Re: [LSF/MM] CXL Boot to Bash - Section 4: Interleave

From: Yuquan Wang
Date: Thu Mar 13 2025 - 04:32:12 EST

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On Tue, Mar 11, 2025 at 08:09:02PM -0400, Gregory Price wrote:
>
> -----------------------------
> Intra-Host-Bridge Interleave.
> -----------------------------
> Now lets consider a system where we've placed 2 CXL devices on the same
> Host Bridge. Maybe each CXL device is only capable of x8 PCIE, and we
> want to make full use of a single x16 link.
>
> This setup only requires the BIOS to create a CEDT CFMWS which reports
> the entire capacity of all devices under the host bridge, but does not
> need to set up any interleaving.
>
> In the follow case, the BIOS has configured as single 4GB memory region
> which only targets the single host bridge, but maps the entire memory
> capacity of both devices (2GB).
>
> ```
> CFMWS:
> Subtable Type : 01 [CXL Fixed Memory Window Structure]
> Reserved : 00
> Length : 002C
> Reserved : 00000000
> Window base address : 0000000300000000 <- Memory Region
> Window size : 0000000080000000 <- 2GB

I think is "Window size : 0000000100000000 <- 4GB" here.

> Interleave Members (2^n) : 00 <- No host bridge interleave
> Interleave Arithmetic : 00
> Reserved : 0000
> Granularity : 00000000
> Restrictions : 0006 <- Bit(2) - Volatile
> QtgId : 0001
> First Target : 00000007 <- Host Bridge _UID
> ```
>
> Assuming no other CEDT or SRAT entries exist, this will result in linux
> creating the following NUMA topology, where all CXL memory is in Node 1.
>
> ```
> NUMA Structure:
> --------- -------- | ----------
> | cpu0 |-----| DRAM |---|----| Node 0 |
> --------- -------- | ----------
> | |
> ------- | ----------
> | HB0 |-----------------|----| Node 1 |
> ------- | ----------
> / \ |
> CXL Dev CXL Dev |
> ```
>
> In this scenario, we program the decoders like so:
> ```
> Decoders
> CXL Root
> |
> decoder0.0
> IW:1 IG:256
> [0x300000000, 0x3FFFFFFFF]
> |
> Host Bridge
> |
> decoder1.0
> IW:2 IG:256
> [0x300000000, 0x3FFFFFFFF]
> / \
> Endpoint 0 Endpoint 1
> | |
> decoder2.0 decoder3.0
> IW:2 IG:256 IW:2 IG:256
> [0x300000000, 0x3FFFFFFFF] [0x300000000, 0x3FFFFFFFF]
> ```
>
> The root decoder in this scenario does not participate in interleave,
> it simply forwards all accesses in this range to the host bridge.
>
> The host bridge then applies the interleave across its connected devices
> and the decodes apply translation accordingly.
>
> -----------------------
> Combination Interleave.
> -----------------------
> Lets consider now a system where 2 Host Bridges have 2 CXL devices each,
> and we want to interleave the entire set. This requires us to make use
> of both inter and intra host bridge interleave.
>
> First, we can interleave this with the a single CEDT entry, the same as
> the first inter-host-bridge CEDT (now assuming 1GB per device).
>
> ```
> Subtable Type : 01 [CXL Fixed Memory Window Structure]
> Reserved : 00
> Length : 002C
> Reserved : 00000000
> Window base address : 0000000300000000 <- Memory Region
> Window size : 0000000100000000 <- 4GB
> Interleave Members (2^n) : 01 <- 2-way interleave
> Interleave Arithmetic : 00
> Reserved : 0000
> Granularity : 00000000
> Restrictions : 0006 <- Bit(2) - Volatile
> QtgId : 0001
> First Target : 00000007 <- Host Bridge _UID
> Next Target : 00000006 <- Host Bridge _UID
> ```
>
> This gives us a NUMA structure as follows:
> ```
> NUMA Structure:
>
> ---------- -------- | ----------
> | cpu0 |-----| DRAM |----|---| Node 0 |
> ---------- -------- | ----------
> / \ |
> ------- ------- | ----------
> | HB0 |-----| HB1 |-------------|---| Node 1 |
> ------- ------- | ----------
> / \ / \ |
> CXL0 CXL1 CXL2 CXL3 |
> ```
>
> And the respective decoder programming looks as follows
> ```
> Decoders:
> CXL Root
> |
> decoder0.0
> IW:2 IG:256
> [0x300000000, 0x3FFFFFFFF]
> / \
> Host Bridge 7 Host Bridge 6
> / \
> decoder1.0 decoder2.0
> IW:2 IG:512 IW:2 IG:512
> [0x300000000, 0x3FFFFFFFFF] [0x300000000, 0x3FFFFFFFF]
> / \ / \
> endpoint0 endpoint1 endpoint2 endpoint3
> | | | |
> decoder3.0 decoder4.0 decoder5.0 decoder6.0
> IW:4 IG:256 IW:4 IG:256
> [0x300000000, 0x3FFFFFFFF] [0x300000000, 0x3FFFFFFFF]
> ```
>
> Notice at both the root and the host bridge, the Interleave Ways is 2.
> There are two targets at each level. The host bridge has a granularity
> of 512 to capture its parent's ways and granularity (`2*256`).
>
> Each decoder is programmed with the total number of targets (4) and the
> overall granularity (256B).

