Re: [RFC PATCH 0/5] add mTHP support for anonymous share pages

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2024/4/25 16:57, David Hildenbrand wrote:
> 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

IMO, for tmpfs this is true, but for anon shmem, this 2 cases should not 
fail.

So I think we should allow this configuration, but for tmpfs huge page 
allocation, we will not check the mTHP.

> 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.

Sounds good to me.