Re: [PATCH v2 4/5] mm: FLEXIBLE_THP for improved performance

[Ryan Roberts]

On 07/07/2023 17:06, David Hildenbrand wrote:
> On 07.07.23 17:13, Ryan Roberts wrote:
>> On 07/07/2023 15:07, David Hildenbrand wrote:
>>> On 07.07.23 15:57, Matthew Wilcox wrote:
>>>> On Fri, Jul 07, 2023 at 01:29:02PM +0200, David Hildenbrand wrote:
>>>>> On 07.07.23 11:52, Ryan Roberts wrote:
>>>>>> On 07/07/2023 09:01, Huang, Ying wrote:
>>>>>>> Although we can use smaller page order for FLEXIBLE_THP, it's hard to
>>>>>>> avoid internal fragmentation completely.  So, I think that finally we
>>>>>>> will need to provide a mechanism for the users to opt out, e.g.,
>>>>>>> something like "always madvise never" via
>>>>>>> /sys/kernel/mm/transparent_hugepage/enabled.  I'm not sure whether it's
>>>>>>> a good idea to reuse the existing interface of THP.
>>>>>>
>>>>>> I wouldn't want to tie this to the existing interface, simply because that
>>>>>> implies that we would want to follow the "always" and "madvise" advice too;
>>>>>> That
>>>>>> means that on a thp=madvise system (which is certainly the case for
>>>>>> android and
>>>>>> other client systems) we would have to disable large anon folios for VMAs
>>>>>> that
>>>>>> haven't explicitly opted in. That breaks the intention that this should be an
>>>>>> invisible performance boost. I think it's important to set the policy for
>>>>>> use of
>>>>>
>>>>> It will never ever be a completely invisible performance boost, just like
>>>>> ordinary THP.
>>>>>
>>>>> Using the exact same existing toggle is the right thing to do. If someone
>>>>> specify "never" or "madvise", then do exactly that.
>>>>>
>>>>> It might make sense to have more modes or additional toggles, but
>>>>> "madvise=never" means no memory waste.
>>>>
>>>> I hate the existing mechanisms.  They are an abdication of our
>>>> responsibility, and an attempt to blame the user (be it the sysadmin
>>>> or the programmer) of our code for using it wrongly.  We should not
>>>> replicate this mistake.
>>>
>>> I don't agree regarding the programmer responsibility. In some cases the
>>> programmer really doesn't want to get more memory populated than requested --
>>> and knows exactly why setting MADV_NOHUGEPAGE is the right thing to do.
>>>
>>> Regarding the madvise=never/madvise/always (sys admin decision), memory waste
>>> (and nailing down bugs or working around them in customer setups) have been very
>>> good reasons to let the admin have a word.
>>>
>>>>
>>>> Our code should be auto-tuning.  I posted a long, detailed outline here:
>>>> https://lore.kernel.org/linux-mm/Y%2FU8bQd15aUO97vS@xxxxxxxxxxxxxxxxxxxx/
>>>>
>>>
>>> Well, "auto-tuning" also should be perfect for everybody, but once reality
>>> strikes you know it isn't.
>>>
>>> If people don't feel like using THP, let them have a word. The "madvise" config
>>> option is probably more controversial. But the "always vs. never" absolutely
>>> makes sense to me.
>>>
>>>>> I remember I raised it already in the past, but you *absolutely* have to
>>>>> respect the MADV_NOHUGEPAGE flag. There is user space out there (for
>>>>> example, userfaultfd) that doesn't want the kernel to populate any
>>>>> additional page tables. So if you have to respect that already, then also
>>>>> respect MADV_HUGEPAGE, simple.
>>>>
>>>> Possibly having uffd enabled on a VMA should disable using large folios,
>>>
>>> There are cases where we enable uffd *after* already touching memory (postcopy
>>> live migration in QEMU being the famous example). That doesn't fly.
>>>
>>>> I can get behind that.  But the notion that userspace knows what it's
>>>> doing ... hahaha.  Just ignore the madvise flags.  Userspace doesn't
>>>> know what it's doing.
>>>
>>> If user space sets MADV_NOHUGEPAGE, it exactly knows what it is doing ... in
>>> some cases. And these include cases I care about messing with sparse VM
>>> memory :)
>>>
>>> I have strong opinions against populating more than required when user space set
>>> MADV_NOHUGEPAGE.
>>
>> I can see your point about honouring MADV_NOHUGEPAGE, so think that it is
>> reasonable to fallback to allocating an order-0 page in a VMA that has it set.
>> The app has gone out of its way to explicitly set it, after all.
>>
>> I think the correct behaviour for the global thp controls (cmdline and sysfs)
>> are less obvious though. I could get on board with disabling large anon folios
>> globally when thp="never". But for other situations, I would prefer to keep
>> large anon folios enabled (treat "madvise" as "always"), with the argument that
>> their order is much smaller than traditional THP and therefore the internal
>> fragmentation is significantly reduced. I really don't want to end up with user
>> space ever having to opt-in (with MADV_HUGEPAGE) to see the benefits of large
>> anon folios.
> 
> I was briefly playing with a nasty idea of an additional "madvise-pmd" option
> (that could be the new default), that would use PMD THP only in madvise'd
> regions, and ordinary everywhere else. But let's disregard that for now. I think
> there is a bigger issue (below).
> 
>>
>> I still feel that it would be better for the thp and large anon folio controls
>> to be independent though - what's the argument for tying them together?
> 
> Thinking about desired 2 MiB flexible THP on aarch64 (64k kernel) vs, 2 MiB PMD
> THP on aarch64 (4k kernel), how are they any different? Just the way they are
> mapped ...

The last patch in the series shows my current approach to that:

int arch_wants_pte_order(struct vm_area_struct *vma)
{
	if (hugepage_vma_check(vma, vma->vm_flags, false, true, true))
		return CONFIG_ARM64_PTE_ORDER_THP; <<< always the contpte size
	else
		return CONFIG_ARM64_PTE_ORDER_NOTHP; <<< limited to 64K
}

But Yu has raised concerns that this type of policy needs to be in the core mm.
So we could have the arch blindly return the preferred order from HW perspective
(which would be contpte size for arm64). Then for !hugepage_vma_check(), mm
could take the min of that value and some determined "acceptable" limit (which
in my mind is 64K ;-).

> 
> It's easy to say "64k vs. 2 MiB" is a difference and we want separate controls,
> but how is "2MiB vs. 2 MiB" different?
> 
> Having that said, I think we have to make up our mind how much control we want
> to give user space. Again, the "2MiB vs. 2 MiB" case nicely shows that it's not
> trivial: memory waste is a real issue on some systems where we limit THP to
> madvise().
> 
> 
> Just throwing it out for discussing:
> 
> What about keeping the "all / madvise / never" semantics (and MADV_NOHUGEPAGE
> ...) but having an additional config knob that specifies in which cases we
> *still* allow flexible THP even though the system was configured for "madvise".
> 
> I can't come up with a good name for that, but something like
> "max_auto_size=64k" could be something reasonable to set. We could have an
> arch+hw specific default.

Ahha, yes, that's essentially what I have above. I personally also like the idea
of the limit being an absolute value rather than an order. Although I know Yu
feels differently (see [1]).

[1]
https://lore.kernel.org/linux-mm/4d4c45a2-0037-71de-b182-f516fee07e67@xxxxxxx/T/#m2aff6eebd7f14d0d0620b48497d26eacecf970e6


> 
> (we all hate config options, I know, but I think there are good reasons to have
> such bare-minimum ones)
>