** Changed in: linux (Ubuntu Bionic)
Status: In Progress => Fix Committed
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https://bugs.launchpad.net/bugs/1770231
Title:
Expose arm64 CPU topology to userspace
Status in linux package in Ubuntu:
In Progress
Status in linux source package in Bionic:
Fix Committed
Bug description:
[Impact]
Applications, particularly those in the HPC domain (e.g. openmpi), can be
optimized for the processor and cache topology. However, the ARM CPU topology
isn't correctly exposed to userspace. The kernel's scheduler also uses this
information to optimize task placement.
[Fix]
The ACPI 6.2 specification introduced a Processor Properties Topology Table
(PPTT). This is what ARM server vendors are using to expose their topology. The
linux kernel needs support for parsing this table, and exposing the parsed
topology to userspace.
[Test Case]
A HiSilicon D06 without the fix. Note that it thinks I'm on a 24-socket
system with 4 cores each. I'm not. I'm on a 2 socket system w/ 48 cores each.
An HPC app that optimized for this (bogus) topology would therefore suffer a
performance penalty.
ubuntu@d06-1:~$ lscpu
Architecture: aarch64
Byte Order: Little Endian
CPU(s): 96
On-line CPU(s) list: 0-95
Thread(s) per core: 1
Core(s) per socket: 4
Socket(s): 24
NUMA node(s): 4
Vendor ID: 0x48
Model: 0
Stepping: 0x0
BogoMIPS: 200.00
NUMA node0 CPU(s): 0-23
NUMA node1 CPU(s): 24-47
NUMA node2 CPU(s): 48-71
NUMA node3 CPU(s): 72-95
Flags: fp asimd evtstrm aes pmull sha1 sha2 crc32 atomics cpuid
asimdrdm dcpop
With the fix (and this is correct):
ubuntu@d06-1:~$ lscpu
Architecture: aarch64
Byte Order: Little Endian
CPU(s): 96
On-line CPU(s) list: 0-95
Thread(s) per core: 1
Core(s) per socket: 48
Socket(s): 2
NUMA node(s): 4
Vendor ID: 0x48
Model: 0
Stepping: 0x0
BogoMIPS: 200.00
L1d cache: 64K
L1i cache: 64K
L2 cache: 512K
L3 cache: 32768K
NUMA node0 CPU(s): 0-23
NUMA node1 CPU(s): 24-47
NUMA node2 CPU(s): 48-71
NUMA node3 CPU(s): 72-95
Flags: fp asimd evtstrm aes pmull sha1 sha2 crc32 atomics cpuid
asimdrdm dcpop
[Regression Risk]
Here's a patch-by patch risk analysis of the changeset:
0001-ACPICA-ACPI-6.2-Additional-PPTT-flags.patch
Just adds new #defines. No functional change.
0002-drivers-base-cacheinfo-move-cache_setup_of_node.patch
Moves code down in a file. No functional change. (I double-checked that the
moved code is the same).
0003-drivers-base-cacheinfo-setup-DT-cache-properties-ear.patch
The code this touches is all #ifdef CONFIG_OF, which only applies to the ARM
& Power ports of Ubuntu. I've tested on arm64 and regression tested on ppc64el
(POWER9). POWER booted fine, and there was no change to lscpu output.
0004-cacheinfo-rename-of_node-to-fw_token.patch
Renames a variable, and changes it's type from "struct device_node *" to
"void *". No functional change.
0005-arm64-acpi-Create-arch-specific-cpu-to-acpi-id-helpe.patch
Adds a new function (but doesn't use it yet).
0006-ACPI-PPTT-Add-Processor-Properties-Topology-Table-pa.patch
Adds new code that isn't called yet.
0007-UBUNTU-Config-CONFIG_ACPI_PPTT-y.patch
Configures on the new code.
0008-ACPI-Enable-PPTT-support-on-ARM64.patch
Kconfig/Makefile bits for the new code.
0009-drivers-base-cacheinfo-Add-support-for-ACPI-based-fi.patch
Finally, we call the new code during initialization, so let's go back and
look at that code. The first thing it does is to check for the presence of a
PPTT table before proceeding, so the regression risk to systems *without* a
PPTT table is negligible. The PPTT table was introduced in the ACPI 6.2
specification, which was released in May 2017. Regression risk should therefore
be restricted to systems manufactured (or firmware updated) after that time
that happened to include a PPTT table. I don't know of anyone other than ARM
licensees doing this - but it's possible there are others. And, it's possible
that there's a table out there that tickles a bug in the parsing code. Should
that be the case, a hotfix would be to use an initrd to override/strip[*] the
PPTT from the XSDT table until a suitable fix is put in place.
0010-arm64-Add-support-for-ACPI-based-firmware-tables.patch
0011-arm64-topology-rename-cluster_id.patch
0012-arm64-topology-enable-ACPI-PPTT-based-CPU-topology.patch
These patches only touch arm64 code. I explicitly tested on an arm64 server.
0013-ACPI-Add-PPTT-to-injectable-table-list.patch
Allows users to override the PPTT table. No change to default behavior.
0014-arm64-topology-divorce-MC-scheduling-domain-from-cor.patch
Only touches arm64 code. I explicitly tested on an arm64 server.
[*]
https://www.kernel.org/doc/Documentation/acpi/initrd_table_override.txt
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