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

[David Hildenbrand <david@redhat.com>]

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@casper.infradead.org/
> 

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.

-- 
Cheers,

David / dhildenb


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