Message-ID: <94ae96f7-79ce-4b3f-a272-6af62d01a3f8@redhat.com>
Date: Thu, 25 Apr 2024 10:26:36 +0200
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org,
 hughd@...gle.com
Cc: willy@...radead.org, wangkefeng.wang@...wei.com, 21cnbao@...il.com,
 ying.huang@...el.com, shy828301@...il.com, ziy@...dia.com,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/5] add mTHP support for anonymous share pages

On 25.04.24 10:17, Ryan Roberts wrote:
> On 25/04/2024 07:20, Baolin Wang wrote:
>>
>>
>> On 2024/4/24 22:20, Ryan Roberts wrote:
>>> On 24/04/2024 14:49, Baolin Wang wrote:
>>>>
>>>>
>>>> On 2024/4/24 18:01, Ryan Roberts wrote:
>>>>> On 24/04/2024 10:55, Baolin Wang wrote:
>>>>>>
>>>>>>
>>>>>> On 2024/4/24 16:26, Ryan Roberts wrote:
>>>>>>> On 24/04/2024 07:55, Baolin Wang wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>> On 2024/4/23 18:41, Ryan Roberts wrote:
>>>>>>>>> On 22/04/2024 08:02, Baolin Wang wrote:
>>>>>>>>>> Anonymous pages have already been supported for multi-size (mTHP)
>>>>>>>>>> allocation
>>>>>>>>>> through commit 19eaf44954df, that can allow THP to be configured
>>>>>>>>>> through the
>>>>>>>>>> sysfs interface located at
>>>>>>>>>> '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'.
>>>>>>>>>>
>>>>>>>>>> However, the anonymous shared pages will ignore the anonymous mTHP rule
>>>>>>>>>> configured through the sysfs interface, and can only use the PMD-mapped
>>>>>>>>>> THP, that is not reasonable. Many implement anonymous page sharing through
>>>>>>>>>> mmap(MAP_SHARED | MAP_ANONYMOUS), especially in database usage scenarios,
>>>>>>>>>> therefore, users expect to apply an unified mTHP strategy for anonymous
>>>>>>>>>> pages,
>>>>>>>>>> also including the anonymous shared pages, in order to enjoy the
>>>>>>>>>> benefits of
>>>>>>>>>> mTHP. For example, lower latency than PMD-mapped THP, smaller memory bloat
>>>>>>>>>> than PMD-mapped THP, contiguous PTEs on ARM architecture to reduce TLB
>>>>>>>>>> miss
>>>>>>>>>> etc.
>>>>>>>>>
>>>>>>>>> This sounds like a very useful addition!
>>>>>>>>>
>>>>>>>>> Out of interest, can you point me at any workloads (and off-the-shelf
>>>>>>>>> benchmarks
>>>>>>>>> for those workloads) that predominantly use shared anon memory?
>>>>>>>>
>>>>>>>> As far as I know, some database related workloads make extensive use of
>>>>>>>> shared
>>>>>>>> anonymous page, such as PolarDB[1] in our Alibaba fleet, or MySQL likely
>>>>>>>> also
>>>>>>>> uses shared anonymous memory. And I still need to do some investigation to
>>>>>>>> measure the performance.
>>>>>>>>
>>>>>>>> [1] https://github.com/ApsaraDB/PolarDB-for-PostgreSQL
>>>>>>>
>>>>>>> Thanks for the pointer!
>>>>>>>
>>>>>>>>
>>>>>>>>>> The primary strategy is that, the use of huge pages for anonymous shared
>>>>>>>>>> pages
>>>>>>>>>> still follows the global control determined by the mount option "huge="
>>>>>>>>>> parameter
>>>>>>>>>> or the sysfs interface at
>>>>>>>>>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled'.
>>>>>>>>>> The utilization of mTHP is allowed only when the global 'huge' switch is
>>>>>>>>>> enabled.
>>>>>>>>>> Subsequently, the mTHP sysfs interface
>>>>>>>>>> (/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled)
>>>>>>>>>> is checked to determine the mTHP size that can be used for large folio
>>>>>>>>>> allocation
>>>>>>>>>> for these anonymous shared pages.
>>>>>>>>>
>>>>>>>>> I'm not sure about this proposed control mechanism; won't it break
>>>>>>>>> compatibility? I could be wrong, but I don't think shmem's use of THP
>>>>>>>>> used to
>>>>>>>>> depend upon the value of /sys/kernel/mm/transparent_hugepage/enabled? So it
>>>>>>>>
>>>>>>>> Yes, I realized this after more testing.
>>>>>>>>
>>>>>>>>> doesn't make sense to me that we now depend upon the
>>>>>>>>> /sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled values (which by
>>>>>>>>> default disables all sizes except 2M, which is set to "inherit" from
>>>>>>>>> /sys/kernel/mm/transparent_hugepage/enabled).
>>>>>>>>>
>>>>>>>>> The other problem is that shmem_enabled has a different set of options
>>>>>>>>> (always/never/within_size/advise/deny/force) to enabled
>>>>>>>>> (always/madvise/never)
>>>>>>>>>
>>>>>>>>> Perhaps it would be cleaner to do the same trick we did for enabled;
>>>>>>>>> Introduce
>>>>>>>>> /mm/transparent_hugepage/hugepage-XXkb/shmem_enabled, which can have all
>>>>>>>>> the
>>>>>>>>> same values as the top-level
>>>>>>>>> /sys/kernel/mm/transparent_hugepage/shmem_enabled,
>>>>>>>>> plus the additional "inherit" option. By default all sizes will be set to
>>>>>>>>> "never" except 2M, which is set to "inherit".
>>>>>>>>
