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

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Message-ID: <87sesl2fc6.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date: Fri, 25 Oct 2024 08:31:21 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Dan Williams <dan.j.williams@...el.com>
Cc: Andy Shevchenko <andriy.shevchenko@...ux.intel.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()

Dan Williams <dan.j.williams@...el.com> writes:

> Andy Shevchenko wrote:
>> On Thu, Oct 24, 2024 at 08:30:39PM +0800, Huang, Ying wrote:
>> > 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.
>> 
>> Yep.
>
> Oh, due to the comment expression, good catch.
>
>> 
>> In property.h nobody cares about evaluation which makes the macro as simple as
>> 
>> #define for_each_resource_XXX(_root, _p)		\
>> 	for (_p = next_resource_XXX(__root, NULL); _p;	\
>> 	     _p = next_resource_XXX(__root, _p))
>> 
>> (Dan,
>>  that's what I called to avoid solving issues we don't have and most likely
>>  will never have.)
>
> Ah, my apologies, I thought the objection was to the macro altogether. 
>
>> but if you want to stick with your variant some improvements can be done:
>> 
>> #define for_each_resource_XXX(_root, _p)				\
>> 	for (typeof(_root) __root = (_root), __p = _p = __root->child;	\
>> 	     __p && _p; _p = next_resource_XXX(__root, _p))
>> 
>> 
>> 1) no need to have local variable in parentheses;
>> 2) no need to have iterator in parentheses, otherwise it would be crazy code
>> that has put something really wrong there and still expect the thing to work.
>
> Why not:
>
> #define for_each_resource_XXX(_root, _p)				\
> 	for (typeof(_root) __root = (_root), __p = _p = __root->child;	\
> 	     _p; _p = next_resource_XXX(__root, _p))
>
> The __p is only to allow for _p to be initialized in the first statement
> without causing a new "_p" shadow to be declared.

I have tries this before.  Compiler will complain with

warning: unused variable ‘__p’ [-Wunused-variable]

--
Best Regards,
Huang, Ying