Message-ID: <db344bf6-a064-4646-9edb-828f3d20d200@arm.com>
Date: Fri, 21 Feb 2025 10:27:14 +0530
From: Dev Jain <dev.jain@....com>
To: Nico Pache <npache@...hat.com>
Cc: Ryan Roberts <ryan.roberts@....com>, linux-kernel@...r.kernel.org,
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Subject: Re: [RFC v2 0/9] khugepaged: mTHP support



On 21/02/25 12:42 am, Nico Pache wrote:
> On Wed, Feb 19, 2025 at 2:01 AM Dev Jain <dev.jain@....com> wrote:
>>
>>
>>
>> On 19/02/25 4:00 am, Nico Pache wrote:
>>> On Tue, Feb 18, 2025 at 9:07 AM Ryan Roberts <ryan.roberts@....com> wrote:
>>>>
>>>> On 11/02/2025 00:30, Nico Pache wrote:
>>>>> The following series provides khugepaged and madvise collapse with the
>>>>> capability to collapse regions to mTHPs.
>>>>>
>>>>> To achieve this we generalize the khugepaged functions to no longer depend
>>>>> on PMD_ORDER. Then during the PMD scan, we keep track of chunks of pages
>>>>> (defined by MTHP_MIN_ORDER) that are utilized. This info is tracked
>>>>> using a bitmap. After the PMD scan is done, we do binary recursion on the
>>>>> bitmap to find the optimal mTHP sizes for the PMD range. The restriction
>>>>> on max_ptes_none is removed during the scan, to make sure we account for
>>>>> the whole PMD range. max_ptes_none will be scaled by the attempted collapse
>>>>> order to determine how full a THP must be to be eligible. If a mTHP collapse
>>>>> is attempted, but contains swapped out, or shared pages, we dont perform the
>>>>> collapse.
>>>>>
>>>>> With the default max_ptes_none=511, the code should keep its most of its
>>>>> original behavior. To exercise mTHP collapse we need to set max_ptes_none<=255.
>>>>> With max_ptes_none > HPAGE_PMD_NR/2 you will experience collapse "creep" and
>>>>
>>>> nit: I think you mean "max_ptes_none >= HPAGE_PMD_NR/2" (greater or *equal*)?
>>>> This is making my head hurt, but I *think* I agree with you that if
>>>> max_ptes_none is less than half of the number of ptes in a pmd, then creep
>>>> doesn't happen.
>>> Haha yea the compressed bitmap does not make the math super easy to
>>> follow, but i'm glad we arrived at the same conclusion :)
>>>>
>>>> To make sure I've understood;
>>>>
>>>>    - to collapse to 16K, you would need >=3 out of 4 PTEs to be present
>>>>    - to collapse to 32K, you would need >=5 out of 8 PTEs to be present
>>>>    - to collapse to 64K, you would need >=9 out of 16 PTEs to be present
>>>>    - ...
>>>>
>>>> So if we start with 3 present PTEs in a 16K area, we collapse to 16K and now
>>>> have 4 PTEs in a 32K area which is insufficient to collapse to 32K.
>>>>
>>>> Sounds good to me!
>>> Great! Another easy way to think about it is, with max_ptes_none =
>>> HPAGE_PMD_NR/2, a collapse will double the size, and we only need half
>>> for it to collapse again. Each size is 2x the last, so if we hit one
>>> collapse, it will be eligible again next round.
>>
>> Please someone correct me if I am wrong.
>>
> 
> max_ptes_none = 204
> scaled_none = (204 >> 9 - 3) = ~3.1
> so 4 pages need to be available in each chunk for the bit to be set, not 5.
> 
> at 204 the bitmap check is
> 512 - 1 - 204 = 307
> (PMD) 307 >> 3 = 38
> (1024k) 307 >> 4 = 19
> (512k) 307 >> 5 = 9
> (256k) 307 >> 6 = 4
> 
>> Consider this; you are collapsing a 256K folio. => #PTEs = 256K/4K = 64
>> => #chunks = 64 / 8 = 8.
>>
>> Let the PTE state within the chunks be as follows:
>>
>> Chunk 0: < 5 filled   Chunk 1: 5 filled   Chunk 2: 5 filled   Chunk 3: 5
>> filled
>>
>> Chunk 4: 5 filled   Chunk 5: < 5 filled   Chunk 6: < 5 filled   Chunk 7:
>> < 5 filled
>>
>> Consider max_ptes_none = 40% (512 * 40 / 100 = 204.8 (round down) = 204
>> < HPAGE_PMD_NR/2).
>> => To collapse we need at least 60% of the PTEs filled.
>>
>> Your algorithm marks chunks in the bitmap if 60% of the chunk is filled.
>> Then, if the number of chunks set is greater than 60%, then we will
>> collapse.
>>
>> Chunk 0 will be marked zero because less than 5 PTEs are filled =>
>> percentage filled <= 50%
>>
>> Right now the state is
>> 0111 1000
>> where the indices are the chunk numbers.
>> Since #1s = 4 => percent filled = 4/8 * 100 = 50%, 256K folio collapse
>> won't happen.
>>
>> For the first 4 chunks, the percent filled is 75%.  So the state becomes
>> 1111 1000
>> after 128K collapse, and now 256K collapse will happen.
>>
>> Either I got this correct, or I do not understand the utility of
>> maintaining chunks :) What you are doing is what I am doing except that
>> my chunk size = 1.
> 
> Ignoring all the math, and just going off the 0111 1000
> We do "creep", but its not the same type of "creep" we've been
> describing. The collapse in the first half will allow the collapse in
> order++ to collapse but it stops there and doesnt keep getting
> promoted to a PMD size. That is unless the adjacent 256k also has some
> bits set, then it can collapse to 512k. So I guess we still can creep,
> but its way less aggressive and only when there is actual memory being
> utilized in the adjacent chunk, so it's not like we are creating a
> huge waste.