Is there any relationship between endpoints'decoder setup(IW&IG) and
others decoder?

>
> We might use this setup if each CXL device is capable of x8 PCIE, and
> we have 2 Host Bridges capable of full x16 - utilizing all bandwidth
> available.
>
> ---------------------------------------------
> Nuance: Hardware Interleave and Memory Holes.
> ---------------------------------------------
> You may encounter a system which cannot place the entire memory capacity
> into a single contiguous System Physical Address range. That's ok,
> because we can just use multiple decoders to capture this nuance.
>
> Most CXL devices allow for multiple decoders.
>
> This may require an SRAT entry to keep these regions on the same node.
> (Obviously the relies on your platform vendor's BIOS)
>
> ```
> CFMWS:
> Subtable Type : 01 [CXL Fixed Memory Window Structure]
> Reserved : 00
> Length : 002C
> Reserved : 00000000
> Window base address : 0000000300000000 <- Memory Region
> Window size : 0000000080000000 <- 2GB
> Interleave Members (2^n) : 00 <- No host bridge interleave
> Interleave Arithmetic : 00
> Reserved : 0000
> Granularity : 00000000
> Restrictions : 0006 <- Bit(2) - Volatile
> QtgId : 0001
> First Target : 00000007 <- Host Bridge 7
>
> Subtable Type : 01 [CXL Fixed Memory Window Structure]
> Reserved : 00
> Length : 002C
> Reserved : 00000000
> Window base address : 0000000400000000 <- Memory Region
> Window size : 0000000080000000 <- 2GB
> Interleave Members (2^n) : 00 <- No host bridge interleave
> Interleave Arithmetic : 00
> Reserved : 0000
> Granularity : 00000000
> Restrictions : 0006 <- Bit(2) - Volatile
> QtgId : 0001
> First Target : 00000007 <- Host Bridge 7
>
> SRAT:
> Subtable Type : 01 [Memory Affinity]
> Length : 28
> Proximity Domain : 00000001 <- NUMA Node 1
> Reserved1 : 0000
> Base Address : 0000000300000000 <- Physical Memory Region
> Address Length : 0000000080000000 <- first 2GB
>
> Subtable Type : 01 [Memory Affinity]
> Length : 28
> Proximity Domain : 00000001 <- NUMA Node 1
> Reserved1 : 0000
> Base Address : 0000000400000000 <- Physical Memory Region
> Address Length : 0000000080000000 <- second 2GB
> ```
>
> The SRAT entries allow us to keep the regions attached to the same node.
> ```
>
> NUMA Structure:
> --------- -------- | ----------
> | cpu0 |-----| DRAM |---|----| Node 0 |
> --------- -------- | ----------
> | |
> ------- | ----------
> | HB0 |-----------------|----| Node 1 |
> ------- | ----------
> / \ |
> CXL Dev CXL Dev |
> ```
>
Hi, Gregory

Seeing this, I have an assumption to discuss.

