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

[Huang, Ying]

Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx> writes:

> 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:  
>> >> >
>> >> > From  https://lpc.events/event/16/contributions/1209/attachments/1042/1995/Live%20In%20a%20World%20With%20Multiple%20Memory%20Types.pdf
>> >> > abstract_distance_offset: override by users to deal with firmware issue.
>> >> >
>> >> > say firmware can configure the cxl node into wrong tiers, similar to
>> >> > that it may also configure all cxl nodes into single memtype, hence
>> >> > all these nodes can fall into a single wrong tier.
>> >> > In this case, per node adistance_offset would be good to have ?  
>> >> 
>> >> I think that it's better to fix the error firmware if possible.  And
>> >> these are only theoretical, not practical issues.  Do you have some
>> >> practical issues?
>> >> 
>> >> I understand that users may want to move nodes between memory tiers for
>> >> different policy choices.  For that, memory_type based adistance_offset
>> >> should be good.
>> >>   
>> >
>> > There's actually an affirmative case to change memory tiering to allow
>> > either movement of nodes between tiers, or at least base placement on
>> > HMAT information. Preferably, membership would be changable to allow
>> > hotplug/DCD to be managed (there's no guarantee that the memory passed
>> > through will always be what HMAT says on initial boot).  
>> 
>> IIUC, from Jonathan Cameron as below, the performance of memory
>> shouldn't change even for DCD devices.
>> 
>> https://lore.kernel.org/linux-mm/20231103141636.000007e4@xxxxxxxxxx/
>> 
>> 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.

Thank you very much for input!  So, to support degraded performance, we
will need to move a NUMA node between memory tiers.  And, per my
understanding, we should do that in kernel.

>> 
>> So, all in all, HMAT + CDAT can help us to put the memory device in
>> appropriate memory tiers.  Now, we have HMAT support in upstream.  We
>> will working on CDAT support.
>> 

--
Best Regards,
Huang, Ying