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Message-ID: <817150f2-abf7-430f-9973-540bd6cdd26f@intel.com>
Date: Tue, 13 Aug 2024 15:09:46 +0800
From: Yin Fengwei <fengwei.yin@...el.com>
To: Mateusz Guzik <mjguzik@...il.com>
CC: David Hildenbrand <david@...hat.com>, 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 8/12/24 00:49, 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.
>
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 5769fe6e4950..2d5d61ab385b 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -194,34 +194,13 @@ enum pageflags {
> #ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
> DECLARE_STATIC_KEY_FALSE(hugetlb_optimize_vmemmap_key);
>
> -/*
> - * Return the real head page struct iff the @page is a fake head page, otherwise
> - * return the @page itself. See Documentation/mm/vmemmap_dedup.rst.
> - */
> +const struct page *_page_fixed_fake_head(const struct page *page);
> +
> static __always_inline const struct page *page_fixed_fake_head(const struct page *page)
> {
> if (!static_branch_unlikely(&hugetlb_optimize_vmemmap_key))
> return page;
> -
> - /*
> - * Only addresses aligned with PAGE_SIZE of struct page may be fake head
> - * struct page. The alignment check aims to avoid access the fields (
> - * e.g. compound_head) of the @page[1]. It can avoid touch a (possibly)
> - * cold cacheline in some cases.
> - */
> - if (IS_ALIGNED((unsigned long)page, PAGE_SIZE) &&
> - test_bit(PG_head, &page->flags)) {
> - /*
> - * We can safely access the field of the @page[1] with PG_head
> - * because the @page is a compound page composed with at least
> - * two contiguous pages.
> - */
> - unsigned long head = READ_ONCE(page[1].compound_head);
> -
> - if (likely(head & 1))
> - return (const struct page *)(head - 1);
> - }
> - return page;
> + return _page_fixed_fake_head(page);
> }
> #else
> static inline const struct page *page_fixed_fake_head(const struct page *page)
> @@ -235,7 +214,7 @@ 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);
>
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index 829112b0a914..3fbc00db607a 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -19,6 +19,33 @@
> #include <asm/tlbflush.h>
> #include "hugetlb_vmemmap.h"
>
> +/*
> + * Return the real head page struct iff the @page is a fake head page, otherwise
> + * return the @page itself. See Documentation/mm/vmemmap_dedup.rst.
> + */
> +const struct page *_page_fixed_fake_head(const struct page *page)
> +{
> + /*
> + * Only addresses aligned with PAGE_SIZE of struct page may be fake head
> + * struct page. The alignment check aims to avoid access the fields (
> + * e.g. compound_head) of the @page[1]. It can avoid touch a (possibly)
> + * cold cacheline in some cases.
> + */
> + if (IS_ALIGNED((unsigned long)page, PAGE_SIZE) &&
> + test_bit(PG_head, &page->flags)) {
> + /*
> + * We can safely access the field of the @page[1] with PG_head
> + * because the @page is a compound page composed with at least
> + * two contiguous pages.
> + */
> + unsigned long head = READ_ONCE(page[1].compound_head);
> +
> + if (likely(head & 1))
> + return (const struct page *)(head - 1);
> + }
> + return page;
> +}
> +
> /**
> * struct vmemmap_remap_walk - walk vmemmap page table
> *
>
The change can resolve the regression (from -3% to 0.5%):
Please note:
9cb28da54643ad464c47585cd5866c30b0218e67 is the parent commit
3f16e4b516ef02d9461b7e0b6c50e05ba0811886 is the commit with above
patch
c0bff412e67b781d761e330ff9578aa9ed2be79e is the commit which
introduced regression
=========================================================================================
tbox_group/testcase/rootfs/kconfig/compiler/nr_threads/testtime/test/cpufreq_governor/debug-setup:
lkp-icl-2sp8/stress-ng/debian-12-x86_64-20240206.cgz/x86_64-rhel-8.3/gcc-12/100%/60s/clone/performance/yfw_test2
commit:
9cb28da54643ad464c47585cd5866c30b0218e67
3f16e4b516ef02d9461b7e0b6c50e05ba0811886
c0bff412e67b781d761e330ff9578aa9ed2be79e
9cb28da54643ad46 3f16e4b516ef02d9461b7e0b6c5 c0bff412e67b781d761e330ff95
---------------- --------------------------- ---------------------------
fail:runs %reproduction fail:runs %reproduction fail:runs
| | | | |
3:3 0% 3:3 0% 3:3
stress-ng.clone.microsecs_per_clone.pass
3:3 0% 3:3 0% 3:3
stress-ng.clone.pass
%stddev %change %stddev %change %stddev
\ | \ | \
2904 -0.6% 2886 +3.7% 3011
stress-ng.clone.microsecs_per_clone
563520 +0.5% 566296 -3.1% 546122
stress-ng.clone.ops
9306 +0.5% 9356 -3.0% 9024
stress-ng.clone.ops_per_sec
BTW, the change needs to export symbol _page_fixed_fake_head otherwise
some modules hit build error.
Regards
Yin, Fengwei
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