Message-ID: <20220512160010.00005bc4@Huawei.com>
Date:   Thu, 12 May 2022 16:00:10 +0100
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
To:     Wei Xu <weixugc@...gle.com>
CC:     Huang Ying <ying.huang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Thelen <gthelen@...gle.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Yang Shi <shy828301@...il.com>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        Jagdish Gediya <jvgediya@...ux.ibm.com>,
        Michal Hocko <mhocko@...nel.org>,
        Tim C Chen <tim.c.chen@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        "Alistair Popple" <apopple@...dia.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Feng Tang <feng.tang@...el.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        "Dan Williams" <dan.j.williams@...el.com>,
        David Rientjes <rientjes@...gle.com>,
        Linux MM <linux-mm@...ck.org>,
        Brice Goglin <brice.goglin@...il.com>,
        "Hesham Almatary" <hesham.almatary@...wei.com>
Subject: Re: RFC: Memory Tiering Kernel Interfaces (v2)

On Wed, 11 May 2022 23:22:11 -0700
Wei Xu <weixugc@...gle.com> wrote:
> 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.
> 
> I'd like to propose revised memory tier kernel interfaces based on
> the discussions in the threads:
> 
> - https://lore.kernel.org/lkml/20220425201728.5kzm4seu7rep7ndr@offworld/T/
> - https://lore.kernel.org/linux-mm/20220426114300.00003ad8@Huawei.com/t/
> - https://lore.kernel.org/linux-mm/867bc216386eb6cbf54648f23e5825830f5b922e.camel@intel.com/T/
> 
> 
> High-level Design Ideas
> =======================
> 
> * Define memory tiers explicitly, not implicitly.
> 
> * Memory tiers are defined based on hardware capabilities of memory
>   nodes, not their relative node distances between each other.
> 
> * The tier assignment of each node is independent from each other.
>   Moving a node from one tier to another tier doesn't affect the tier
>   assignment of any other node.
> 
> * The node-tier association is stable. A node can be reassigned to a
>   different tier only under the specific conditions that don't block
>   future tier-based memory cgroup accounting.
> 
> * A node can demote its pages to any nodes of any lower tiers. The
>   demotion target node selection follows the allocation fallback order
>   of the source node, which is built based on node distances.  The
>   demotion targets are also restricted to only the nodes from the tiers
>   lower than the source node.  We no longer need to maintain a separate
>   per-node demotion order (node_demotion[]).
> 

Hi Wei,

This proposal looks good to me, though we'll be having fun
white boarding topologies from our roadmaps for the next few days :)

A few comments inline. It also seems likely to me that there is little
benefit in starting with 3 tiers as the maximum.  Seems unlikely the
code will be substantially simpler for 3 than it would be for 4 or 5.
I've drawn out one simple case that needs 4 to do sensible things.

> 
> Sysfs Interfaces
> ================
> 
> * /sys/devices/system/memtier/memtierN/nodelist
> 
>   where N = 0, 1, 2 (the kernel supports only 3 tiers for now).
> 
>   Format: node_list
> 
>   Read-only.  When read, list the memory nodes in the specified tier.
> 
>   Tier 0 is the highest tier, while tier 2 is the lowest tier.
> 
>   The absolute value of a tier id number has no specific meaning.
>   What matters is the relative order of the tier id numbers.
> 
>   When a memory tier has no nodes, the kernel can hide its memtier
>   sysfs files.
> 
> * /sys/devices/system/node/nodeN/memtier
> 
>   where N = 0, 1, ...
> 
>   Format: int or empty
> 
>   When read, list the memory tier that the node belongs to.  Its value
>   is empty for a CPU-only NUMA node.
> 
>   When written, the kernel moves the node into the specified memory
>   tier if the move is allowed.  The tier assignment of all other nodes
>   are not affected.
> 
>   Initially, we can make this interface read-only.
> 
> 
> Kernel Representation
> =====================
> 
> * All memory tiering code is guarded by CONFIG_TIERED_MEMORY.
> 
> * #define MAX_MEMORY_TIERS 3
> 
>   Support 3 memory tiers for now.
> 
> * #define MEMORY_DEFAULT_TIER 1
> 
>   The default tier that a memory node is assigned to.
> 
> * nodemask_t memory_tiers[MAX_MEMORY_TIERS]
> 
>   Store memory nodes by tiers.
> 
> * int node_tier_map[MAX_NUMNODES]
> 
>   Map a node to its tier.
> 
>   For each CPU-only node c, node_tier_map[c] = -1.
> 
> 
> Memory Tier Initialization
> ==========================
> 
> By default, all memory nodes are assigned to the default tier
> (MEMORY_DEFAULT_TIER).

