Re: [PATCH v3 0/9] mm: introduce Designated Movable Blocks

From: Florian Fainelli
Date: Tue Apr 18 2023 - 16:09:34 EST

On 1/5/23 05:29, David Hildenbrand wrote:
On 04.01.23 20:00, Florian Fainelli wrote:
On 1/4/23 07:56, David Hildenbrand wrote:
On 04.01.23 00:43, Florian Fainelli wrote:
On 10/20/22 14:53, Doug Berger wrote:
MOTIVATION:
Some Broadcom devices (e.g. 7445, 7278) contain multiple memory
controllers with each mapped in a different address range within
a Uniform Memory Architecture. Some users of these systems have
expressed the desire to locate ZONE_MOVABLE memory on each
memory controller to allow user space intensive processing to
make better use of the additional memory bandwidth.
Unfortunately, the historical monotonic layout of zones would
mean that if the lowest addressed memory controller contains
ZONE_MOVABLE memory then all of the memory available from
memory controllers at higher addresses must also be in the
ZONE_MOVABLE zone. This would force all kernel memory accesses
onto the lowest addressed memory controller and significantly
reduce the amount of memory available for non-movable
allocations.

The main objective of this patch set is therefore to allow a
block of memory to be designated as part of the ZONE_MOVABLE
zone where it will always only be used by the kernel page
allocator to satisfy requests for movable pages. The term
Designated Movable Block is introduced here to represent such a
block. The favored implementation allows extension of the
'movablecore' kernel parameter to allow specification of a base
address and support for multiple blocks. The existing
'movablecore' mechanisms are retained.

BACKGROUND:
NUMA architectures support distributing movablecore memory
across each node, but it is undesirable to introduce the
overhead and complexities of NUMA on systems that don't have a
Non-Uniform Memory Architecture.

Commit 342332e6a925 ("mm/page_alloc.c: introduce kernelcore=mirror
option")
also depends on zone overlap to support sytems with multiple
mirrored ranges.

Commit c6f03e2903c9 ("mm, memory_hotplug: remove zone restrictions")
embraced overlapped zones for memory hotplug.

This commit set follows their lead to allow the ZONE_MOVABLE
zone to overlap other zones. Designated Movable Blocks are made
absent from overlapping zones and present within the
ZONE_MOVABLE zone.

I initially investigated an implementation using a Designated
Movable migrate type in line with comments[1] made by Mel Gorman
regarding a "sticky" MIGRATE_MOVABLE type to avoid using
ZONE_MOVABLE. However, this approach was riskier since it was
much more instrusive on the allocation paths. Ultimately, the
progress made by the memory hotplug folks to expand the
ZONE_MOVABLE functionality convinced me to follow this approach.


Mel, David, does the sub-thread discussion with Doug help ensuring that
all of the context is gathered before getting into a more detailed patch
review on a patch-by-patch basis?

Eventually we may need a fairly firm answer as to whether the proposed
approach has any chance of landing upstream in order to either commit to
in subsequent iterations of this patch set, or find an alternative.


As raised, I'd appreciate if less intrusive alternatives could be
evaluated (e.g., fake NUMA nodes and being ablee to just use mbind(),
moving such memory to ZONE_MOVABLE after boot via something like daxctl).

This is not an option with the environment we have to ultimately fit in
which is Android TV utilizing the GKI kernel which does not enable NUMA
and probably never will, and for similar reasons bringing a whole swath
of user-space tools like daxctl may not be practical either, from both a
logistical perspective (simply getting the tools built with bionic,
accepted etc.) as well as system configuration perspective.

(looks like I never replied to this email, whoops)


Adding feature A because people don't want to (! whoever the "people" are) enable feature B? I hope I don't have to tell you what I think about statements like this :)

It is not just that NUMA is not wanted, it is also not a great fit, the ARM CPU cluster and most peripherals that Linux cares about do have an uniform memory access to the available DRAM controllers/DRAM chips.

Only a subset of the peripherals, especially the real-time and high bandwidth ones like video decoders and display that may not be uniformly accessing DRAM. This stems from the fact that the memory controller(s) on the System-on-Chip we work with have a star topology and they schedule the accesses of each DRAM client (CPU, GPU, video decoder, display, Ethernet, PCIe, etc) differently in order to guarantee a certain quality of service.

On a system with multiple DRAM controller / DRAM chips, you will typically see video decoder + display instances #0 be serviced by DRAM controller 0, and video decoder + display instance #1 be servied by DRAM controller 1, and this is the only way to allow dual decode + display as they are very bandwidth hungry.

