfht_node *lookup_bucket(struct cds_lfht *ht, unsigned long size,
> +		unsigned long hash)
> +{
> +	assert(size > 0);
> +	return bucket_at(ht, hash & (size - 1));
> +}
> +
>  /*
>   * Remove all logically deleted nodes from a bucket up to a certain node key.
>   */
> @@ -1105,19 +1109,18 @@ static
>  void init_table_populate_partition(struct cds_lfht *ht, unsigned long i,
>  				   unsigned long start, unsigned long len)
>  {
> -	unsigned long j;
> +	unsigned long j, size = 1UL << (i - 1);
>  
>  	assert(i > ht->min_alloc_order);
>  	ht->cds_lfht_rcu_read_lock();
> -	for (j = start; j < start + len; j++) {
> -		struct cds_lfht_node *new_node = &ht->t.tbl[i]->nodes[j];
> -
> -		dbg_printf("init populate: i %lu j %lu hash %lu\n",
> -			   i, j, (1UL << (i - 1)) + j);
> -		new_node->reverse_hash =
> -				bit_reverse_ulong((1UL << (i - 1)) + j);
> -		_cds_lfht_add(ht, NULL, NULL, 1UL << (i - 1),
> -				new_node, NULL, 1);
> +	for (j = start + size; j < size + start + len; j++) {

Please use "j = size + start; j < size + start + len;"

> +		struct cds_lfht_node *new_node = bucket_at(ht, j);
> +
> +		assert(j >= size && j < (size << 1));
> +		dbg_printf("init populate: order %lu index %lu hash %lu\n",
> +			   i, j, j);
> +		new_node->reverse_hash = bit_reverse_ulong(j);
> +		_cds_lfht_add(ht, NULL, NULL, size, new_node, NULL, 1);
>  	}
>  	ht->cds_lfht_rcu_read_unlock();
>  }
> @@ -1205,18 +1208,18 @@ static
>  void remove_table_partition(struct cds_lfht *ht, unsigned long i,
>  			    unsigned long start, unsigned long len)
>  {
> -	unsigned long j;
> +	unsigned long j, size = 1UL << (i - 1);
>  
>  	assert(i > ht->min_alloc_order);
>  	ht->cds_lfht_rcu_read_lock();
> -	for (j = start; j < start + len; j++) {
> -		struct cds_lfht_node *fini_node = &ht->t.tbl[i]->nodes[j];
> -
> -		dbg_printf("remove entry: i %lu j %lu hash %lu\n",
> -			   i, j, (1UL << (i - 1)) + j);
> -		fini_node->reverse_hash =
> -			bit_reverse_ulong((1UL << (i - 1)) + j);
> -		(void) _cds_lfht_del(ht, 1UL << (i - 1), fini_node, 1);
> +	for (j = size + start; j < size + start + len; j++) {
> +		struct cds_lfht_node *fini_node = bucket_at(ht, j);
> +
> +		assert(j >= size && j < (size << 1));
> +		dbg_printf("remove entry: order %lu index %lu hash %lu\n",
> +			   i, j, j);
> +		fini_node->reverse_hash = bit_reverse_ulong(j);
> +		(void) _cds_lfht_del(ht, size, fini_node, 1);
>  	}
>  	ht->cds_lfht_rcu_read_unlock();
>  }
> @@ -1293,14 +1296,15 @@ static
>  void cds_lfht_create_bucket(struct cds_lfht *ht, unsigned long size)
>  {
>  	struct cds_lfht_node *prev, *node;
> -	unsigned long order, len, i, j;
> +	unsigned long order, len, i;
>  
>  	ht->t.tbl[0] = calloc(1, ht->min_alloc_size * sizeof(struct cds_lfht_node));
>  	assert(ht->t.tbl[0]);
>  
> -	dbg_printf("create bucket: order %lu index %lu hash %lu\n", 0, 0, 0);
> -	ht->t.tbl[0]->nodes[0].next = flag_bucket(get_end());
> -	ht->t.tbl[0]->nodes[0].reverse_hash = 0;
> +	dbg_printf("create bucket: order 0 index 0 hash 0\n");
> +	node = bucket_at(ht, 0);
> +	node->next = flag_bucket(get_end());
> +	node->reverse_hash = 0;
>  
>  	for (order = 1; order < get_count_order_ulong(size) + 1; order++) {
>  		len = 1UL << (order - 1);
> @@ -1311,22 +1315,15 @@ void cds_lfht_create_bucket(struct cds_lfht *ht, unsigned long size)
>  			assert(ht->t.tbl[order]);
>  		}
>  

below is the change I fail to understand. I think I'd need to be walked
through this change with an explanation, which could be added as a
comment telling what this function is doing.

> -		i = 0;
> -		prev = ht->t.tbl[i]->nodes;
> -		for (j = 0; j < len; j++) {
> -			if (j & (j - 1)) {	/* Between power of 2 */
> -				prev++;
> -			} else if (j) {		/* At each power of 2 */
> -				i++;
> -				prev = ht->t.tbl[i]->nodes;
> -			}
> +		for (i = 0; i < len; i++) {
> +			prev = bucket_at(ht, i);
> +			node = bucket_at(ht, i + len);

Using "len + i" would help us follow the code, as i changes, but not
len. I usually try to put the iterators that change the most (innermost
indexes) at the right.


>  
> -			node = &ht->t.tbl[order]->nodes[j];
>  			dbg_printf("create bucket: order %lu index %lu hash %lu\n",
> -				   order, j, j + len);
> +				   order, i + len, i + len);

Same here,

>  			node->next = prev->next;
>  			assert(is_bucket(node->next));
> -			node->reverse_hash = bit_reverse_ulong(j + len);
> +			node->reverse_hash = bit_reverse_ulong(i + len);

Same here,

Thanks!

Mathieu


>  			prev->next = flag_bucket(node);
>  		}
>  	}
> @@ -1476,14 +1473,11 @@ void cds_lfht_next(struct cds_lfht *ht, struct cds_lfht_iter *iter)
>  
>  void cds_lfht_first(struct cds_lfht *ht, struct cds_lfht_iter *iter)
>  {
> -	struct cds_lfht_node *lookup;
> -
>  	/*
>  	 * Get next after first bucket node. The first bucket node is the
>  	 * first node of the linked list.
>  	 */
> -	lookup = &ht->t.tbl[0]->nodes[0];
> -	iter->next = lookup->next;
> +	iter->next = bucket_at(ht, 0)->next;
>  	cds_lfht_next(ht, iter);
>  }
>  
> @@ -1569,7 +1563,7 @@ int cds_lfht_delete_bucket(struct cds_lfht *ht)
>  	unsigned long order, i, size;
>  
>  	/* Check that the table is empty */
> -	node = &ht->t.tbl[0]->nodes[0];
> +	node = bucket_at(ht, 0);
>  	do {
>  		node = clear_flag(node)->next;
>  		if (!is_bucket(node))
> @@ -1648,7 +1642,7 @@ void cds_lfht_count_nodes(struct cds_lfht *ht,
>  	*removed = 0;
>  
>  	/* Count non-bucket nodes in the table */
> -	node = &ht->t.tbl[0]->nodes[0];
> +	node = bucket_at(ht, 0);
>  	do {
>  		next = rcu_dereference(node->next);
>  		if (is_removed(next)) {
> -- 
> 1.7.4.4
> 

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com