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

[Hao Xiang]

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.

>
> Jonathan
>
>
>
> >
> > >
> > > </2cents>
> > >
> > > ~Gregory
>