linux-kernel - Re: [RFC] resource: Avoid unnecessary resource tree walking in __region

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Message-ID: <38566dbf-1293-4fd5-9cbd-385e6c35344c@redhat.com>
Date: Fri, 11 Oct 2024 10:02:43 +0200
From: David Hildenbrand <david@...hat.com>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, linux-cxl@...r.kernel.org,
 Dan Williams <dan.j.williams@...el.com>, Davidlohr Bueso
 <dave@...olabs.net>, Jonathan Cameron <jonathan.cameron@...wei.com>,
 Alistair Popple <apopple@...dia.com>,
 Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
 Bjorn Helgaas <bhelgaas@...gle.com>, Baoquan He <bhe@...hat.com>,
 Dave Jiang <dave.jiang@...el.com>,
 Alison Schofield <alison.schofield@...el.com>
Subject: Re: [RFC] resource: Avoid unnecessary resource tree walking in
 __region_intersects()

On 11.10.24 03:06, Huang, Ying wrote:
> David Hildenbrand <david@...hat.com> writes:
> 
>> On 10.10.24 08:55, Huang Ying wrote:
>>> Currently, if __region_intersects() finds any overlapped but unmatched
>>> resource, it walks the descendant resource tree to check for
>>> overlapped and matched descendant resources.  This is achieved using
>>> for_each_resource(), which iterates not only the descent tree, but
>>> also subsequent sibling trees in certain scenarios.  While this
>>> doesn't introduce bugs, it makes code hard to be understood and
>>> potentially inefficient.
>>> So, the patch renames next_resource() to __next_resource() and
>>> modified it to return NULL after traversing all descent resources.
>>> Test shows that this avoids unnecessary resource tree walking in
>>> __region_intersects().
>>> It appears even better to revise for_each_resource() to traverse the
>>> descendant resource tree of "_root" only.  But that will cause "_root"
>>> to be evaluated twice, which I don't find a good way to eliminate.
>>
>> I'm not sure I'm enjoying below code, it makes it harder for me to
>> understand what's happening.
>>
>> I'm also not 100% sure why "p" becomes "root" and "dp" becomes "p" when
>> calling the function :) Likely this works as intended, but it's confusing
>> (IOW, bad naming, especially for dp).
>>
>>
>> I think you should just leave next_resource() alone and rather add
>> a new function that doesn't conditionally consume NULL pointers
>> (and also no skip_children because you're passing false either way).
>>
>> static struct resource *next_resource_XXX(struct resource *root,
>> 		struct resource *p)
>> {
>> 	while (!p->sibling && p->parent) {
>> 		p = p->parent;
>> 		if (p == root)
>> 			return NULL;
>> 	}
>> 	return p->sibling;
>> }
>>
>> Maybe even better, add a new for_each_resource() macro that expresses the intended semantics.
>>
>> #define for_each_resource_XXX(_root, _p) \
>> 	for ((_p) = (_root)->child; (_p); (_p) = next_resource_XXX(_root, _p))
> 
> Yes.  This can improve code readability.
> 
> A possible issue is that "_root" will be evaluated twice in above macro
> definition. 

Do you mean that we would process it twice in the loop body, or what 
exactly do you mean with "evaluate" ?


And just I understand what we want to achieve: we want to walk the 
subtree below "root" and prevent going to root->sibling or root->parent 
if "root" is not actually the "real root", correct?

X
|--------|
A----D   E
|
B--C


So assume we start walking at A, we want to evaluate A,B,C but not D,E,X.

Does that sum up what we want to achieve?

-- 
Cheers,

David / dhildenb