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

Aneesh Kumar K V
2022-09-02T05:24:20Z

On 9/2/22 10:39 AM, Wei Xu wrote:
> On Thu, Sep 1, 2022 at 5:33 PM Huang, Ying <ying.huang@intel.com> wrote:
>>
>> Aneesh Kumar K V <aneesh.kumar@linux.ibm.com> writes:
>>
>>> On 9/1/22 12:31 PM, Huang, Ying wrote:
>>>> "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com> 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.
>>>>

I missed replying to this earlier. I will keep memory_tiering as subsystem name in v4
because we would want it to a susbsystem where all memory tiering related details can be found
including memory type in the future. This is as per discussion

https://lore.kernel.org/linux-mm/CAAPL-u9TKbHGztAF=r-io3gkX7gorUunS2UfstudCWuihrA=0g@mail.gmail.com

>>>>>
>>>>> 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.
>
> I have asked for this interface to allow the userspace to query a list
> of top-tier nodes as the targets of userspace-driven promotions. The
> idea is that demotion can gradually go down tier by tier, but we
> promote hot pages directly to the top-tier and bypass the immediate
> tiers.
>
> Certainly, this can be viewed as a policy choice. Given that now we
> have a clearly defined memory tier hierarchy in sysfs and the
> toptier_nodes content can be constructed from this memory tier
> hierarchy and other information from the node sysfs interfaces, I am
> fine if we want to remove toptier_nodes and keep the current memory
> tier sysfs interfaces to the minimal.
>

Ok I can do a v4 with toptier_nodes dropped.

-aneesh