Date:   Tue, 12 Jul 2022 10:39:48 +0530
From:   Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        Wei Xu <weixugc@...gle.com>, Yang Shi <shy828301@...il.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Tim C Chen <tim.c.chen@...el.com>,
        Michal Hocko <mhocko@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Hesham Almatary <hesham.almatary@...wei.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Alistair Popple <apopple@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Johannes Weiner <hannes@...xchg.org>, jvgediya.oss@...il.com
Subject: Re: [PATCH v8 00/12] mm/demotion: Memory tiers and demotion

On 7/12/22 10:12 AM, Aneesh Kumar K V wrote:
> On 7/12/22 6:46 AM, Huang, Ying wrote:
>> Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com> writes:
>>
>>> On 7/5/22 9:59 AM, Huang, Ying wrote:
>>>> Hi, Aneesh,
>>>>
>>>> "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com> writes:
>>>>
>>>>> The current kernel has the basic memory tiering support: Inactive
>>>>> pages on a higher tier NUMA node can be migrated (demoted) to a lower
>>>>> tier NUMA node to make room for new allocations on the higher tier
>>>>> NUMA node.  Frequently accessed pages on a lower tier NUMA node can be
>>>>> migrated (promoted) to a higher tier NUMA node to improve the
>>>>> performance.
>>>>>
>>>>> In the current kernel, memory tiers are defined implicitly via a
>>>>> demotion path relationship between NUMA nodes, which is created during
>>>>> the kernel initialization and updated when a NUMA node is hot-added or
>>>>> hot-removed.  The current implementation puts all nodes with CPU into
>>>>> the top tier, and builds the tier hierarchy tier-by-tier by establishing
>>>>> the per-node demotion targets based on the distances between nodes.
>>>>>
>>>>> This current memory tier kernel interface needs to be improved for
>>>>> several important use cases:
>>>>>
>>>>> * The current tier initialization code always initializes
>>>>>   each memory-only NUMA node into a lower tier.  But a memory-only
>>>>>   NUMA node may have a high performance memory device (e.g. a DRAM
>>>>>   device attached via CXL.mem or a DRAM-backed memory-only node on
>>>>>   a virtual machine) and should be put into a higher tier.
>>>>>
>>>>> * The current tier hierarchy always puts CPU nodes into the top
>>>>>   tier. But on a system with HBM (e.g. GPU memory) devices, these
>>>>>   memory-only HBM NUMA nodes should be in the top tier, and DRAM nodes
>>>>>   with CPUs are better to be placed into the next lower tier.
>>>>>
>>>>> * Also because the current tier hierarchy always puts CPU nodes
>>>>>   into the top tier, when a CPU is hot-added (or hot-removed) and
>>>>>   triggers a memory node from CPU-less into a CPU node (or vice
>>>>>   versa), the memory tier hierarchy gets changed, even though no
>>>>>   memory node is added or removed.  This can make the tier
>>>>>   hierarchy unstable and make it difficult to support tier-based
>>>>>   memory accounting.
>>>>>
>>>>> * A higher tier node can only be demoted to selected nodes on the
>>>>>   next lower tier as defined by the demotion path, not any other
>>>>>   node from any lower tier.  This strict, hard-coded demotion order
>>>>>   does not work in all use cases (e.g. some use cases may want to
>>>>>   allow cross-socket demotion to another node in the same demotion
>>>>>   tier as a fallback when the preferred demotion node is out of
>>>>>   space), and has resulted in the feature request for an interface to
>>>>>   override the system-wide, per-node demotion order from the
>>>>>   userspace.  This demotion order is also inconsistent with the page
>>>>>   allocation fallback order when all the nodes in a higher tier are
>>>>>   out of space: The page allocation can fall back to any node from
>>>>>   any lower tier, whereas the demotion order doesn't allow that.
>>>>>
>>>>> * There are no interfaces for the userspace to learn about the memory
>>>>>   tier hierarchy in order to optimize its memory allocations.
>>>>>
>>>>> This patch series make the creation of memory tiers explicit under
>>>>> the control of userspace or device driver.
>>>>>
>>>>> Memory Tier Initialization
>>>>> ==========================
>>>>>
>>>>> By default, all memory nodes are assigned to the default tier with
>>>>> tier ID value 200.
>>>>>
>>>>> A device driver can move up or down its memory nodes from the default
>>>>> tier.  For example, PMEM can move down its memory nodes below the
>>>>> default tier, whereas GPU can move up its memory nodes above the
>>>>> default tier.
>>>>>
>>>>> The kernel initialization code makes the decision on which exact tier
>>>>> a memory node should be assigned to based on the requests from the
>>>>> device drivers as well as the memory device hardware information
>>>>> provided by the firmware.
>>>>>
>>>>> Hot-adding/removing CPUs doesn't affect memory tier hierarchy.
>>>>>
>>>>> Memory Allocation for Demotion
>>>>> ==============================
