Re: [PATCH v3 updated] mm/demotion: Expose memory tier details via sysfs

[Huang, Ying]

Aneesh Kumar K V <aneesh.kumar@xxxxxxxxxxxxx> writes:

> On 9/1/22 12:31 PM, Huang, Ying wrote:
>> "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxx> writes:
>> 
>>> This patch adds /sys/devices/virtual/memory_tiering/ where all memory tier
>>> related details can be found. All allocated memory tiers will be listed
>>> there as /sys/devices/virtual/memory_tiering/memory_tierN/
>>>
>>> The nodes which are part of a specific memory tier can be listed via
>>> /sys/devices/virtual/memory_tiering/memory_tierN/nodes
>> 
>> I think "memory_tier" is a better subsystem/bus name than
>> memory_tiering.  Because we have a set of memory_tierN devices inside.
>> "memory_tier" sounds more natural.  I know this is subjective, just my
>> preference.
>> 
>>>
>>> A directory hierarchy looks like
>>> :/sys/devices/virtual/memory_tiering$ tree memory_tier4/
>>> memory_tier4/
>>> ├── nodes
>>> ├── subsystem -> ../../../../bus/memory_tiering
>>> └── uevent
>>>
>>> All toptier nodes are listed via
>>> /sys/devices/virtual/memory_tiering/toptier_nodes
>>>
>>> :/sys/devices/virtual/memory_tiering$ cat toptier_nodes
>>> 0,2
>>> :/sys/devices/virtual/memory_tiering$ cat memory_tier4/nodes
>>> 0,2
>> 
>> I don't think that it is a good idea to show toptier information in user
>> space interface.  Because it is just a in kernel implementation
>> details.  Now, we only promote pages from !toptier to toptier.  But
>> there may be multiple memory tiers in toptier and !toptier, we may
>> change the implementation in the future.  For example, we may promote
>> pages from DRAM to HBM in the future.
>> 
>
>
> In the case you describe above and others, we will always have a list of
> NUMA nodes from which memory promotion is not done.
> /sys/devices/virtual/memory_tiering/toptier_nodes shows that list.

I don't think we will need that interface if we don't restrict promotion
in the future.  For example, he can just check the memory tier with
smallest number.

TBH, I don't know why do we need that interface.  What is it for?  We
don't want to expose unnecessary information to restrict our in kernel
implementation in the future.

So, please remove that interface at least before we discussing it
thoroughly.

>> Do we need a way to show the default memory tier in sysfs?  That is, the
>> memory tier that the DRAM nodes belong to.
>> 
>
> I will hold adding that until we have support for modifying memory tier details from
> userspace. That is when userspace would want to know about the default memory tier. 
>
> For now, the user interface is a simpler hierarchy of memory tiers, it's associated
> nodes and the list of nodes from which promotion is not done.

OK.

Best Regards,
Huang, Ying