Re: radix_tree_next_chunk: redundant search for next slot in hole

[Valdis Klētnieks]

On Fri, 03 May 2019 19:00:26 -0500, Probir Roy said:
> > > 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.

Is this doing static analysis, or actually doing run-time tracing?

> > 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?

Actually, that calling context doesn't tell us much of anything till depth 7.

Yes, next_chunk() and next_slot() can get called repeatedly, especially if it's
a large radix tree. The important question is: Is it being called with the
*same value* of 'slot' repeatedly? Looking at the code, it's pretty obvious
that 'slot' will be updated at least once through every pass through the
for_each_slot(), unless the radix tree is corrupted.

If you're trying to do static analysis, your code may be confused by either the
'slot || next_chunk()' iterator, or the fact that 'slot' is assigned both in 
the for loops
iterator and in the body of the loop, and thus failing to detect that slot is 
updated.




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Re: radix_tree_next_chunk: redundant search for next slot in hole

[Probir Roy]

> > 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

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Re: radix_tree_next_chunk: redundant search for next slot in hole

[Valdis Klētnieks]

On Fri, 03 May 2019 16:03:46 -0500, Probir Roy said:

> While searching for next slot in a hole, it walks through the same
> slots over n over.

How did you determine this?

>   while (++offset < RADIX_TREE_MAP_SIZE) {
> void *slot = rcu_dereference_raw(   /* redundant slot 
> walk */
> node->slots[offset]);
> if (slot)
> break;
> }

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

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.


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