Re: [PATCH] resource: Avoid unnecessary resource tree walking in __region_intersects()

From: Huang, Ying
Date: Tue Oct 22 2024 - 04:56:06 EST


Dan Williams <dan.j.williams@xxxxxxxxx> writes:

> Huang Ying wrote:
> [..]
>> For the example resource tree as follows,
>>
>> X
>> |
>> A----D----E
>> |
>> B--C
>>
>> if 'A' is the overlapped but unmatched resource, original kernel
>> iterates 'B', 'C', 'D', 'E' when it walks the descendant tree. While
>> the patched kernel iterates only 'B', 'C'.
>>
>> It appears even better to revise for_each_resource() to traverse the
>> resource subtree under "_root" only. But that will cause "_root" to
>> be evaluated twice, which I don't find a good way to eliminate.
>>
>> Thanks David Hildenbrand for providing a good resource tree example.
>
> Should this have a Reported-by: and a Closes: tags for that report?
> Seems useful to capture that in the history.

IIUC, David didn't reported an issue. He just provided an example to
explain the different traversal behavior.

>> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
>> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
>> Cc: David Hildenbrand <david@xxxxxxxxxx>
>> Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
>> Cc: Jonathan Cameron <jonathan.cameron@xxxxxxxxxx>
>> Cc: Alistair Popple <apopple@xxxxxxxxxx>
>> Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
>> Cc: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
>> Cc: Baoquan He <bhe@xxxxxxxxxx>
>> Cc: Dave Jiang <dave.jiang@xxxxxxxxx>
>> Cc: Alison Schofield <alison.schofield@xxxxxxxxx>
>> ---
>>
>> Changes:
>>
>> RFC->v1:
>>
>> - Revised patch description and comments, Thanks David and Andy!
>>
>> - Link to RFC: https://lore.kernel.org/linux-mm/20241010065558.1347018-1-ying.huang@xxxxxxxxx/
>>
>> ---
>> kernel/resource.c | 26 +++++++++++++++++++++++---
>> 1 file changed, 23 insertions(+), 3 deletions(-)
>>
>> diff --git a/kernel/resource.c b/kernel/resource.c
>> index b730bd28b422..bd217d57fb09 100644
>> --- a/kernel/resource.c
>> +++ b/kernel/resource.c
>> @@ -50,15 +50,34 @@ EXPORT_SYMBOL(iomem_resource);
>>
>> static DEFINE_RWLOCK(resource_lock);
>>
>> -static struct resource *next_resource(struct resource *p, bool skip_children)
>> +/*
>> + * Return the next node of @p in pre-order tree traversal. If
>> + * @skip_children is true, skip the descendant nodes of @p in
>> + * traversal. If @p is a descendant of @subtree_root, only traverse
>> + * the subtree under @subtree_root.
>> + */
>> +static struct resource *__next_resource(struct resource *p, bool skip_children,
>> + struct resource *subtree_root)
>> {
>> if (!skip_children && p->child)
>> return p->child;
>> - while (!p->sibling && p->parent)
>> + while (!p->sibling && p->parent) {
>> p = p->parent;
>> + if (p == subtree_root)
>> + return NULL;
>> + }
>> return p->sibling;
>> }
>>
>> +static struct resource *next_resource(struct resource *p, bool skip_children)
>> +{
>> + return __next_resource(p, skip_children, NULL);
>> +}
>> +
>> +/*
>> + * Traverse the whole resource tree with @_root as root in pre-order.
>> + * NOTE: @_root should be the topmost node, that is, @_root->parent == NULL.
>> + */
>> #define for_each_resource(_root, _p, _skip_children) \
>> for ((_p) = (_root)->child; (_p); (_p) = next_resource(_p, _skip_children))
>>
>> @@ -572,7 +591,8 @@ static int __region_intersects(struct resource *parent, resource_size_t start,
>> covered = false;
>> ostart = max(res.start, p->start);
>> oend = min(res.end, p->end);
>> - for_each_resource(p, dp, false) {
>> + /* Traverse the subtree under 'p'. */
>> + for (dp = p->child; dp; dp = __next_resource(dp, false, p)) {
>
> Perhaps a new for_each_resource_descendant() to clarify this new
> iterator from for_each_resource()?

Yes. That's a good idea. The problem is that it's hard to avoid double
evaluation in an elegant way. We have discussed this in

https://lore.kernel.org/linux-mm/ZwkCt_ip5VOGWp4u@xxxxxxxxxxxxxxxxxx/

I have proposed something like,

#define for_each_resource_descendant(_root, _p) \
for (typeof(_root) __root = (_root), __p = (_p) = (__root)->child; \
__p && (_p); (_p) = __next_resource(_p, false, __root))

But this doesn't look elegant.

> Otherwise looks good to me:
>
> Acked-by: Dan Williams <dan.j.williams@xxxxxxxxx>

Thanks!

--
Best Regards,
Huang, Ying