Re: [PATCH v2 1/3] radix-tree: introduce bit-optimized iterator

On Fri, 10 Feb 2012 23:25:42 +0400
Konstantin Khlebnikov [off-list ref] wrote:

This patch implements clean, simple and effective radix-tree iteration routine.

Iterating divided into two phases:
* lookup next chunk in radix-tree leaf node
* iterating through slots in this chunk

Main iterator function radix_tree_next_chunk() returns pointer to first slot,
and stores in the struct radix_tree_iter index of next-to-last slot.
For tagged-iterating it also constuct bitmask of tags for retunted chunk.
All additional logic implemented as static-inline functions and macroses.

Also patch adds radix_tree_find_next_bit() static-inline variant of
find_next_bit() optimized for small constant size arrays, because
find_next_bit() too heavy for searching in an array with one/two long elements.

...

+
+static inline
+void **radix_tree_iter_init(struct radix_tree_iter *iter, unsigned long start)

Nit: if we're going to line break a function definition/declaration
line like this then the usual way is to split it before the function
name, so

static inline void **
radix_tree_iter_init(struct radix_tree_iter *iter, unsigned long start)

Old-school people did this so they could find the function with
/^radix_tree_iter_init in vi ;)

+{
+	iter->index = 0; /* to bypass next_index overflow protection */
+	iter->next_index = start;
+	return NULL;
+}

Why didn't it initialize .tags?

In fact .tags only ever gets initialized deep inside
radix_tree_next_chunk(), if !radix_tree_is_indirect_ptr().  Is this
correct?

...

+/**
+ * radix_tree_next_slot - find next slot in chunk
+ *
+ * @slot	pointer to slot
+ * @iter	iterator state
+ * @flags	RADIX_TREE_ITER_*
+ *
+ * Returns pointer to next slot, or NULL if no more left.
+ */
+static __always_inline
+void **radix_tree_next_slot(void **slot, struct radix_tree_iter *iter,
+			    unsigned flags)
+{
+	unsigned size, offset;

'offset' could be made local to the single code block which uses it. 
personally I find that this leads to clearer code.

+	size = radix_tree_chunk_size(iter) - 1;

radix_tree_chunk_size() returns unsigned long, and we just threw away
the upper 32 bits.  I'm unsure if that's a bug, but it's messy and
possibly inefficient.

+	if (flags & RADIX_TREE_ITER_TAGGED) {
+		iter->tags >>= 1;
+		if (likely(iter->tags & 1ul)) {
+			iter->index++;
+			return slot + 1;
+		}
+		if ((flags & RADIX_TREE_ITER_CONTIG) && size)
+			return NULL;
+		if (likely(iter->tags)) {
+			offset = __ffs(iter->tags);
+			iter->tags >>= offset;
+			iter->index += offset + 1;
+			return slot + offset + 1;
+		}
+	} else {
+		while (size--) {
+			slot++;
+			iter->index++;
+			if (likely(*slot))
+				return slot;
+			if (flags & RADIX_TREE_ITER_CONTIG)
+				return NULL;
+		}
+	}
+	return NULL;
+}

This is a whopping big function.  Why was it inlined?  Are you sure
that was a correct decision?

+/**
+ * radix_tree_for_each_chunk - iterate over chunks
+ *
+ * @slot:	the void** for pointer to chunk first slot
+ * @root	the struct radix_tree_root pointer
+ * @iter	the struct radix_tree_iter pointer
+ * @start	starting index
+ * @flags	RADIX_TREE_ITER_* and tag index

Some of the arguments have a colon, others don't.

+ * Locks can be released and reasquired between iterations.

"reacquired"

+ */
+#define radix_tree_for_each_chunk(slot, root, iter, start, flags)	\
+	for ( slot = radix_tree_iter_init(iter, start) ;		\
+	      (slot = radix_tree_next_chunk(root, iter, flags)) ; )

I don't think I understand this whole interface :(

The term "chunk" has not been defined anywhere in the code, which
doesn't help.

Neither radix_tree_for_each_chunk() nor
radix_tree_for_each_chunk_slot() get used anywhere in this patchset so
one can't go look at call sites to work out what they're for.

It's a strange iterator - it never terminates.  It requires that the
caller have an open-coded `break' in the search loop.