I get you. You will creep when the adjacent chunk has at least 1 bit 
set. I don't really have a strong opinion on this one.

> 
> 
>>
>>>>
>>>>> constantly promote mTHPs to the next available size.
>>>>>
>>>>> Patch 1:     Some refactoring to combine madvise_collapse and khugepaged
>>>>> Patch 2:     Refactor/rename hpage_collapse
>>>>> Patch 3-5:   Generalize khugepaged functions for arbitrary orders
>>>>> Patch 6-9:   The mTHP patches
>>>>>
>>>>> ---------
>>>>>    Testing
>>>>> ---------
>>>>> - Built for x86_64, aarch64, ppc64le, and s390x
>>>>> - selftests mm
>>>>> - I created a test script that I used to push khugepaged to its limits while
>>>>>      monitoring a number of stats and tracepoints. The code is available
>>>>>      here[1] (Run in legacy mode for these changes and set mthp sizes to inherit)
>>>>>      The summary from my testings was that there was no significant regression
>>>>>      noticed through this test. In some cases my changes had better collapse
>>>>>      latencies, and was able to scan more pages in the same amount of time/work,
>>>>>      but for the most part the results were consistant.
>>>>> - redis testing. I tested these changes along with my defer changes
>>>>>     (see followup post for more details).
>>>>> - some basic testing on 64k page size.
>>>>> - lots of general use. These changes have been running in my VM for some time.
>>>>>
>>>>> Changes since V1 [2]:
>>>>> - Minor bug fixes discovered during review and testing
>>>>> - removed dynamic allocations for bitmaps, and made them stack based
>>>>> - Adjusted bitmap offset from u8 to u16 to support 64k pagesize.
>>>>> - Updated trace events to include collapsing order info.
>>>>> - Scaled max_ptes_none by order rather than scaling to a 0-100 scale.
>>>>> - No longer require a chunk to be fully utilized before setting the bit. Use
>>>>>      the same max_ptes_none scaling principle to achieve this.
>>>>> - Skip mTHP collapse that requires swapin or shared handling. This helps prevent
>>>>>      some of the "creep" that was discovered in v1.
>>>>>
>>>>> [1] - https://gitlab.com/npache/khugepaged_mthp_test
>>>>> [2] - https://lore.kernel.org/lkml/20250108233128.14484-1-npache@redhat.com/
>>>>>
>>>>> Nico Pache (9):
>>>>>     introduce khugepaged_collapse_single_pmd to unify khugepaged and
>>>>>       madvise_collapse
>>>>>     khugepaged: rename hpage_collapse_* to khugepaged_*
>>>>>     khugepaged: generalize hugepage_vma_revalidate for mTHP support
>>>>>     khugepaged: generalize alloc_charge_folio for mTHP support
>>>>>     khugepaged: generalize __collapse_huge_page_* for mTHP support
>>>>>     khugepaged: introduce khugepaged_scan_bitmap for mTHP support
>>>>>     khugepaged: add mTHP support
>>>>>     khugepaged: improve tracepoints for mTHP orders
>>>>>     khugepaged: skip collapsing mTHP to smaller orders
>>>>>
>>>>>    include/linux/khugepaged.h         |   4 +
>>>>>    include/trace/events/huge_memory.h |  34 ++-
>>>>>    mm/khugepaged.c                    | 422 +++++++++++++++++++----------
>>>>>    3 files changed, 306 insertions(+), 154 deletions(-)
>>>>>
>>>>
>>>
>>>
>>
> 

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