Re: [linus:master] [mm] c0bff412e6: stress-ng.clone.ops_per_sec -2.9% regression

[David Hildenbrand <david@redhat.com>]

On 12.08.24 10:18, Mateusz Guzik wrote:
> On Mon, Aug 12, 2024 at 10:12 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 12.08.24 06:49, Mateusz Guzik wrote:
>>> On Mon, Aug 12, 2024 at 12:43:08PM +0800, Yin Fengwei wrote:
>>>> Hi David,
>>>>
>>>> On 8/1/24 09:44, David Hildenbrand wrote:
>>>>> On 01.08.24 15:37, Mateusz Guzik wrote:
>>>>>> On Thu, Aug 1, 2024 at 3:34 PM David Hildenbrand <david@redhat.com>
>>>>>> wrote:
>>>>>>>
>>>>>>> On 01.08.24 15:30, Mateusz Guzik wrote:
>>>>>>>> On Thu, Aug 01, 2024 at 08:49:27AM +0200, David Hildenbrand wrote:
>>>>>>>>> Yes indeed. fork() can be extremely sensitive to each
>>>>>>>>> added instruction.
>>>>>>>>>
>>>>>>>>> I even pointed out to Peter why I didn't add the
>>>>>>>>> PageHuge check in there
>>>>>>>>> originally [1].
>>>>>>>>>
>>>>>>>>> "Well, and I didn't want to have runtime-hugetlb checks in
>>>>>>>>> PageAnonExclusive code called on certainly-not-hugetlb code paths."
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> We now have to do a page_folio(page) and then test for hugetlb.
>>>>>>>>>
>>>>>>>>>         return folio_test_hugetlb(page_folio(page));
>>>>>>>>>
>>>>>>>>> Nowadays, folio_test_hugetlb() will be faster than at
>>>>>>>>> c0bff412e6 times, so
>>>>>>>>> maybe at least part of the overhead is gone.
>>>>>>>>>
>>>>>>>>
>>>>>>>> I'll note page_folio expands to a call to _compound_head.
>>>>>>>>
>>>>>>>> While _compound_head is declared as an inline, it ends up being big
>>>>>>>> enough that the compiler decides to emit a real function instead and
>>>>>>>> real func calls are not particularly cheap.
>>>>>>>>
>>>>>>>> I had a brief look with a profiler myself and for single-threaded usage
>>>>>>>> the func is quite high up there, while it manages to get out with the
>>>>>>>> first branch -- that is to say there is definitely performance lost for
>>>>>>>> having a func call instead of an inlined branch.
>>>>>>>>
>>>>>>>> The routine is deinlined because of a call to page_fixed_fake_head,
>>>>>>>> which itself is annotated with always_inline.
>>>>>>>>
>>>>>>>> This is of course patchable with minor shoveling.
>>>>>>>>
>>>>>>>> I did not go for it because stress-ng results were too unstable for me
>>>>>>>> to confidently state win/loss.
>>>>>>>>
>>>>>>>> But should you want to whack the regression, this is what I would look
>>>>>>>> into.
>>>>>>>>
>>>>>>>
>>>>>>> This might improve it, at least for small folios I guess:
>>>> Do you want us to test this change? Or you have further optimization
>>>> ongoing? Thanks.
>>>
>>> I verified the thing below boots, I have no idea about performance. If
>>> it helps it can be massaged later from style perspective.
>>
>> As quite a lot of setups already run with the vmemmap optimization enabled, I
>> wonder how effective this would be (might need more fine tuning, did not look
>> at the generated code):
>>
>>
>> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
>> index 085dd8dcbea2..7ddcdbd712ec 100644
>> --- a/include/linux/page-flags.h
>> +++ b/include/linux/page-flags.h
>> @@ -233,7 +233,7 @@ static __always_inline int page_is_fake_head(const struct page *page)
>>           return page_fixed_fake_head(page) != page;
>>    }
>>
>> -static inline unsigned long _compound_head(const struct page *page)
>> +static __always_inline unsigned long _compound_head(const struct page *page)
>>    {
>>           unsigned long head = READ_ONCE(page->compound_head);
>>
>>
> 
> Well one may need to justify it with bloat-o-meter which is why I did
> not just straight up inline the entire thing.
> 
> But if you are down to fight opposition of the sort I agree this is
> the patch to benchmark. :)

I spotted that we already to that for 
PageHead()/PageTail()/page_is_fake_head(). So we effectively 
force-inline it everywhere except into _compound_head() I think.

But yeah, measuring the bloat would be a necessary exercise.

-- 
Cheers,

David / dhildenb