Re: ANON_LARGE_FOLIOS meeting follow-up & refined proposal

[David Hildenbrand <david@redhat.com>]

On 22.09.23 17:48, Ryan Roberts wrote:
> On 14/09/2023 09:16, Ryan Roberts wrote:
>> Hi All,
>>
>> Thanks for participating in the discussion yesterday - it finally feels like we
>> are converging on the MVP feature set. Below are my notes from the call and a
>> modified proposal for controls and stats. It would be great if we can continue
>> to review and refine over email. I'm also planning to post an implementation
>> within the next couple of weeks, which I hope will also accelerate convergence.
> 
> I never had any feedback on the below; I'm not sure if that means everyone is
> happy or that nobody read it??
> 
> I've got an implementation for all of the below ready to go, with a few tweeks
> to the details (the main change is that anon_orders is now a bitfield where each
> set bit represents an order in the set, rather than the originally proposed
> comma-separated list of orders).
> 
> BUT I've had yet another idea on the controls front, which would enable exposing
> this to user space as an extension to transparent_hugepage, while continuing to
> support THP as is and also be able to control THP and ALF (anon large folio)
> usage independently. On reflection, I think it is cleaner to do it this way for
> a couple of reasons:
> 
>    - We don't have to introduce a whole new feature (ALF) to the user. Most of
>      the concepts and controls overlap a lot with THP anyway, so if we can make
>      it look like an extension, I think it would be easier to communicate.
> 
>    - The approach I have in mind would make it easy to extend to orders greater
>      than PMD_ORDER in future if that's a direction we want to eventually go.
>      Because >PMD_ORDER implies multiple PMD entries, it would half belong to THP
>      and half belong to ALF in the current proposal, which is nasty.
> 
> I'll lay out the new proposal now, but I suspect this will ultimately warrant
> another mm alignment meeting...
> 
> 
> Add 2 controls to sysfs:
> 
> /sys/kernel/mm/transparent_hugepage/anon_orders
>    - bitfield where set bits are orders that will be tried during allocation
>    - defaults to 1<<PMD_ORDER, which gives current THP behaviour with no ALF
>    - For now, 1<<PMD_ORDER is highest settable bit, but easy to expand in future
>    - To enable ALF, set the appropriate lower bits
>    - To disable THP, clear 1<<PMD_ORDER
>    - (In future we could add an "auto" option too)
> 

I recall discussing that in the past -- where we also thought about 
adding exactly that to debugfs -- but the consensus was that we should 
keep THP alone.

I agree at this point that we better keep THP alone.

THP might be a subset of ALF (at some point, hopefully), but not the 
other way around IMHO.

-- 
Cheers,

David / dhildenb