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

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Message-ID: <87wmhx3cpc.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date: Thu, 24 Oct 2024 20:30:39 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: Dan Williams <dan.j.williams@...el.com>,  David Hildenbrand
 <david@...hat.com>,  Andrew Morton <akpm@...ux-foundation.org>,
  <linux-mm@...ck.org>,  <linux-kernel@...r.kernel.org>,
  <linux-cxl@...r.kernel.org>,  Davidlohr Bueso <dave@...olabs.net>,
  "Jonathan Cameron" <jonathan.cameron@...wei.com>,  Alistair Popple
 <apopple@...dia.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()

Andy Shevchenko <andriy.shevchenko@...ux.intel.com> writes:

> On Wed, Oct 23, 2024 at 02:07:52PM -0700, Dan Williams wrote:
>> Andy Shevchenko wrote:
>> > On Fri, Oct 11, 2024 at 09:06:37AM +0800, Huang, Ying wrote:
>> > > David Hildenbrand <david@...hat.com> writes:
>> > > > On 10.10.24 08:55, Huang Ying wrote:
>
> ...
>
>> > > > 	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.  IMO, this should be avoided.
>> > 
>> > Ideally, yes. But how many for_each type of macros you see that really try hard
>> > to achieve that? I believe we shouldn't worry right now about this and rely on
>> > the fact that root is the given variable. Or do you have an example of what you
>> > suggested in the other reply, i.e. where it's an evaluation of the heavy call?
>> > 
>> > > Do you have some idea about
>> > > how to do that?  Something like below?
>> > > 
>> > > #define for_each_resource_XXX(_root, _p)                                \
>> > > 	for (typeof(_root) __root = (_root), __p = (_p) = (__root)->child; \
>> > > 	     __p && (_p); (_p) = next_resource_XXX(__root, _p))
>> > 
>> > This is a bit ugly :-( I would avoid ugliness as long as we have no problem to
>> > solve (see above).
>> 
>> Using a local defined variable to avoid double evaluation is standard
>> practice. I do not understand "avoid ugliness as long as we have no problem to
>> solve", the problem to solve will be if someone accidentally does
>> something like "for_each_resource_descendant(root++, res)". *That* will
>> be a problem when someone finally realizes that the macro is hiding a
>> double evaluation.
>
> Can you explain, why do we need __p and how can we get rid of that?
> I understand the part of the local variable for root.

If don't use '__p', the macro becomes

#define for_each_resource_XXX(_root, _p)                                \
	for (typeof(_root) __root = (_root), (_p) = (__root)->child; \
	     (_p); (_p) = next_resource_XXX(__root, _p))

Where, '_p' must be a variable name, and it will be a new variable
inside for loop and mask the variable with same name outside of macro.
IIUC, this breaks the macro convention in kernel and has subtle variable
masking semantics.

>> So no, this proposal is not "ugly", it is a best practice. See the
>> definition of min_not_zero() for example.
>
> I know that there are a lot of macros that look uglier that this one.

--
Best Regards,
Huang, Ying