A bit more description and perhaps a usage example would help.

+/**
+ * radix_tree_for_each_chunk_slot - iterate over slots in one chunk
+ *
+ * @slot:	the void** for pointer to slot
+ * @iter	the struct radix_tree_iter pointer
+ * @flags	RADIX_TREE_ITER_*
+ */
+#define radix_tree_for_each_chunk_slot(slot, iter, flags)	\
+	for ( ; slot ; slot = radix_tree_next_slot(slot, iter, flags) )

Similar observations here.

+/**
+ * radix_tree_for_each_slot - iterate over all slots
+ *
+ * @slot:	the void** for pointer to slot
+ * @root	the struct radix_tree_root pointer
+ * @iter	the struct radix_tree_iter pointer
+ * @start	starting index
+ */
+#define radix_tree_for_each_slot(slot, root, iter, start)	\
+	for ( slot = radix_tree_iter_init(iter, start) ;	\
+	      slot || (slot = radix_tree_next_chunk(root, iter, 0)) ; \
+	      slot = radix_tree_next_slot(slot, iter, 0) )

All of these macros reference some of their arguments more than once. 
So wierd and wrong things will happen if they are invoked with an
expression-with-side-effects.  Also they lack parenthesisation, so

	radix_tree_for_each_slot(myslot + 1, ...)

won't compile.  The first problem is more serious than the second.

This is always a pain with complex macros and fixing it here would
deeply uglify the code.  It's unlikely that anyone will be invoking
these with expression-with-side-effects so I'd be inclined to just live
with the dangers.

otoh, someone *might* do

	radix_tree_for_each_slot(slot,
				 expensive_function_which_returns_a_root(),
				 iter, start);

and we'd call expensive_function_which_returns_a_root() each time
around the loop.  But I don't think this is fixable.

Anyway, have a think about it all.

+/**
+ * radix_tree_for_each_contig - iterate over all contiguous slots

Now what does this mean?  Given a slot, iterate over that slot and all
contiguous successor slots until we encounter a hole?

Maybe.  Again, better interface descriptions are needed, please.

+ * @slot:	the void** for pointer to slot
+ * @root	the struct radix_tree_root pointer
+ * @iter	the struct radix_tree_iter pointer
+ * @start	starting index
+ */
+#define radix_tree_for_each_contig(slot, root, iter, start)		\
+	for ( slot = radix_tree_iter_init(iter, start) ;		\
+	      slot || (slot = radix_tree_next_chunk(root, iter,		\
+				RADIX_TREE_ITER_CONTIG)) ;		\
+	      slot = radix_tree_next_slot(slot, iter,			\
+				RADIX_TREE_ITER_CONTIG) )
+
+/**
+ * radix_tree_for_each_tagged - iterate over all tagged slots
+ *
+ * @slot:	the void** for pointer to slot
+ * @root	the struct radix_tree_root pointer
+ * @iter	the struct radix_tree_iter pointer
+ * @start	starting index
+ * @tag		tag index
+ */
+#define radix_tree_for_each_tagged(slot, root, iter, start, tag)	\
+	for ( slot = radix_tree_iter_init(iter, start) ;		\
+	      slot || (slot = radix_tree_next_chunk(root, iter,		\
+			      RADIX_TREE_ITER_TAGGED | tag)) ;		\
+	      slot = radix_tree_next_slot(slot, iter,			\
+				RADIX_TREE_ITER_TAGGED) )
+
 #endif /* _LINUX_RADIX_TREE_H */

...

+static inline unsigned long radix_tree_find_next_bit(const unsigned long *addr,
+		unsigned long size, unsigned long offset)
+{
+	if (!__builtin_constant_p(size))
+		return find_next_bit(addr, size, offset);
+
+	if (offset < size) {
+		unsigned long tmp;
+
+		addr += offset / BITS_PER_LONG;
+		tmp = *addr >> (offset % BITS_PER_LONG);
+		if (tmp)
+			return __ffs(tmp) + offset;
+		offset = (offset + BITS_PER_LONG) & ~(BITS_PER_LONG - 1);
+		while (offset < size) {
+			tmp = *++addr;
+			if (tmp)
+				return __ffs(tmp) + offset;
+			offset += BITS_PER_LONG;
+		}
+	}
+	return size;
+}

Beware that gcc will freely ignore your "inline" directive.