The splitting or load balancing is done on a PFN basis, DRAM pages below a certain address are serviced by DRAM controller #0 and those above another cut off are servied by DRAM controller #1.


If feature B is a problem, try stripping it down such that it can be enabled. If it's to hard to configure for your use case, maybe we can extend configuration options to avoid tools like daxctl for some special cases.

I do not see the splitting of the notion of a 'memory node' object away from CONFIG_NUMA going anywhere, and sorry to put that way, but this would be requiring many months for a result that is not even clear, but would be undone anytime someone is not aware of that larger effort.


But of course, only if feature B actually solves the problem.

One issue I have with DMB is actual use cases / users / requirements. Maybe requirements are defined somewhere cleanly and I missed them.

That part is entirely fair, the requirements would be as follows:

- we need to be able to control precisely across the available DRAM range which specific PFNs fall within specific zones, and the consequence is that we should also be able to have a non-monotonically increasing definition of zones such that there is an appropriate balance between zones and the underlying PFNs / backing DRAM controller instance

- device driver(s) should be able to be allocate (via alloc_contig_range() and friends) memory from specific regions of DRAM which should be covered by an underlying zone/fallback/migrate set of heuristics which maximizes the re-use of such memory when the driver is not using it

- the underlying zone/fallback/migrate type heuristics should not "excessively" memory in reserve (CMA I am looking at you) but rather should allow for all of the memory in ideal conditions to be "claimed" by the device driver(s) if they desire so

- it is acceptable to spend time compacting/reclaiming memory under tight memory pressure since the transitions requiring said driver(s) to allocate are slow path/control events

We have other "soft" requirements which are mainly logistical such that:

- the least amount of files are changed

- there is no need for custom user-space to be running in order to set-up the regions, aka plug & play is highly desirable

- there is no dependency upon CONFIG_NUMA in order to simplify the deployment

- there is no overhead to the other users of the patch set and the behavior is entirely opt-in


If we have clear requirements, we can talk about possible solutions. If we have a specific solution, it's harder to talk about requirements.

 >>
I'm not convinced that these intrusive changes are worth it at this
point. Further, some of the assumptions (ZONE_MOVABLE == user space) are
not really future proof as I raised.

I find this patch set reasonably small in contrast to a lot of other mm/
changes, what did you find intrusive specifically?

AFAICT, there only assumption that is being made is that ZONE_MOVABLE
contains memory that can be moved, but even if it did not in the future,
there should hopefully be enough opportunities, given a large enough DMB
region to service the allocation requests of its users. I will go back
and read your comment to make sure I don't misunderstand it.

Let me clarify what ZONE_MOVABLE can and cannot do:

* We cannot assume that specific user space allocations are served from
  it, neither can we really modify behavior.
* We cannot assume that user space allocations won't be migrated off
  that zone to another zone.
* We cannot assume that no other (kernel) allocations will end up on it.
* We cannot make specific processes preferably consume memory from it.

Designing a feature that relies on any of these assumptions is IMHO wrong.

If you want an application to consume memory from a specific address range, there are some possible ways I can see:

(1) Model the special memory areas using fake NUMA nodes. e.g., mbind()
    the applications to these nodes. Use ZONE_MOVABLE to make sure we
    don't get unmovable allocations. The buddy will take care of it.
(2) Use some driver that manages that memory and provides that memory
    to an application by mmap()'ing it. The buddy won't manage it (no
    swap, migration ...). DEVDAX is one possible such driver.
(3) Use hugetlb and reserve them from the selected memory ranges.
    Make the application consume these hugetlb pages.

For a single node, without a special driver, it gets more complicated: We'd need new way to tell the buddy that these memory ranges are "special". I don't want to use the word "new zone" but that's most likely what it would have to be. Further, one would need a way to specify that only specific allocations should end up on these ranges.

Maybe I'm overthinking this. Having clear requirements such that we can try discussing solutions and exploring alternatives would be great.

This was helpful. We do not need or want to control precisely or exactly where and how applications are allocating memory from, we just need the kernel to do a good enough job at re-using the memory defined in ZONE_MOVABLE when there is no other consumer of the memory residing there. We are perfectly fine with a rather opportunistic and not deterministic approach as it does not require any specific support on the user-space side. For instance using page_alloc.shuffle=1 as Doug showed would be entirely acceptable even if it is fairly naive currently.

Hope this helps.
--
Florian