Re: [PATCH v6 21/21] Documentation: arm64: describe asymmetric 32-bit support

On Fri, May 21, 2021 at 06:37:21PM +0100, Qais Yousef wrote:

On 05/18/21 10:47, Will Deacon wrote:

quoted

Document support for running 32-bit tasks on asymmetric 32-bit systems
and its impact on the user ABI when enabled.

Signed-off-by: Will Deacon <will@kernel.org>
---
 .../admin-guide/kernel-parameters.txt         |   3 +
 Documentation/arm64/asymmetric-32bit.rst      | 149 ++++++++++++++++++
 Documentation/arm64/index.rst                 |   1 +
 3 files changed, 153 insertions(+)
 create mode 100644 Documentation/arm64/asymmetric-32bit.rst

[...]

quoted

+Cpusets
+-------
+
+The affinity of a 32-bit task may include CPUs that are not explicitly
+allowed by the cpuset to which it is attached. This can occur as a
+result of the following two situations:
+
+  - A 64-bit task attached to a cpuset which allows only 64-bit CPUs
+    executes a 32-bit program.
+
+  - All of the 32-bit-capable CPUs allowed by a cpuset containing a
+    32-bit task are offlined.
+
+In both of these cases, the new affinity is calculated according to step
+(2) of the process described in `execve(2)`_ and the cpuset hierarchy is
+unchanged irrespective of the cgroup version.

nit: Should we call out that we're breaking cpuset-v1 behavior? Don't feel
strongly about it.

I think the text is pretty clear that the new behaviour documented here
applies to cpuset-v1 and I wouldn't say we're breaking anything as we're not
changing any of the existing behaviours.

Will

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