radix_tree_next_chunk: redundant search for next slot in hole

Probir Roy proy.cse at gmail.com
Fri May 3 20:00:26 EDT 2019

> > While searching for next slot in a hole, it walks through the same
> > slots over n over.
> How did you determine this?

I am working on a tool that identifies repeated load of an address.
Often these repeated loads are redundant and can be avoided with data
structure modification. The tool points me to this line.

> Looks to me like the ++offset will walk through each potential slot once,
> and break out if it finds one.

This function is being called by the radix_tree_for_each_slot
iterator, defined as follows:

#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))

Here is the calling context I get:
|_ depth: 1 :0, method: ext4_block_write_begin+0x335/0x4f0(),
  |_ depth: 2 :0, method: alloc_buffer_head+0x21/0x60(),
   |_ depth: 3 :0, method: ext4_da_get_block_prep+0x1a6/0x490(),
    |_ depth: 4 :0, method: clean_bdev_aliases+0x9a/0x210(),
     |_ depth: 5 :0, method: pagevec_lookup_range+0x24/0x30(),
      |_ depth: 6 :0, method: find_get_pages_range+0x151/0x2d0(),
       |_ depth: 7 :0, method: radix_tree_next_chunk+0x10f/0x360()

Does it explain the case?

> I haven't looked at the code closely, perhaps what you're seeing is repeated
> scan/merge/rescan behavior?  Often, compacting a data structure requires
> multiple passes.

If that's the case, can this repeated search for the next valid slot
be avoided by caching/storing the next valid slot?

Probir Roy

More information about the Kernelnewbies mailing list