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

[Huang, Ying]

Hao Xiang <hao.xiang@xxxxxxxxxxxxx> writes:

> On Wed, Jan 10, 2024 at 6:18 AM Jonathan Cameron
> <Jonathan.Cameron@xxxxxxxxxx> wrote:
>>
>> On Tue, 9 Jan 2024 16:28:15 -0800
>> Hao Xiang <hao.xiang@xxxxxxxxxxxxx> wrote:
>>
>> > On Tue, Jan 9, 2024 at 9:59 AM Gregory Price <gregory.price@xxxxxxxxxxxx> wrote:
>> > >
>> > > On Tue, Jan 09, 2024 at 03:50:49PM +0000, Jonathan Cameron wrote:
>> > > > On Tue, 09 Jan 2024 11:41:11 +0800
>> > > > "Huang, Ying" <ying.huang@xxxxxxxxx> wrote:
>> > > > > Gregory Price <gregory.price@xxxxxxxxxxxx> writes:
>> > > > > > On Thu, Jan 04, 2024 at 02:05:01PM +0800, Huang, Ying wrote:
>> > > > > 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.
>> > > >
>> > > > Not supported, but ACPI has _HMA methods to in theory allow changing
>> > > > HMAT values based on firmware notifications...  So we 'could' make
>> > > > it work for HMAT based description.
>> > > >
>> > > > Ultimately my current thinking is we'll end up emulating CXL type3
>> > > > devices (hiding topology complexity) and you can update CDAT but
>> > > > IIRC that is only meant to be for degraded situations - so if you
>> > > > want multiple performance regions, CDAT should describe them form the start.
>> > > >
>> > >
>> > > That was my thought.  I don't think it's particularly *realistic* for
>> > > HMAT/CDAT values to change at runtime, but I can imagine a case where
>> > > it could be valuable.
>> > >
>> > > > > > 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.
>> > > >
>> > > > I'd flex that a bit - we will end up with a solution for virtualization but
>> > > > it isn't the emulation that is there today because it's not possible to
>> > > > emulate some of the topology in a peformant manner (interleaving with sub
>> > > > page granularity / interleaving at all (to a lesser degree)). There are
>> > > > ways to do better than we are today, but they start to look like
>> > > > software dissagregated memory setups (think lots of page faults in the host).
>> > > >
>> > >
>> > > Agreed, the emulated device as-is can't be the virtualization device,
>> > > but it doesn't mean it can't be the basis for it.
>> > >
>> > > My thought is, if you want to pass host CXL *memory* through to the
>> > > guest, you don't actually care to pass CXL *control* through to the
>> > > guest.  That control lies pretty squarely with the host/hypervisor.
>> > >
>> > > So, at least in theory, you can just cut the type3 device out of the
>> > > QEMU configuration entirely and just pass it through as a distinct numa
>> > > node with specific hmat qualities.
>> > >
>> > > Barring that, if we must go through the type3 device, the question is
>> > > how difficult would it be to just make a stripped down type3 device
>> > > to provide the informational components, but hack off anything
>> > > topology/interleave related? Then you just do direct passthrough as you
>> > > described below.
>> > >
>> > > qemu/kvm would report errors if you tried to touch the naughty bits.
>> > >
>> > > The second question is... is that device "compliant" or does it need
>> > > super special handling from the kernel driver :D?  If what i described
>> > > is not "compliant", then it's probably a bad idea, and KVM/QEMU should
>> > > just hide the CXL device entirely from the guest (for this use case)
>> > > and just pass the memory through as a numa node.
>> > >
>> > > Which gets us back to: The memory-tiering component needs a way to
>> > > place nodes in different tiers based on HMAT/CDAT/User Whim. All three
>> > > of those seem like totally valid ways to go about it.
>> > >
>> > > > > >
>> > > > > > 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.
>> > > >
>> > > > This I fully agree with.  There will be systems with a bunch of normal DDR with different
>> > > > access characteristics irrespective of CXL. + likely HMAT solutions will be used
>> > > > before we get anything more complex in place for CXL.
>> > > >
>> > >
>> > > Had not even considered this, but that's completely accurate as well.
>> > >
>> > > And more discretely: What of devices that don't provide HMAT/CDAT? That
>> > > isn't necessarily a violation of any standard.  There probably could be
>> > > a release valve for us to still make those devices useful.
>> > >
>> > > The concern I have with not implementing a movement mechanism *at all*
>> > > is that a one-size-fits-all initial-placement heuristic feels gross
>> > > when we're, at least ideologically, moving toward "software defined memory".
>> > >
>> > > Personally I think the movement mechanism is a good idea that gets folks
>> > > where they're going sooner, and it doesn't hurt anything by existing We
>> > > can change the initial placement mechanism too.
>> >
>> > I think providing users a way to "FIX" the memory tiering is a backup
>> > option. Given that DDRs with different access characteristics provide
>> > the relevant CDAT/HMAT information, the kernel should be able to
>> > correctly establish memory tiering on boot.
>>
>> Include hotplug and I'll be happier!  I know that's messy though.
>>
>> > Current memory tiering code has
>> > 1) memory_tier_init() to iterate through all boot onlined memory
>> > nodes. All nodes are assumed to be fast tier (adistance
>> > MEMTIER_ADISTANCE_DRAM is used).
>> > 2) dev_dax_kmem_probe to iterate through all devdax controlled memory
>> > nodes. This is the place the kernel reads the memory attributes from
>> > HMAT and recognizes the memory nodes into the correct tier (devdax
>> > controlled CXL, pmem, etc).
>> > If we want DDRs with different memory characteristics to be put into
>> > the correct tier (as in the guest VM memory tiering case), we probably
>> > need a third path to iterate the boot onlined memory nodes and also be
>> > able to read their memory attributes. I don't think we can do that in
>> > 1) because the ACPI subsystem is not yet initialized.
>>
>> Can we move it later in general?  Or drag HMAT parsing earlier?
>> ACPI table availability is pretty early, it's just that we don't bother
>> with HMAT because nothing early uses it.
>> IIRC SRAT parsing occurs way before memory_tier_init() will be called.
>
> I tested the call sequence under a debugger earlier. hmat_init() is
> called after memory_tier_init(). Let me poke around and see what our
> options are.

This sounds reasonable.

Please keep in mind that we need a way to identify the base line memory
type(default_dram_type).  A simple method is to use NUMA nodes with CPU
attached.  But I remember that Aneesh said that some NUMA nodes without
CPU will need to be put in default_dram_type too on their systems.  We
need a way to identify that.

--
Best Regards,
Huang, Ying