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

[Bob Liu]

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.

> 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.


>>> 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).

--
Regards,
Bob Liu

> regular memory. It will be reported as a regular PCIE resources and then the
> device driver will be able to determine through some flags if the link between
> the CPU(s) and the device is cache coherent or not. At that point the device
> driver can use register it with HMM helper.
> 
> 
> The whole NUMA discussion happen several time in the past i suggest looking
> on mm list archive for them. But it was rule out for several reasons. Top of
> my head:
>   - people hate CPU less node and device memory is inherently CPU less
>   - device driver want total control over memory and thus to be isolated from
>     mm mecanism and doing all those special cases was not welcome
>   - existing NUMA migration mecanism are ill suited for this memory as
>     access by the device to the memory is unknown to core mm and there
>     is no easy way to report it or track it (this kind of depends on the
>     platform and hardware)
> 
> I am likely missing other big points.
> 
> Cheers,
> Jérôme
> 
> .
>