Re: Suspicious error for CMA stress test

Joonsoo Kim
2016-03-14T14:10:45Z

2016-03-14 21:30 GMT+09:00 Vlastimil Babka <vbabka@suse.cz>:
> 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 at 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.

Now I see why this happen. I enabled CONFIG_DEBUG_PAGEALLOC
and it makes difference.

I tested on x86_64, gcc (Ubuntu 4.8.4-2ubuntu1~14.04.1) 4.8.4.

With CONFIG_CMA + CONFIG_DEBUG_PAGEALLOC
./scripts/bloat-o-meter page_alloc_base.o page_alloc_vlastimil_orig.o
add/remove: 0/0 grow/shrink: 2/0 up/down: 510/0 (510)
function old new delta
free_one_page 1050 1334 +284
free_pcppages_bulk 1396 1622 +226

./scripts/bloat-o-meter page_alloc_base.o page_alloc_mine.o
add/remove: 0/0 grow/shrink: 2/0 up/down: 351/0 (351)
function old new delta
free_one_page 1050 1230 +180
free_pcppages_bulk 1396 1567 +171

With CONFIG_CMA + !CONFIG_DEBUG_PAGEALLOC
(pa_b is base, pa_v is yours and pa_m is mine)

./scripts/bloat-o-meter pa_b.o pa_v.o
add/remove: 0/0 grow/shrink: 1/1 up/down: 88/-23 (65)
function old new delta
free_one_page 761 849 +88
free_pcppages_bulk 1117 1094 -23

./scripts/bloat-o-meter pa_b.o pa_m.o
add/remove: 0/0 grow/shrink: 2/0 up/down: 329/0 (329)
function old new delta
free_one_page 761 1031 +270
free_pcppages_bulk 1117 1176 +59

Still, it has difference but less than before.
Maybe, we are still using different configuration. Could you
check if CONFIG_DEBUG_VM is enabled or not? In my case, it's not
enabled. And, do you think this bloat isn't acceptable?

Thanks.