Re: [Patch v2] mm/rmap.c: split huge pmd when it really is

[Wei Yang <richardw.yang@linux.intel.com>]

On Tue, Jan 07, 2020 at 03:03:33PM +0300, Kirill A. Shutemov wrote:
>On Fri, Jan 03, 2020 at 10:01:28PM +0800, Wei Yang wrote:
>> On Fri, Jan 03, 2020 at 04:26:50PM +0300, Kirill A. Shutemov wrote:
>> >On Fri, Jan 03, 2020 at 09:05:54PM +0800, Wei Yang wrote:
>> >> On Fri, Jan 03, 2020 at 03:18:46PM +0800, Wei Yang wrote:
>> >> >On Tue, Dec 24, 2019 at 06:28:56AM +0800, Wei Yang wrote:
>> >> >>When page is not NULL, function is called by try_to_unmap_one() with
>> >> >>TTU_SPLIT_HUGE_PMD set. There are two cases to call try_to_unmap_one()
>> >> >>with TTU_SPLIT_HUGE_PMD set:
>> >> >>
>> >> >>  * unmap_page()
>> >> >>  * shrink_page_list()
>> >> >>
>> >> >>In both case, the page passed to try_to_unmap_one() is PageHead() of the
>> >> >>THP. If this page's mapping address in process is not HPAGE_PMD_SIZE
>> >> >>aligned, this means the THP is not mapped as PMD THP in this process.
>> >> >>This could happen when we do mremap() a PMD size range to an un-aligned
>> >> >>address.
>> >> >>
>> >> >>Currently, this case is handled by following check in __split_huge_pmd()
>> >> >>luckily.
>> >> >>
>> >> >>  page != pmd_page(*pmd)
>> >> >>
>> >> >>This patch checks the address to skip some work.
>> >> >
>> >> >I am sorry to forget address Kirill's comment in 1st version.
>> >> >
>> >> >The first one is the performance difference after this change for a PTE
>> >> >mappged THP.
>> >> >
>> >> >Here is the result:(in cycle)
>> >> >
>> >> >        Before     Patched
>> >> >
>> >> >        963        195
>> >> >        988        40
>> >> >        895        78
>> >> >
>> >> >Average 948        104
>> >> >
>> >> >So the change reduced 90% time for function split_huge_pmd_address().
>> >
>> >Right.
>> >
>> >But do we have a scenario, where the performance of
>> >split_huge_pmd_address() matters? I mean, it it called as part of rmap
>> >walk, attempt to split huge PMD where we don't have huge PMD should be
>> >within noise.
>> 
>> Sorry for my poor English.
>> 
>> I don't catch the meaning of the last sentence. "within noise" here means
>> non-huge PMD is an expected scenario and we could tolerate this? 
>
>Basically, I doubt that this change would bring any measurable perfromance
>benefits on a real workload.
>

Ok, let's leave it now.

>-- 
> Kirill A. Shutemov

-- 
Wei Yang
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