Re: FW: [LSF/MM/BPF TOPIC] SMDK inspired MM changes for CXL

["Huang, Ying" <ying.huang@intel.com>]

Dragan Stancevic <dragan@stancevic.com> writes:

> Hi Mike,
>
> On 4/3/23 03:44, Mike Rapoport wrote:
>> Hi Dragan,
>> On Thu, Mar 30, 2023 at 05:03:24PM -0500, Dragan Stancevic wrote:
>>> On 3/26/23 02:21, Mike Rapoport wrote:
>>>> Hi,
>>>>
>>>> [..] >> One problem we experienced was occured in the combination of
>>> hot-remove and kerelspace allocation usecases.
>>>>> ZONE_NORMAL allows kernel context allocation, but it does not allow hot-remove because kernel resides all the time.
>>>>> ZONE_MOVABLE allows hot-remove due to the page migration, but it only allows userspace allocation.
>>>>> Alternatively, we allocated a kernel context out of ZONE_MOVABLE by adding GFP_MOVABLE flag.
>>>>> In case, oops and system hang has occasionally occured because ZONE_MOVABLE can be swapped.
>>>>> We resolved the issue using ZONE_EXMEM by allowing seletively choice of the two usecases.
>>>>> As you well know, among heterogeneous DRAM devices, CXL DRAM is the first PCIe basis device, which allows hot-pluggability, different RAS, and extended connectivity.
>>>>> So, we thought it could be a graceful approach adding a new zone and separately manage the new features.
>>>>
>>>> This still does not describe what are the use cases that require having
>>>> kernel allocations on CXL.mem.
>>>>
>>>> I believe it's important to start with explanation *why* it is important to
>>>> have kernel allocations on removable devices.
>>>
>>> Hi Mike,
>>>
>>> not speaking for Kyungsan here, but I am starting to tackle hypervisor
>>> clustering and VM migration over cxl.mem [1].
>>>
>>> And in my mind, at least one reason that I can think of having kernel
>>> allocations from cxl.mem devices is where you have multiple VH connections
>>> sharing the memory [2]. Where for example you have a user space application
>>> stored in cxl.mem, and then you want the metadata about this
>>> process/application that the kernel keeps on one hypervisor be "passed on"
>>> to another hypervisor. So basically the same way processors in a single
>>> hypervisors cooperate on memory, you extend that across processors that span
>>> over physical hypervisors. If that makes sense...
>> Let me reiterate to make sure I understand your example.
>> If we focus on VM usecase, your suggestion is to store VM's memory and
>> associated KVM structures on a CXL.mem device shared by several nodes.
>
> Yes correct. That is what I am exploring, two different approaches:
>
> Approach 1: Use CXL.mem for VM migration between hypervisors. In this
> approach the VM and the metadata executes/resides on a traditional
> NUMA node (cpu+dram) and only uses CXL.mem to transition between
> hypervisors. It's not kept permanently there. So basically on
> hypervisor A you would do something along the lines of migrate_pages
> into cxl.mem and then on hypervisor B you would migrate_pages from
> cxl.mem and onto the regular NUMA node (cpu+dram).
>
> Approach 2: Use CXL.mem to cluster hypervisors to improve high
> availability of VMs. In this approach the VM and metadata would be
> kept in CXL.mem permanently and each hypervisor accessing this shared
> memory could have the potential to schedule/run the VM if the other
> hypervisor experienced a failure.
>
>> Even putting aside the aspect of keeping KVM structures on presumably
>> slower memory, 
>
> Totally agree, presumption of memory speed dully noted. As far as I am
> aware, CXL.mem at this point has higher latency than DRAM, and
> switched CXL.mem has an additional latency. That may or may not change
> in the future, but even with actual CXL induced latency I think there
> are benefits to the approaches.
>
> In the example #1 above, I think even if you had a very noisy VM that
> is dirtying pages at a high rate, once migrate_pages has occurred, it 
> wouldn't have to be quiesced for the migration to happen. A migration
> could basically occur in-between the CPU slices, once VCPU is done
> with it's slice on hypervisor A, the next slice could be on hypervisor
> B.
>
> And the example #2 above, you are trading memory speed for
> high-availability. Where either hypervisor A or B could run the CPU
> load of the VM. You could even have a VM where some of the VCPUs are 
> executing on hypervisor A and others on hypervisor B to be able to
> shift CPU load across hypervisors in quasi real-time.
>
>
>> what ZONE_EXMEM will provide that cannot be accomplished
>> with having the cxl memory in a memoryless node and using that node to
>> allocate VM metadata?
>
> It has crossed my mind to perhaps use NUMA node distance for the two
> approaches above. But I think that is not sufficient because we can
> have varying distance, and distance in itself doesn't indicate 
> switched/shared CXL.mem or non-switched/non-shared CXL.mem. Strictly
> speaking just for myself here, with the two approaches above, the 
> crucial differentiator in order for #1 and #2 to work would be that
> switched/shared CXL.mem would have to be indicated as such in a way. 
> Because switched memory would have to be treated and formatted in some
> kind of ABI way that would allow hypervisors to cooperate and follow 
> certain protocols when using this memory.
>
>
> I can't answer what ZONE_EXMEM will provide since we haven's seen
> Kyungsan's talk yet, that's why I myself was very curious to find out 
> more about ZONE_EXMEM proposal and if it includes some provisions for
> CXL switched/shared memory.
>
> To me, I don't think it makes a difference if pages are coming from
> ZONE_NORMAL, or ZONE_EXMEM but the part that I was curious about was
> if I could allocate from or migrate_pages to (ZONE_EXMEM | type 
> "SWITCHED/SHARED"). So it's not the zone that is crucial for me,  it's
> the typing. That's what I meant with my initial response but I guess
> it wasn't clear enough, "_if_ ZONE_EXMEM had some typing mechanism, in
> my case, this is where you'd have kernel allocations on CXL.mem"
>

We have 2 choices here.

a) Put CXL.mem in a separate NUMA node, with an existing ZONE type
(normal or movable).  Then you can migrate pages there with
move_pages(2) or migrate_pages(2).  Or you can run your workload on the
CXL.mem with numactl.

b) Put CXL.mem in an existing NUMA node, with a new ZONE type.  To
control your workloads in user space, you need a set of new ABIs.
Anything you cannot do in a)?

Best Regards,
Huang, Ying