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

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Message-ID: <66c4fcc5-47f6-438c-a73a-3af6e19c3200@redhat.com>
Date: Mon, 12 Aug 2024 10:12:49 +0200
From: David Hildenbrand <david@...hat.com>
To: Mateusz Guzik <mjguzik@...il.com>, Yin Fengwei <fengwei.yin@...el.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

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@...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.
> 
> 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);
  


-- 
Cheers,

David / dhildenb