Dragan Stancevic <dragan@xxxxxxxxxxxxx> 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