>>>>> This patch series keep the demotion target page allocation logic same.
>>>>> The demotion page allocation pick the closest NUMA node in the
>>>>> next lower tier to the current NUMA node allocating pages from.
>>>>>
>>>>> This will be later improved to use the same page allocation strategy
>>>>> using fallback list.
>>>>>
>>>>> Sysfs Interface:
>>>>> -------------
>>>>> Listing current list of memory tiers details:
>>>>>
>>>>> :/sys/devices/system/memtier$ ls
>>>>> default_tier max_tier  memtier1  power  uevent
>>>>> :/sys/devices/system/memtier$ cat default_tier
>>>>> memtier200
>>>>> :/sys/devices/system/memtier$ cat max_tier 
>>>>> 400
>>>>> :/sys/devices/system/memtier$ 
>>>>>
>>>>> Per node memory tier details:
>>>>>
>>>>> For a cpu only NUMA node:
>>>>>
>>>>> :/sys/devices/system/node# cat node0/memtier 
>>>>> :/sys/devices/system/node# echo 1 > node0/memtier 
>>>>> :/sys/devices/system/node# cat node0/memtier 
>>>>> :/sys/devices/system/node# 
>>>>>
>>>>> For a NUMA node with memory:
>>>>> :/sys/devices/system/node# cat node1/memtier 
>>>>> 1
>>>>> :/sys/devices/system/node# ls ../memtier/
>>>>> default_tier  max_tier  memtier1  power  uevent
>>>>> :/sys/devices/system/node# echo 2 > node1/memtier 
>>>>> :/sys/devices/system/node# 
>>>>> :/sys/devices/system/node# ls ../memtier/
>>>>> default_tier  max_tier  memtier1  memtier2  power  uevent
>>>>> :/sys/devices/system/node# cat node1/memtier 
>>>>> 2
>>>>> :/sys/devices/system/node# 
>>>>>
>>>>> Removing a memory tier
>>>>> :/sys/devices/system/node# cat node1/memtier 
>>>>> 2
>>>>> :/sys/devices/system/node# echo 1 > node1/memtier
>>>>
>>>> Thanks a lot for your patchset.
>>>>
>>>> Per my understanding, we haven't reach consensus on
>>>>
>>>> - how to create the default memory tiers in kernel (via abstract
>>>>   distance provided by drivers?  Or use SLIT as the first step?)
>>>>
>>>> - how to override the default memory tiers from user space
>>>>
>>>> As in the following thread and email,
>>>>
>>>> https://lore.kernel.org/lkml/YqjZyP11O0yCMmiO@cmpxchg.org/
>>>>
>>>> I think that we need to finalized on that firstly?
>>>
>>> I did list the proposal here 
>>>
>>> https://lore.kernel.org/linux-mm/7b72ccf4-f4ae-cb4e-f411-74d055482026@linux.ibm.com
>>>
>>> So both the kernel default and driver-specific default tiers now become kernel parameters that can be updated
>>> if the user wants a different tier topology. 
>>>
>>> All memory that is not managed by a driver gets added to default_memory_tier which got a default value of 200
>>>
>>> For now, the only driver that is updated is dax kmem, which adds the memory it manages to memory tier 100.
>>> Later as we learn more about the device attributes (HMAT or something similar) that we might want to use
>>> to control the tier assignment this can be a range of memory tiers. 
>>>
>>> Based on the above, I guess we can merge what is posted in this series and later fine-tune/update
>>> the memory tier assignment based on device attributes.
>>
>> Sorry for late reply.
>>
>> As the first step, it may be better to skip the parts that we haven't
>> reached consensus yet, for example, the user space interface to override
>> the default memory tiers.  And we can use 0, 1, 2 as the default memory
>> tier IDs.  We can refine/revise the in-kernel implementation, but we
>> cannot change the user space ABI.
>>
> 
> Can you help list the use case that will be broken by using tierID as outlined in this series?
> One of the details that were mentioned earlier was the need to track top-tier memory usage in a
> memcg and IIUC the patchset posted https://lore.kernel.org/linux-mm/cover.1655242024.git.tim.c.chen@linux.intel.com
> can work with tier IDs too. Let me know if you think otherwise. So at this point
> I am not sure which area we are still debating w.r.t the userspace interface.
> 
> I will still keep the default tier IDs with a large range between them. That will allow
> us to go back to tierID based demotion order if we can. That is much simpler than using tierID and rank
> together. If we still want to go back to rank based approach the tierID value won't have much
> meaning anyway.
> 
> Any feedback on patches 1 - 5, so that I can request Andrew to merge them? 
> 

Looking at this again, I guess we just need to drop patch 7
mm/demotion: Add per node memory tier attribute to sysfs ? 

We do agree to use the device model to expose memory tiers to userspace so patch 6 can still be included.
It also exposes max_tier, default_tier, and node list of a memory tier. All these are useful
and agreed upon. Hence patch 6 can be merged? 

patch 8 - 10 -> are done based on the request from others and is independent of how memory tiers
are exposed/created from userspace. Hence that can be merged? 

If you agree I can rebase the series moving patch 7,11,12 as the last patches in the series so
that we can skip merging them based on what we conclude w.r.t usage of rank.

-aneesh

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