Re: [HMM-v25 19/19] mm/hmm: add new helper to hotplug CDM memory region v3

[Jerome Glisse]

On Tue, Sep 05, 2017 at 11:50:57AM +0800, Bob Liu wrote:
> On 2017/9/5 10:38, Jerome Glisse wrote:
> > On Tue, Sep 05, 2017 at 09:13:24AM +0800, Bob Liu wrote:
> >> On 2017/9/4 23:51, Jerome Glisse wrote:
> >>> On Mon, Sep 04, 2017 at 11:09:14AM +0800, Bob Liu wrote:
> >>>> On 2017/8/17 8:05, Jérôme Glisse wrote:
> >>>>> Unlike unaddressable memory, coherent device memory has a real
> >>>>> resource associated with it on the system (as CPU can address
> >>>>> it). Add a new helper to hotplug such memory within the HMM
> >>>>> framework.
> >>>>>
> >>>>
> >>>> Got an new question, coherent device( e.g CCIX) memory are likely reported to OS 
> >>>> through ACPI and recognized as NUMA memory node.
> >>>> Then how can their memory be captured and managed by HMM framework?
> >>>>
> >>>
> >>> Only platform that has such memory today is powerpc and it is not reported
> >>> as regular memory by the firmware hence why they need this helper.
> >>>
> >>> I don't think anyone has defined anything yet for x86 and acpi. As this is
> >>
> >> Not yet, but now the ACPI spec has Heterogeneous Memory Attribute
> >> Table (HMAT) table defined in ACPI 6.2.
> >> The HMAT can cover CPU-addressable memory types(though not non-cache
> >> coherent on-device memory).
> >>
> >> Ross from Intel already done some work on this, see:
> >> https://lwn.net/Articles/724562/
> >>
> >> arm64 supports APCI also, there is likely more this kind of device when CCIX
> >> is out (should be very soon if on schedule).
> > 
> > HMAT is not for the same thing, AFAIK HMAT is for deep "hierarchy" memory ie
> > when you have several kind of memory each with different characteristics:
> >   - HBM very fast (latency) and high bandwidth, non persistent, somewhat
> >     small (ie few giga bytes)
> >   - Persistent memory, slower (both latency and bandwidth) big (tera bytes)
> >   - DDR (good old memory) well characteristics are between HBM and persistent
> > 
> 
> Okay, then how the kernel handle the situation of "kind of memory each with different characteristics"?
> Does someone have any suggestion?  I thought HMM can do this.
> Numa policy/node distance is good but perhaps require a few extending, e.g a HBM node can't be
> swap, can't accept DDR fallback allocation.

I don't think there is any consensus for this. I put forward the idea that NUMA
needed to be extended as with deep hierarchy it is not only the distance between
two nodes but also others factors like persistency, bandwidth, latency ...


> > So AFAICT this has nothing to do with what HMM is for, ie device memory. Note
> > that device memory can have a hierarchy of memory themself (HBM, GDDR and in
> > maybe even persistent memory).
> > 
> 
> This looks like a subset of HMAT when CPU can address device memory directly in cache-coherent way.

It is not, it is much more complex than that. Linux kernel has no idea on what is
going on a device and thus do not have any usefull informations to make proper
decission regarding device memory. Here device is real device ie something with
processing capability, not something like HBM or persistent memory even if the
latter is associated with a struct device inside linux kernel.

> 
> 
> >>> memory on PCIE like interface then i don't expect it to be reported as NUMA
> >>> memory node but as io range like any regular PCIE resources. Device driver
> >>> through capabilities flags would then figure out if the link between the
> >>> device and CPU is CCIX capable if so it can use this helper to hotplug it
> >>> as device memory.
> >>>
> >>
> >> From my point of view,  Cache coherent device memory will popular soon and
> >> reported through ACPI/UEFI. Extending NUMA policy still sounds more reasonable
> >> to me.
> > 
> > Cache coherent device will be reported through standard mecanisms defined by
> > the bus standard they are using. To my knowledge all the standard are either
> > on top of PCIE or are similar to PCIE.
> > 
> > It is true that on many platform PCIE resource is manage/initialize by the
> > bios (UEFI) but it is platform specific. In some case we reprogram what the
> > bios pick.
> > 
> > So like i was saying i don't expect the BIOS/UEFI to report device memory as
> 
> But it's happening.
> In my understanding, that's why HMAT was introduced.
> For reporting device memory as regular memory(with different characteristics).

That is not my understanding but only Intel can confirm. HMAT was introduced
for things like HBM or persistent memory. Which i do not consider as device
memory. Sure persistent memory is assign a device struct because it is easier
for integration with the block system i assume. But it does not make it a
device in my view. For me a device is a piece of hardware that has some
processing capabilities (network adapter, sound card, GPU, ...)

But we can argue about semantic and what a device is. For all intent and purposes
device in HMM context is some piece of hardware with processing capabilities and
local device memory.

Cheers,
Jérôme