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

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Message-ID: <12c0a19e-784d-4ac0-8d3c-d5242bcd3723@redhat.com>
Date: Fri, 11 Oct 2024 13:30:15 +0200
From: David Hildenbrand <david@...hat.com>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.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 11.10.24 13:19, Andy Shevchenko wrote:
> On Fri, Oct 11, 2024 at 02:15:55PM +0300, Andy Shevchenko wrote:
>> 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.
> 
> For any context as Ying provided an example with calls, they have to be
> idempotent, or you definitely get two different pointers for these, which is
> bigger issue that what I described above.

Ah, now I understood what Ying meant: if the root pointer is modified 
within the loop body we'd be in trouble.

-- 
Cheers,

David / dhildenb