Subject: Suspicious error for CMA stress test

From: [email-masked] (Vlastimil Babka)
Date: Mon, 14 Mar 2016 13:30:09 +0100

On 03/14/2016 08:18 AM, Joonsoo Kim wrote:
> On Mon, Mar 14, 2016 at 08:06:16AM +0100, Vlastimil Babka wrote:
>> On 03/14/2016 07:49 AM, Joonsoo Kim wrote:
>>> On Fri, Mar 11, 2016 at 06:07:40PM +0100, Vlastimil Babka wrote:
>>>> On 03/11/2016 04:00 PM, Joonsoo Kim wrote:
>>>>
>>>> How about something like this? Just and idea, probably buggy (off-by-one etc.).
>>>> Should keep away cost from <pageblock_order iterations@the expense of the
>>>> relatively fewer >pageblock_order iterations.
>>>
>>> Hmm... I tested this and found that it's code size is a little bit
>>> larger than mine. I'm not sure why this happens exactly but I guess it would be
>>> related to compiler optimization. In this case, I'm in favor of my
>>> implementation because it looks like well abstraction. It adds one
>>> unlikely branch to the merge loop but compiler would optimize it to
>>> check it once.
>>
>> I would be surprised if compiler optimized that to check it once, as
>> order increases with each loop iteration. But maybe it's smart
>> enough to do something like I did by hand? Guess I'll check the
>> disassembly.
>
> Okay. I used following slightly optimized version and I need to
> add 'max_order = min_t(unsigned int, MAX_ORDER, pageblock_order + 1)'
> to yours. Please consider it, too.

Hmm, so this is bloat-o-meter on x86_64, gcc 5.3.1. CONFIG_CMA=y

next-20160310 vs my patch (with added min_t as you pointed out):
add/remove: 0/0 grow/shrink: 1/1 up/down: 69/-5 (64)
function                                     old     new   delta
free_one_page                                833     902     +69
free_pcppages_bulk                          1333    1328      -5

next-20160310 vs your patch:
add/remove: 0/0 grow/shrink: 2/0 up/down: 577/0 (577)
function                                     old     new   delta
free_one_page                                833    1187    +354
free_pcppages_bulk                          1333    1556    +223

my patch vs your patch:
add/remove: 0/0 grow/shrink: 2/0 up/down: 513/0 (513)
function                                     old     new   delta
free_one_page                                902    1187    +285
free_pcppages_bulk                          1328    1556    +228

The increase of your version is surprising, wonder what the compiler 
did. Otherwise I would like simpler/maintainable version, but this is crazy.
Can you post your results? I wonder if your compiler e.g. decided to 
stop inlining page_is_buddy() or something.