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

[Bob Liu]

On 2017/9/6 2:54, Ross Zwisler wrote:
> On Mon, Sep 04, 2017 at 10:38:27PM -0400, 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
>>
>> 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).
>>
>>>> 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
>> 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
> 
> With the introduction of the HMAT in ACPI 6.2 one of the things that was added
> was the ability to have an ACPI proximity domain that isn't associated with a
> CPU.  This can be seen in the changes in the text of the "Proximity Domain"
> field in table 5-73 which describes the "Memory Affinity Structure".  One of
> the major features of the HMAT was the separation of "Initiator" proximity
> domains (CPUs, devices that initiate memory transfers), and "target" proximity
> domains (memory regions, be they attached to a CPU or some other device).
> 
> ACPI proximity domains map directly to Linux NUMA nodes, so I think we're
> already in a place where we have to support CPU-less NUMA nodes.
> 
>>   - device driver want total control over memory and thus to be isolated from
>>     mm mecanism and doing all those special cases was not welcome
> 
> I agree that the kernel doesn't have enough information to be able to
> accurately handle all the use cases for the various types of heterogeneous
> memory.   The goal of my HMAT enabling is to allow that memory to be reserved
> from kernel use via the "Reservation Hint" in the HMAT's Memory Subsystem
> Address Range Structure, then provide userspace with enough information to be
> able to distinguish between the various types of memory in the system so it
> can allocate & utilize it appropriately.
> 

Does this mean require an user space memory management library to deal with all alloc/free/defragment..
But how to do with virtual <-> physical address mapping from userspace?

--
Regards,
Bob Liu

>>   - 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