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

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Message-ID: <ZwkI62qBAbc02O8C@smile.fi.intel.com>
Date: Fri, 11 Oct 2024 14:15:55 +0300
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: David Hildenbrand <david@...hat.com>
Cc: "Huang, Ying" <ying.huang@...el.com>,
	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>,
	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 Fri, Oct 11, 2024 at 12:51:09PM +0200, David Hildenbrand wrote:
> On 11.10.24 12:49, 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).
> 
> Fully agreed, I didn't quite understand the concern about "evaluation" at
> first.

It's a basic concept for macros and a good mine field even for the simple
cases.

> If it's just reading a variable twice, it doesn't matter at all right
> now.

The problem (even if it's a variable) is that the content of variable can be
changed when run in non-atomic context, i.e. two evaluations will give two
different results. Most "simple" for_each macros leave this exercise to the
caller. That's what I also suggest for now.

-- 
With Best Regards,
Andy Shevchenko