Date:   Tue, 12 Jul 2022 23:39:33 -0700
From:   Wei Xu <weixugc@...gle.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Yang Shi <shy828301@...il.com>,
        Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>,
        Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        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 Tue, Jul 12, 2022 at 8:42 PM Huang, Ying <ying.huang@...el.com> 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.
>
> Best Regards,
> Huang, Ying
>

I also agree that it is a good idea to split this patch series into
the kernel and userspace parts.

The current sysfs interface provides more dynamic memtiers than what I
have expected.  Let's have more discussions on that after the kernel
space changes are finalized.

Wei

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