[LSF/MM/BPF TOPIC] Locally attached memory tiering

[David Rientjes <rientjes@google.com>]

Hi all,

I think it would be very worthwhile to have a block set aside for 
discussion on locally attached memory tiering extensions at LSF/MM/BPF 
2024.

Primarily interested in discussing Linux enlightenment for CXL 1.1 and 
later type-3 memory expansion devices (CXL.mem).  I think we could touch 
on CXL 2.0 and later memory pooling architectures if we have time and 
there is interest, but the primary focus here would be local attached.

Based on the premise for a Memory Tiering Working Group[1], there is 
widespread interest in the foundational topics for generally useful Linux 
enlightenment:

 - Decoupling CPU balancing from memory balancing (or obsoleting CPU
   balancing entirely)

   + John Hubbard notes this would be useful for GPUs:

      a) GPUs have their own processors that are invisible to the kernel's
         NUMA "which tasks are active on which NUMA nodes" calculations,
         and

      b) Similar to where CXL is generally going, we have already built
         fully memory-coherent hardware, which include memory-only NUMA
         nodes.

 - In-kernel hot memory abstraction, informed by hardware hinting drivers
   (incl some architectures like Power10), usable as a NUMA Balancing
   backend for promotion and other areas of the kernel like transparent
   hugepage utilization

 - NUMA and memory tiering enlightenment for accelerators, such as for
   optimal use of GPU memory, extremely important for a cloud provider
   (hint hint :)

 - Asynchronous memory promotion independent of task_numa_fault() while
   considering the cost of page migration (due to identifying cold memory)

 - What the role of userspace plays in this decision-making and how we can
   extend the default policy and mechanisms in the kernel to allow for it
   if necessary

Additional topics that you find interesting are also very helpful!

I'm biased toward a generally useful solution that would leverage the 
kernel as the ultimate source of truth for page hotness that can be 
extended for multiple use caes, one of which is memory tiering support.  
But certainly if there are other approaches, we can discuss that as well.

A few main goals from this discussion:

 - Ensure that proposals address, or can be extended to address, the 
   emerging needs of the various use cases that users may have

 - Surface any constraints that stakeholders may find to be prohibitive
   for support in the core MM subsystem

 - Alignment and division of work for developers who are actively looking
   to contribute to this area

As I'm just one of many stakeholders for this discussion, I'd nominate 
Michal Hocko to moderate it if he's willing to do so.  If he's so willing, 
we'd be in good hands :)

 [1] https://lore.kernel.org/linux-mm/45d850ec-623b-7c07-c266-e948cdbf1f62@linux.com/T/