>>>>>>>> Sounds good to me. But I do not want to copy all same values from top-level
>>>>>>>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled':
>>>>>>>> always within_size advise never deny force
>>>>>>>>
>>>>>>>> For mTHP's shmem_enabled interface, we can just keep below values:
>>>>>>>> always within_size advise never
>>>>>>>>
>>>>>>>> Cause when checking if mTHP can be used for anon shmem, 'deny' is equal to
>>>>>>>> 'never', and 'force' is equal to 'always'.
>>>>>>>
>>>>>>> I'll admit it wasn't completely clear to me after reading the docs, but my
>>>>>>> rough
>>>>>>> understanding is:
>>>>>>>
>>>>>>>      - /sys/kernel/mm/transparent_hugepage/shmem_enabled controls
>>>>>>>        mmap(SHARED|ANON) allocations (mostly; see rule 3)
>>>>>>>      - huge=... controls tmpfs allocations
>>>>>>>      - deny and force in shmem_enabled are equivalent to never and always for
>>>>>>>        mmap(SHARED|ANON) but additionally override all tmpfs mounts so they
>>>>>>> act as
>>>>>>>        if they were mounted with huge=never or huge=always
>>>>>>>
>>>>>>> Is that correct? If so, then I think it still makes sense to support per-size
>>>>>>
>>>>>> Correct.
>>>>>>
>>>>>>> deny/force. Certainly if a per-size control is set to "inherit" and the
>>>>>>> top-level control is set to deny or force, you would need that to mean
>>>>>>> something.
>>>>>>
>>>>>> IMHO, the '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled' interface
>>>>>> should only control the anonymous shmem. And 'huge=' controls tmpfs
>>>>>> allocation,
>>>>>> so we should not use anonymous control to override tmpfs control, which
>>>>>> seems a
>>>>>> little mess?
>>>>>
>>>>> I agree it would be cleaner to only handle mmap(SHARED|ANON) here, and leave
>>>>> the
>>>>> tmpfs stuff for another time. But my point is that
>>>>> /mm/transparent_hugepage/shmem_enabled already interferes with tmpfs if the
>>>>> value is deny or force. So if you have:
>>>>>
>>>>> echo deny > /mm/transparent_hugepage/shmem_enabled
>>>>
>>>> IIUC, this global control will cause shmem_is_huge() to always return false, so
>>>> no matter how '/mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled' is set,
>>>> anonymous shmem will not use mTHP. No?
>>>
>>> No, that's not how '/mm/transparent_hugepage/hugepage-xxxkB/enabled' works, and
>>> I think '/mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled' should follow
>>> the established pattern.
>>>
>>> For anon-private, each size is controlled by its
>>> /mm/transparent_hugepage/hugepage-xxxkB/enabled value. Unless that value is
>>> "inherit", in which case the value in /mm/transparent_hugepage/enabled is used
>>> for that size.
>>>
>>> That approach enables us to 1) maintain back-compat and 2) control each size
>>> independently
>>>
>>> 1) is met because the default is that all sizes are initially set to "never",
>>> except the PMD-size (e.g. /mm/transparent_hugepage/hugepage-2048kB/enabled)
>>> which is initially set to inherit. So any mTHP unaware SW can still modify
>>> /mm/transparent_hugepage/enabled and it will still only apply to PMD size.
>>>
>>> 2) is met because mTHP aware SW can come along and e.g. enable the 64K size
>>> (echo always > /mm/transparent_hugepage/hugepage-64kB/enabled) without having to
>>> modify the value in /mm/transparent_hugepage/enabled.
>>
>> Thanks for explanation. Initially, I want to make
>> ‘/mm/transparent_hugepage/shmem_enabled’ be a global control for huge page, but
>> I think it should follow the same strategy as anon mTHP as you said.
>>
>>>>> echo inherit > /mm/transparent_hugepage/hugepage-64kB/shmem_enabled
>>>>>
>>>>> What does that mean?
>>>
>>> So I think /mm/transparent_hugepage/hugepage-xxxkB/shmem_enabled will need to
>>> support the deny and force values. When applied to non-PMD sizes, "deny" can
>>> just be a noop for now, because there was no way to configure a tmpfs mount for
>>> non-PMD size THP in the first place. But I'm not sure what to do with "force"?
>>
>> OK. And I also prefer that "force" should be a noop too, since anon shmem
>> control should not configure tmpfs huge page allocation.
> 
> I guess technically they won't be noops, but (for the non-PMD-sizes) "force"
> will be an alias for "always" and "deny" will be an alias for "never"?
> 
> I was just a bit concerned about later changing that behavior to also impact
> tmpfs once tmpfs supports mTHP; could that cause breaks? But thinking about it,
> I don't see that as a problem.

Is the question what should happen if we "inherit" "force" or if someone 
specifies "force" for a mTP size explicitly?

-- 
Cheers,

David / dhildenb

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