This is tighter than it needs to be.  In many cases we can easily
establish if there is any possibility of CPU being hotplugged into
a memory node.  If it's CXL attached no way CPUs are going to be
turning up their later :)  If CPU HP into a given node can't happen
we can be more flexible and I think that often results in better decisions.
See example below, though obviously I could just use the userspace
interface to fix that up anyway or have a CXL driver move it around
if that's relevant.  In some other cases I'm fairly sure we know in
advance where CPUs can be added but I'd need to check all the
relevant specs to be sure there aren't any corner cases.  I 'think'
for ARM for example we know where all possible CPUs can be hotplugged
(constraint coming from the interrupt controller + the fact that only
virtual CPU HP is defined).

> 
> 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.
> 
> 
> Memory Tier Reassignment
> ========================
> 
> After a memory node is hot-removed, it can be hot-added back to a
> different memory tier.  This is useful for supporting dynamically
> provisioned CXL.mem NUMA nodes, which may connect to different
> memory devices across hot-plug events.  Such tier changes should
> be compatible with tier-based memory accounting.
> 
> The userspace may also reassign an existing online memory node to a
> different tier.  However, this should only be allowed when no pages
> are allocated from the memory node or when there are no non-root
> memory cgroups (e.g. during the system boot).  This restriction is
> important for keeping memory tier hierarchy stable enough for
> tier-based memory cgroup accounting.
> 
> Hot-adding/removing CPUs doesn't affect memory tier hierarchy.
> 
> 
> Memory Allocation for Demotion
> ==============================
> 
> To allocate a new page as the demotion target for a page, the kernel
> calls the allocation function (__alloc_pages_nodemask) with the
> source page node as the preferred node and the union of all lower
> tier nodes as the allowed nodemask.  The actual target node selection
> then follows the allocation fallback order that the kernel has
> already defined.
> 
> The pseudo code looks like:
> 
>     targets = NODE_MASK_NONE;
>     src_nid = page_to_nid(page);
>     src_tier = node_tier_map[src_nid];
>     for (i = src_tier + 1; i < MAX_MEMORY_TIERS; i++)
>             nodes_or(targets, targets, memory_tiers[i]);
>     new_page = __alloc_pages_nodemask(gfp, order, src_nid, targets);
> 
> The memopolicy of cpuset, vma and owner task of the source page can
> be set to refine the demotion target nodemask, e.g. to prevent
> demotion or select a particular allowed node as the demotion target.
> 
> 
> Memory Allocation for Promotion
> ===============================
> 
> The page allocation for promotion is similar to demotion, except that (1)
> the target nodemask uses the promotion tiers, (2) the preferred node can
> be the accessing CPU node, not the source page node.
> 
> 
> Examples
> ========
> 

...

> * Example 3:
> 
> Node 0 & 1 are DRAM nodes, Node 2 is a memory-only DRAM node.

Node2 is drawn as pmem.

> 
> All nodes are in the same tier.
> 
>                   20
>   Node 0 (DRAM)  ----  Node 1 (DRAM)
>          \                 /
>           \ 30            / 30
>            \             /
>              Node 2 (PMEM)
> 
> node distances:
> node   0    1    2
>    0  10   20   30
>    1  20   10   30
>    2  30   30   10
> 
> $ cat /sys/devices/system/memtier/memtier*/nodelist
> <empty>
> 0-2
> <empty>
> 
> $ cat /sys/devices/system/node/node*/memtier
> 1
> 1
> 1
> 
> Demotion fallback order:
> node 0: empty
> node 1: empty
> node 2: empty
> 
> 
> * Example 4:
> 
> Node 0 is a DRAM node with CPU.
> Node 1 is a PMEM node.
> Node 2 is a GPU node.
> 
>                   50
>   Node 0 (DRAM)  ----  Node 2 (GPU)
>          \                 /
>           \ 30            / 60
>            \             /
>              Node 1 (PMEM)
> 
> node distances:
> node   0    1    2
>    0  10   30   50
>    1  30   10   60
>    2  50   60   10
> 
> $ cat /sys/devices/system/memtier/memtier*/nodelist
> 2
> 0
> 1
> 
> $ cat /sys/devices/system/node/node*/memtier
> 1
> 2
> 0
> 
> Demotion fallback order:
> node 0: 1
> node 1: empty
> node 2: 0, 1
> 
> 
> * Example 5:
> 
> Node 0 is a DRAM node with CPU.
> Node 1 is a GPU node.
> Node 2 is a PMEM node.
> Node 3 is a large, slow DRAM node without CPU.
> 
> 
>      Node 2 (PMEM)  ----
>    /      |              \
>   /       | 30            \ 120
>  |        |         100    \
>  |   Node 0 (DRAM)  ----  Node 1 (GPU)
>   \         \                 /
>     \        \ 40            / 110
>   80  \       \             /
>         ---  Node 3 (Slow DRAM)

