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

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Message-ID: <671ac2d2b7bea_10e59294f2@dwillia2-xfh.jf.intel.com.notmuch>
Date: Thu, 24 Oct 2024 14:57:38 -0700
From: Dan Williams <dan.j.williams@...el.com>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>, "Huang, Ying"
	<ying.huang@...el.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 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.