Re: [PATCH v3 0/3] create sysfs representation of ACPI HMAT

[Liubo(OS Lab)]

On 2017/12/23 6:31, Ross Zwisler wrote:
> On Fri, Dec 22, 2017 at 08:39:41AM +0530, Anshuman Khandual wrote:
>> On 12/14/2017 07:40 AM, Ross Zwisler wrote:
> <>
>>> We solve this issue by providing userspace with performance information on
>>> individual memory ranges.  This performance information is exposed via
>>> sysfs:
>>>
>>>   # grep . mem_tgt2/* mem_tgt2/local_init/* 2>/dev/null
>>>   mem_tgt2/firmware_id:1
>>>   mem_tgt2/is_cached:0
>>>   mem_tgt2/local_init/read_bw_MBps:40960
>>>   mem_tgt2/local_init/read_lat_nsec:50
>>>   mem_tgt2/local_init/write_bw_MBps:40960
>>>   mem_tgt2/local_init/write_lat_nsec:50
> <>
>> We will enlist properties for all possible "source --> target" on the system?
> 
> Nope, just 'local' initiator/target pairs.  I talk about the reasoning for
> this in the cover letter for patch 3:
> 
> https://lists.01.org/pipermail/linux-nvdimm/2017-December/013574.html
> 
>> Right now it shows only bandwidth and latency properties, can it accommodate
>> other properties as well in future ?
> 
> We also have an 'is_cached' attribute for the memory targets if they are
> involved in a caching hierarchy, but right now those are all the things we
> expose.  We can potentially expose whatever we want that is present in the
> HMAT, but those seemed like a good start.
> 
> I noticed that in your presentation you had some other examples of attributes
> you cared about:
> 
>  * reliability
>  * power consumption
>  * density
> 
> The HMAT doesn't provide this sort of information at present, but we
> could/would add them to sysfs if the HMAT ever grew support for them.
> 
>>> This allows applications to easily find the memory that they want to use.
>>> We expect that the existing NUMA APIs will be enhanced to use this new
>>> information so that applications can continue to use them to select their
>>> desired memory.
>>
>> I had presented a proposal for NUMA redesign in the Plumbers Conference this
>> year where various memory devices with different kind of memory attributes
>> can be represented in the kernel and be used explicitly from the user space.
>> Here is the link to the proposal if you feel interested. The proposal is
>> very intrusive and also I dont have a RFC for it yet for discussion here.
>>
>> https://linuxplumbersconf.org/2017/ocw//system/presentations/4656/original/Hierarchical_NUMA_Design_Plumbers_2017.pdf
>>
>> Problem is, designing the sysfs interface for memory attribute detection
>> from user space without first thinking about redesigning the NUMA for
>> heterogeneous memory may not be a good idea. Will look into this further.
> 
> I took another look at your presentation, and overall I think that if/when a
> NUMA redesign like this takes place ACPI systems with HMAT tables will be able
> to participate.  But I think we are probably a ways away from that, and like I

I'm afraid not, there are cache-coherent bus like CCIX/OpenCAPI come out soon.
No matter to say System-on-Chip already with internal bus linked DDRãHBMãCPUãAccelerator..

> said in my previous mail ACPI systems with memory-only NUMA nodes are going to
> exist and need to be supported with the current NUMA scheme.  Hence I don't

And not only memory-only, but the accelerators can also be a master like CPU.

> think that this patch series conflicts with your proposal.

Didn't see conflict neither, but perhaps we should think for a longer-term solution and cover more
situations/platforms.
Anshuman's proposal is really a good start point to us.

Cheers,
Bob Liu