If the same system uses tables like below:

CFMWS:
Subtable Type : 01 [CXL Fixed Memory Window Structure]
Reserved : 00
Length : 002C
Reserved : 00000000
Window base address : 0000000300000000 <- Memory Region
Window size : 0000000080000000 <- 2GB
Interleave Members (2^n) : 00 <- No host bridge interleave
Interleave Arithmetic : 00
Reserved : 0000
Granularity : 00000000
Restrictions : 0006 <- Bit(2) - Volatile
QtgId : 0001
First Target : 00000007 <- Host Bridge 7

Subtable Type : 01 [CXL Fixed Memory Window Structure]
Reserved : 00
Length : 002C
Reserved : 00000000
Window base address : 0000000400000000 <- Memory Region
Window size : 0000000080000000 <- 2GB
Interleave Members (2^n) : 00 <- No host bridge interleave
Interleave Arithmetic : 00
Reserved : 0000
Granularity : 00000000
Restrictions : 0006 <- Bit(2) - Volatile
QtgId : 0001
First Target : 00000007 <- Host Bridge 7

SRAT:
Subtable Type : 01 [Memory Affinity]
Length : 28
Proximity Domain : 00000000 <- NUMA Node 0
Reserved1 : 0000
Base Address : 0000000300000000 <- Physical Memory Region
Address Length : 0000000080000000 <- first 2GB

Subtable Type : 01 [Memory Affinity]
Length : 28
Proximity Domain : 00000001 <- NUMA Node 1
Reserved1 : 0000
Base Address : 0000000400000000 <- Physical Memory Region
Address Length : 0000000080000000 <- second 2GB

The first 2GB cxl memory region would locate at node0 with DRAM.

NUMA Structure:

