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Message-ID: <4F618347.8080400@openvz.org>
Date: Thu, 15 Mar 2012 09:51:03 +0400
From: Konstantin Khlebnikov <khlebnikov@...nvz.org>
To: Andrew Morton <akpm@...ux-foundation.org>
CC: Hugh Dickins <hughd@...gle.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH v2 1/3] radix-tree: introduce bit-optimized iterator
Andrew Morton wrote:
> On Fri, 10 Feb 2012 23:25:42 +0400
> Konstantin Khlebnikov<khlebnikov@...nvz.org> 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 ;)
Thanks for watching! I'll try to fix all style problems.
>
>> +{
>> + 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?
Yes, it correct.
.tags can be used only after success radix_tree_next_chunk() calling with RADIX_TREE_ITER_TAGGED
It initialized in two places: one for trivial single-entry tree and one at the end for normal chunk.
>
>>
>> ...
>>
>> +/**
>> + * 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.
Would it be better to convert all to unsigned long?
As I remember, I was played there a lot trying to shrink code size for x86_64.
This function is really hot, so it should be carefully optimized.
>
>> + 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?
Ok, I'll split it in two: for cases with/without RADIX_TREE_ITER_TAGGED,
and split radix_tree_for_each_chunk_slot() macro in two: tagged and non-tagged.
>
>> +/**
>> + * 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.
Ok, description must be fixed.
"chunk" is array of slot-pointers from radix tree leaf node.
radix_tree_for_each_chunk() iterates through radix-tree with with long steps.
Iterator body can work with chunk as with array: slot[0..radix_tree_chunk_size(iter)-1]
or iterate through it with help radix_tree_for_each_chunk_slot()
>
> 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 was used it in "[PATCH 3/4] shmem: use radix-tree iterator in shmem_unuse_inode()" from
"[PATCH 0/4] shmem: radix-tree cleanups and swapoff optimizations"
>
> It's a strange iterator - it never terminates. It requires that the
> caller have an open-coded `break' in the search loop.
It terminates if radix_tree_next_chunk() returns NULL.
>
> 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, ...)
slot must be lvalue, like "pos" in list_for_each_entry()
Description not explains arguments' roles...
>
> 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.
list_for_each_entry() do the same for "head" argument, I think this is ok.
>
>> +/**
>> + * 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?
It start from "start" index and iterate till first empty slot.
>
> 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.
>
>> /*
>> * This assumes that the caller has performed appropriate preallocation, and
>> * that the caller has pinned this thread of control to the current CPU.
>> @@ -613,6 +649,117 @@ int radix_tree_tag_get(struct radix_tree_root *root,
>> EXPORT_SYMBOL(radix_tree_tag_get);
>>
>> /**
>> + * radix_tree_next_chunk - find next chunk of slots for iteration
>> + *
>> + * @root: radix tree root
>> + * @iter: iterator state
>> + * @flags RADIX_TREE_ITER_* flags and tag index
>> + *
>> + * Returns pointer to first slots in chunk, or NULL if there no more left
>> + */
>> +void **radix_tree_next_chunk(struct radix_tree_root *root,
>> + struct radix_tree_iter *iter, unsigned flags)
>> +{
>> + unsigned shift, tag = flags& RADIX_TREE_ITER_TAG_MASK;
>> + struct radix_tree_node *rnode, *node;
>> + unsigned long i, index;
>
> 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.
=) Ok, I can make it "int"
>
>> + 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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