linux-kernel - Re: [linus:master] [mm] c0bff412e6: stress-ng.clone.ops_per

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Message-ID: <c7e0d029-0a64-4b27-bd62-cf9a3577d7ff@intel.com>
Date: Mon, 12 Aug 2024 12:43:08 +0800
From: Yin Fengwei <fengwei.yin@...el.com>
To: David Hildenbrand <david@...hat.com>, Mateusz Guzik <mjguzik@...il.com>
CC: kernel test robot <oliver.sang@...el.com>, Peter Xu <peterx@...hat.com>,
	<oe-lkp@...ts.linux.dev>, <lkp@...el.com>, <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>, Huacai Chen
	<chenhuacai@...nel.org>, Jason Gunthorpe <jgg@...dia.com>, Matthew Wilcox
	<willy@...radead.org>, Nathan Chancellor <nathan@...nel.org>, Ryan Roberts
	<ryan.roberts@....com>, WANG Xuerui <kernel@...0n.name>,
	<linux-mm@...ck.org>, <ying.huang@...el.com>, <feng.tang@...el.com>
Subject: Re: [linus:master] [mm] c0bff412e6: stress-ng.clone.ops_per_sec -2.9%
 regression

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@...hat.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.

Regards
Yin, Fengwei

>>>
>>> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
>>> index 5769fe6e4950..7796ae116018 100644
>>> --- a/include/linux/page-flags.h
>>> +++ b/include/linux/page-flags.h
>>> @@ -1086,7 +1086,7 @@ PAGE_TYPE_OPS(Zsmalloc, zsmalloc, zsmalloc)
>>>     */
>>>    static inline bool PageHuge(const struct page *page)
>>>    {
>>> -       return folio_test_hugetlb(page_folio(page));
>>> +       return PageCompound(page) && 
>>> folio_test_hugetlb(page_folio(page));
>>>    }
>>>
>>>    /*
>>>
>>>
>>> We would avoid the function call for small folios.
>>>
>>
>> why not massage _compound_head back to an inlineable form instead? for
>> all i know you may even register a small win in total
> 
> Agreed, likely it will increase code size a bit which is why the 
> compiler decides to not inline. We could force it with __always_inline.
> 
> Finding ways to shrink page_fixed_fake_head() might be even better.
>