--------- -------- | ----------
| cpu0 |-----| DRAM |---|------------| Node 0 |
--------- -------- | / ----------
| | /first 2GB
------- | / ----------
| HB0 |-----------------|------------| Node 1 |
------- |second 2GB ----------
/ \ |
CXL Dev CXL Dev |
```

Is above configuration and structure valid?

Yuquan
> And the decoder programming would look like so
> ```
> Decoders:
> CXL Root
> / \
> decoder0.0 decoder0.1
> IW:1 IG:256 IW:1 IG:256
> [0x300000000, 0x37FFFFFFF] [0x400000000, 0x47FFFFFFF]
> \ /
> Host Bridge
> / \
> decoder1.0 decoder1.1
> IW:2 IG:256 IW:2 IG:256
> [0x300000000, 0x37FFFFFFF] [0x400000000, 0x47FFFFFFF]
> / \ / \
> Endpoint 0 Endpoint 1 Endpoint 0 Endpoint 1
> | | | |
> decoder2.0 decoder3.0 decoder2.1 decoder3.1
> IW:2 IG:256 IW:2 IG:256
> [0x300000000, 0x37FFFFFFF] [0x400000000, 0x47FFFFFFF]
> ```
>
> Linux manages decoders in relation to the associated component, so
> decoders are N.M where N is the component and M is the decoder number.
>
> If you look, you'll see each side of this tree looks individually
> equivalent to the intra-host-bridge interleave example, just with one
> half of the total memory each (matching the CFMWS ranges).
>
> Each of the root decoders still has an interleave width of 1 because
> they both only target one host bridge (despite it being the same one).
>
>
> --------------------------------
> Software Interleave (Mempolicy).
> --------------------------------
> Linux provides a software mechanism to allow tasks to to interleave its
> memory across NUMA nodes - which may have different performance
> characteristics. This component is called `mempolicy`, and is primarily
> operated on with the `set_mempolicy()` and `mbind()` syscalls.
>
> These syscalls take a nodemask (bitmask representing NUMA node ids) as
> an argument to describe the intended allocation policy of the task.
>
> The following policies are presently supported (as of v6.13)
> ```
> enum {
> MPOL_DEFAULT,
> MPOL_PREFERRED,
> MPOL_BIND,
> MPOL_INTERLEAVE,
> MPOL_LOCAL,
> MPOL_PREFERRED_MANY,
> MPOL_WEIGHTED_INTERLEAVE,
> };
> ```
>
> Let's look at `MPOL_INTERLEAVE` and `MPOL_WEIGHTED_INTERLEAVE`.
>
> To quote the man page:
> ```
> MPOL_INTERLEAVE
> This mode interleaves page allocations across the nodes specified
> in nodemask in numeric node ID order. This optimizes for bandwidth
> instead of latency by spreading out pages and memory accesses to those
> pages across multiple nodes. However, accesses to a single page will
> still be limited to the memory bandwidth of a single node.
>
> MPOL_WEIGHTED_INTERLEAVE (since Linux 6.9)
> This mode interleaves page allocations across the nodes specified in
> nodemask according to the weights in
> /sys/kernel/mm/mempolicy/weighted_interleave
> For example, if bits 0, 2, and 5 are set in nodemask and the contents of
> /sys/kernel/mm/mempolicy/weighted_interleave/node0
> /sys/ ... /node2
> /sys/ ... /node5
> are 4, 7, and 9, respectively, then pages in this region will be
> allocated on nodes 0, 2, and 5 in a 4:7:9 ratio.
> ```
>
> To put it simply, MPOL_INTERLEAVE will interleave allocations at a page
> granularity (4KB, 2MB, etc) across nodes in a 1:1 ratio, while
> MPOL_WEIGHTED_INTERLEAVE takes into account weights - which presumably
> map to the bandwidth of each respective node.
>
> Or more concretely:
>
> MPOL_INTERLEAVE
> 1:1 Interleave between two nodes.
> malloc(4096) -> node0
> malloc(4096) -> node1
> malloc(4096) -> node0
> malloc(4096) -> node1
> ... and so on ...
>
> MPOL_WEIGHTED_INTERLEAVE
> 2:1 Interleave between two nodes.
> malloc(4096) -> node0
> malloc(4096) -> node0
> malloc(4096) -> node1
> malloc(4096) -> node0
> malloc(4096) -> node0
> malloc(4096) -> node1
> ... and so on ...
>
> This is the preferred mechanism for *heterogeneous interleave* on Linux,
> as it allows for predictable performance based on the explicit (and
> visible) placement of memory.
>
> It also allows for memory ZONE restrictions to enable better performance
> predictability (e.g. keeping kernel locks out of CXL while allowing
> workloads to leverage it for expansion or bandwidth).
>
> ======================
> Mempolicy Limitations.
> ======================
> Mempolicy is a *per-task* allocation policy that is inherited by
> child-tasks on clone/fork. It can only be changed by the task itself,
> though cgroups may affect the effective nodemask via cpusets.
>
> This means once a task has been launched, and external actor cannot
> change the policy of a running task - except possibly by migrating that
> task between cgroups or changing the cpusets.mems value of the cgroup
> the task lives in.
>
> Additionally, If capacity on a given node is not available, allocations
> will fall back to another node in the node mask - which may cause
> interleave to become unbalanced.
>
> ================================
> Hardware Interleave Limitations.
> ================================
> Granularities:
> granularities are limited on hardware
> (typically 256B up to 16KB by power of 2)
>
> Ways:
> Ways are limited by the CXL configuration to:
> 2,4,8,16,3,6,12
>
> Balance:
> Linux does not allow imbalanced interleave configurations
> (e.g. 3-way interleave where 2 targets are on 1 HB and 1 on another)
>
> Depending on your platform vendor and type of interleave, you may not
> be able to deconstruct an interleave region at all (decoders may be
> locked). In this case, you may not have the flexiblity to convert
> operation from interleaved to non-interleave via the driver interface.
>
> In the scenario where your interleave configuration is entirely driver
> managed, you cannot adjust the size of an interleave set without
> deconstructing the entire set.
>
> ------------------------------------------------------------------------
>
> Next we'll discuss how memory allocations occur in a CXL-enabled system,
> which may be affected by things like Reclaim and Tiering systems.
>
> ~Gregory

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In reply to: Gregory Price: "[LSF/MM] CXL Boot to Bash - Section 4: Interleave"
Next in thread: Gregory Price: "Re: [LSF/MM] CXL Boot to Bash - Section 4: Interleave"
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