When I compiled it, gcc did appear to inline it.  Then I added
__always_inline and it was still inlined, but the text section in the
.o file got 20 bytes larger.  Odd.

When a c programmer sees a variable called "i", he solidly expects it
to have type "int".  Please choose a better name for this guy! 
Perferably something which helps the reader understand what the
variable's role is.

+	if ((flags & RADIX_TREE_ITER_TAGGED) && !root_tag_get(root, tag))
+		return NULL;
+
+	/*
+	 * Catch next_index overflow after ~0UL.
+	 * iter->index can be zero only at the beginning.
+	 * Because RADIX_TREE_MAP_SHIFT < BITS_PER_LONG we cannot
+	 * oveflow iter->next_index in single step.
+	 */
+	index = iter->next_index;
+	if (!index && iter->index)
+		return NULL;
+
+	rnode = rcu_dereference_raw(root->rnode);
+	if (radix_tree_is_indirect_ptr(rnode)) {
+		rnode = indirect_to_ptr(rnode);
+	} else if (rnode && !index) {
+		/* Single-slot tree */
+		iter->index = 0;
+		iter->next_index = 1;
+		iter->tags = 1;
+		return (void **)&root->rnode;
+	} else
+		return NULL;
+
+restart:
+	shift = (rnode->height - 1) * RADIX_TREE_MAP_SHIFT;
+	i = index >> shift;
+
+	/* Index ouside of the tree */
+	if (i >= RADIX_TREE_MAP_SIZE)
+		return NULL;
+
+	node = rnode;
+	while (1) {
+		if ((flags & RADIX_TREE_ITER_TAGGED) ?
+				!test_bit(i, node->tags[tag]) :
+				!node->slots[i]) {
+			/* Hole detected */
+			if (flags & RADIX_TREE_ITER_CONTIG)
+				return NULL;
+
+			if (flags & RADIX_TREE_ITER_TAGGED)
+				i = radix_tree_find_next_bit(node->tags[tag],
+						RADIX_TREE_MAP_SIZE, i + 1);
+			else
+				while (++i < RADIX_TREE_MAP_SIZE &&
+						!node->slots[i]);
+
+			index &= ~((RADIX_TREE_MAP_SIZE << shift) - 1);
+			index += i << shift;
+			/* Overflow after ~0UL */
+			if (!index)
+				return NULL;
+			if (i == RADIX_TREE_MAP_SIZE)
+				goto restart;
+		}
+
+		/* This is leaf-node */
+		if (!shift)
+			break;
+
+		node = rcu_dereference_raw(node->slots[i]);
+		if (node == NULL)
+			goto restart;
+		shift -= RADIX_TREE_MAP_SHIFT;
+		i = (index >> shift) & RADIX_TREE_MAP_MASK;
+	}
+
+	/* Update the iterator state */
+	iter->index = index;
+	iter->next_index = (index | RADIX_TREE_MAP_MASK) + 1;
+
+	/* Construct iter->tags bitmask from node->tags[tag] array */
+	if (flags & RADIX_TREE_ITER_TAGGED) {
+		unsigned tag_long, tag_bit;
+
+		tag_long = i / BITS_PER_LONG;
+		tag_bit  = i % BITS_PER_LONG;
+		iter->tags = node->tags[tag][tag_long] >> tag_bit;
+		/* This never happens if RADIX_TREE_TAG_LONGS == 1 */
+		if (tag_long < RADIX_TREE_TAG_LONGS - 1) {
+			/* Pick tags from next element */
+			if (tag_bit)
+				iter->tags |= node->tags[tag][tag_long + 1] <<
+						(BITS_PER_LONG - tag_bit);
+			/* Clip chunk size, here only BITS_PER_LONG tags */
+			iter->next_index = index + BITS_PER_LONG;
+		}
+	}
+
+	return node->slots + i;
+}
+EXPORT_SYMBOL(radix_tree_next_chunk);
+
+/**
  * radix_tree_range_tag_if_tagged - for each item in given range set given
  *				   tag if item has another tag set
  * @root:		radix tree root

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