Message-ID: <bf005e0f-6fda-4068-8af6-5f8c00257de7@redhat.com>
Date: Thu, 25 Apr 2024 10:57:03 +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 10:46, Ryan Roberts wrote:
> On 25/04/2024 09:26, David Hildenbrand wrote:
>> 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?
> 
> Well I think it amounts to the same thing; there isn't much point in forbidding
> "force" to be set directly because it can still be set indirectly through
> "inherit". We can't forbid indirectly setting it, because "inherit" could be set
> first, then the top-level shmem_enabled changed to "force" after - and we
> wouldn't want to fail that.

The default for PMD should be "inherit", for the other mTHP sizes it 
should be "never".

So we should fail if:
* Setting top-level to "force" when any non-PMD size is "inherit"
* Setting "inherit" of a non-PMD size when the top-level is force

Both will only happen if someone messes with the mTHP configuration 
manually.

And we should only offer "force" as an option for PMD-sized mTHP as long 
as the others are not supported. See below.

> 
> So I think the question is just 'what should happen when "force" is configured
> for a non-PMD-sized mTHP'?

We should hide it and not offer a configuration toggle that is inactive.

If someone wants to sense support for other mTHP "force" settings in the 
future, they can just parse if the "shmem_enabled" toggle offers "force" 
as an option. Then they know that it can actually be enabled and will 
also do what is promised.

-- 
Cheers,

David / dhildenb

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