Re: [EXT] Re: [RFC PATCH v2 0/2] Node migration between memory tiers

[Huang, Ying]

Gregory Price <gregory.price@xxxxxxxxxxxx> writes:

> On Thu, Jan 04, 2024 at 02:05:01PM +0800, Huang, Ying wrote:
>> >
>> > From  https://lpc.events/event/16/contributions/1209/attachments/1042/1995/Live%20In%20a%20World%20With%20Multiple%20Memory%20Types.pdf
>> > abstract_distance_offset: override by users to deal with firmware issue.
>> >
>> > say firmware can configure the cxl node into wrong tiers, similar to
>> > that it may also configure all cxl nodes into single memtype, hence
>> > all these nodes can fall into a single wrong tier.
>> > In this case, per node adistance_offset would be good to have ?
>> 
>> I think that it's better to fix the error firmware if possible.  And
>> these are only theoretical, not practical issues.  Do you have some
>> practical issues?
>> 
>> I understand that users may want to move nodes between memory tiers for
>> different policy choices.  For that, memory_type based adistance_offset
>> should be good.
>> 
>
> There's actually an affirmative case to change memory tiering to allow
> either movement of nodes between tiers, or at least base placement on
> HMAT information. Preferably, membership would be changable to allow
> hotplug/DCD to be managed (there's no guarantee that the memory passed
> through will always be what HMAT says on initial boot).

IIUC, from Jonathan Cameron as below, the performance of memory
shouldn't change even for DCD devices.

https://lore.kernel.org/linux-mm/20231103141636.000007e4@xxxxxxxxxx/

It's possible to change the performance of a NUMA node changed, if we
hot-remove a memory device, then hot-add another different memory
device.  It's hoped that the CDAT changes too.

So, all in all, HMAT + CDAT can help us to put the memory device in
appropriate memory tiers.  Now, we have HMAT support in upstream.  We
will working on CDAT support.

--
Best Regards,
Huang, Ying

> https://lore.kernel.org/linux-cxl/CAAYibXjZ0HSCqMrzXGv62cMLncS_81R3e1uNV5Fu4CPm0zAtYw@xxxxxxxxxxxxxx/
>
> This group wants to enable passing CXL memory through to KVM/QEMU
> (i.e. host CXL expander memory passed through to the guest), and
> allow the guest to apply memory tiering.
>
> There are multiple issues with this, presently:
>
> 1. The QEMU CXL virtual device is not and probably never will be
>    performant enough to be a commodity class virtualization.  The
>    reason is that the virtual CXL device is built off the I/O
>    virtualization stack, which treats memory accesses as I/O accesses.
>
>    KVM also seems incompatible with the design of the CXL memory device
>    in general, but this problem may or may not be a blocker.
>
>    As a result, access to virtual CXL memory device leads to QEMU
>    crawling to a halt - and this is unlikely to change.
>
>    There is presently no good way forward to create a performant virtual
>    CXL device in QEMU.  This means the memory tiering component in the
>    kernel is functionally useless for virtual CXL memory, because...
>
> 2. When passing memory through as an explicit NUMA node, but not as
>    part of a CXL memory device, the nodes are lumped together in the
>    DRAM tier.
>
> None of this has to do with firmware.
>
> Memory-type is an awful way of denoting membership of a tier, but we
> have HMAT information that can be passed through via QEMU:
>
> -object memory-backend-ram,size=4G,id=ram-node0 \
> -object memory-backend-ram,size=4G,id=ram-node1 \
> -numa node,nodeid=0,cpus=0-4,memdev=ram-node0 \
> -numa node,initiator=0,nodeid=1,memdev=ram-node1 \
> -numa hmat-lb,initiator=0,target=0,hierarchy=memory,data-type=access-latency,latency=10 \
> -numa hmat-lb,initiator=0,target=0,hierarchy=memory,data-type=access-bandwidth,bandwidth=10485760 \
> -numa hmat-lb,initiator=0,target=1,hierarchy=memory,data-type=access-latency,latency=20 \
> -numa hmat-lb,initiator=0,target=1,hierarchy=memory,data-type=access-bandwidth,bandwidth=5242880
>
> Not only would it be nice if we could change tier membership based on
> this data, it's realistically the only way to allow guests to accomplish
> memory tiering w/ KVM/QEMU and CXL memory passed through to the guest.
>
> ~Gregory