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

[Dragan Stancevic <dragan@stancevic.com>]

Hi Ying-

On 4/4/23 01:47, Huang, Ying wrote:
> 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)?

I like the CXL.mem as a NUMA node approach, and also think it's best to 
do this with move/migrate_pages and numactl and those a & b are good 
choices.

I think there is an option c too though, which is an amalgamation of a & 
b. Here is my thinking, and please do let me know what you think about 
this approach.

If you think about CXL 3.0 shared/switched memory as a portal for a VM 
to move from one hypervisor to another, I think each switched memory 
should be represented by it's own node and have a distinct type so the 
migration path becomes more deterministic. I was thinking along the 
lines that there would be some kind of user space clustering/migration 
app/script that runs on all the hypervisors. Which would read, let's say 
/proc/pagetypeinfo to find these "portals":
Node 4, zone Normal, type Switched ....
Node 6, zone Normal, type Switched ....

Then it would build a traversal Graph, find per hypervisor reach and 
critical connections, where critical connections are cross-rack or 
cross-pod, perhaps something along the lines of this pseudo/python code:
class Graph:
	def __init__(self, mydict):
		self.dict = mydict
		self.visited = set()
		self.critical = list()
		self.reach = dict()
		self.id = 0
	def depth_first_search(self, vertex, parent):
		self.visited.add(vertex)
		if vertex not in self.reach:
			self.reach[vertex] = {'id':self.id, 'reach':self.id}
			self.id += 1
		for next_vertex in self.dict[vertex] - {parent}:
			if next_vertex not in self.visited:
				self.depth_first_search(next_vertex, vertex)
			if self.reach[next_vertex]['reach'] < self.reach[vertex]['reach']:
				self.reach[vertex]['reach'] = self.reach[next_vertex]['reach']
		if parent != None and self.reach[vertex]['id'] == 
self.reach[vertex]['reach']:
			self.critical.append([parent, vertex])
		return self.critical

critical = mygraph.depth_first_search("hostname-foo4", None)

that way you could have a VM migrate between only two hypervisors 
sharing switched memory, or pass through a subset of hypervisors (that 
don't necessarily share switched memory) to reach it's destination. This 
may be rack confined, or across a rack or even a pod using critical 
connections.

Long way of saying that if you do a) then the clustering/migration 
script only sees a bunch of nodes and a bunch of normal zones it 
wouldn't know how to build the "flight-path" and where to send a VM. 
You'd probably have to add an additional interface in the kernel for the 
script to query the paths somehow, where on the other hand pulling 
things from proc/sys is easy.


And then if you do b) and put it in an existing NUMA and with a 
"Switched" type, you could potentially end up with several "Switched" 
types under the same node. So when you numactl/move/migrate pages they 
could go in either direction and you could send some pages through one 
"portal" and others through another "portal", which is not what you want 
to do.

That's why I think the c option might be the most optimal, where each 
switched memory has it's own node number. And then displaying type as 
"Switched" just makes it easier to detect and Graph the topology.


And with regards to an ABI, I was referring to an ABI needed between the 
kernels running on separate hypervisors. When hypervisor B boots, it 
needs to detect through an ABI if this switched/shared memory is already 
initialized and if there are VMs in there which are used by another 
hypervisor, say A. Also during the migration, hypervisors A and B would 
have to use this ABI to synchronize the hand-off between the two 
physical hosts. Not an all-inclusive list, but I was referring to those 
types of scenarios.

What do you think?


--
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of universal enlightenment -Dr. Nikola Tesla