Date:   Wed, 13 Jul 2022 16:20:34 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>
Cc:     Yang Shi <shy828301@...il.com>, Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Wei Xu <weixugc@...gle.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

Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com> writes:

> On 7/13/22 9:12 AM, Huang, Ying wrote:
>> Yang Shi <shy828301@...il.com> writes:
>> 
>>> On Mon, Jul 11, 2022 at 10:10 PM Aneesh Kumar K V
>>> <aneesh.kumar@...ux.ibm.com> wrote:
>>>>
>>>> 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.
>>>
>>> I think the most controversial part is the user visible interfaces so
>>> far. And IIUC the series could be split roughly into two parts, patch
>>> 1 - 5 and others. The patch 1 -5 added the explicit memory tier
>>> support and fixed the issue reported by Jagdish. I think we are on the
>>> same page for this part. But I haven't seen any thorough review on
>>> those patches yet since we got distracted by spending most time
>>> discussing about the user visible interfaces.
>>>
>>> So would it help to move things forward to submit patch 1 - 5 as a
>>> standalone series to get thorough review then get merged?
>> 
>> Yes.  I think this is a good idea.  We can discuss the in kernel
>> implementation (without user space interface) in details and try to make
>> it merged.
>> 
>> And we can continue our discussion of user space interface in a separate
>> thread.
>
> Thanks. I will post patch 1 - 5 as a series for review.

I think that you should add 8-10 too, that is, all in-kernel
implementation except the user space interface part.  Although I think
we should squash 8/12 personally.  We can discuss that further during
review.

Best Regards,
Huang, Ying

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