This is close but not quite what was intended for Hesham's
example... (note we just checked that Hesham's original node0-1
timing didn't make any sense.).


> 
> node distances:
> node    0    1    2    3
>    0   10  100   30   40
>    1  100   10  120  110
>    2   30  120   10   80
>    3   40  110   80   10
> 
> $ cat /sys/devices/system/memtier/memtier*/nodelist
> 1
> 0,3
> 2
> 
> $ cat /sys/devices/system/node/node*/memtier
> 1
> 0
> 2
> 1
> 
> Demotion fallback order:
> node 0: 2
> node 1: 0, 3, 2
> node 2: empty
> node 3: 2

This is close but not quite the same as the example
Hesham gave (note the node timing 1 to 0 on in the table
with that example didn't make sense).  I added another
level of switching to make the numbers more obviously
different and show how critical it might be.

* Example 6:
 
Node 0 is a DRAM node with CPU.
Node 1 is a GPU node.
Node 2 is a PMEM node.
Node 3 is an extremely large, DRAM node without CPU.
  (Key point here being that it probably never makes sense
   to demote to anywhere else from this memory).


I've redone the timings wrt to example 5.
Basis for this is 0 and 2 are directly connected
via controllers in an SoC. 1 and 3 are connected
via a a common switch one switch down switch
(each hop via this is 100)
All drams cost 10 once you've reached correct node
and pmem costs 30 from SoC.
Numbers get too large as a result but meh, I'm making
a point not providing real numbers :)

         PMEM Node 2
            |(30)
        CPU + DRAM Node0
            |(100)
         Switch 1
            |(100)
          Switch 2
    (100)  |      |(100) 
Node 1 GPU     Node3 Large memory.


With one level of s

     Node 2 (PMEM)  ----
    /      |              \
   /       | 30            \ 330
  |        |         310    \
  |   Node 0 (DRAM)  ----  Node 1 (GPU)
   \         \                 /
     \        \ 310           / 210
   330 \       \             /
         ---  Node 3 (Extremely large DRAM)

To my mind, we should potentially also take into account
the fact that Node3 can be known to never contain CPUs
(in at least some architectures we know where the CPUs
 might be added later, they can't just magically turn up
 anywhere in the topology).
 
node distances:
node    0    1    2    3
    0   10   310  30   310
    1   310  10   330  210
    2   30   330  10   330
    3   310  210  330   10

So, my ideal would treat node 3 different from other dram nodes
as we know it can't have CPUs. Trying to come up with an
always correct order for nodes 3 and 2 is tricky as to a certain
extent depends on capacity. If node 2 was  big enough to take
any demotion from node 0 and still have lots of room then demoting
there form node 3 would make sense and visa versa.

 
 $ cat /sys/devices/system/memtier/memtier*/nodelist
 1
 0
 2
 3


 $ cat /sys/devices/system/node/node*/memtier
  1
  0
  2
  3
 
 Demotion fallback order:
 node 0: 2, 3
 node 1: 3, 0, 2 (key being we will almost always have less pressure on node 3)
 node 2: 3
 node 3: empty 

or as Hesham just pointed out this can be done with 3 tiers
because we can put the GPU and CPU in the same tier because
their is little reason to demote from one to the other. 

We are also a bit worried about ABI backwards compatibility because
of potential need to make more space in tiers lower in number than
CPU attached DDR. I rather liked the negative proposal with
default as 0 that Huang, Ying made.

Jonathan

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