Re: [PATCH 2/2] cpuset: Call set_cpus_allowed_ptr() with appropriate mask for task

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jan 31, 2023 at 10:17:19PM +0000, Will Deacon wrote:
> set_cpus_allowed_ptr() will fail with -EINVAL if the requested
> affinity mask is not a subset of the task_cpu_possible_mask() for the
> task being updated. Consequently, on a heterogeneous system with cpusets
> spanning the different CPU types, updates to the cgroup hierarchy can
> silently fail to update task affinities when the effective affinity
> mask for the cpuset is expanded.
> 
> For example, consider an arm64 system with 4 CPUs, where CPUs 2-3 are
> the only cores capable of executing 32-bit tasks. Attaching a 32-bit
> task to a cpuset containing CPUs 0-2 will correctly affine the task to
> CPU 2. Extending the cpuset to CPUs 0-3, however, will fail to extend
> the affinity mask of the 32-bit task because update_tasks_cpumask() will
> pass the full 0-3 mask to set_cpus_allowed_ptr().
> 
> Extend update_tasks_cpumask() to take a temporary 'cpumask' paramater
> and use it to mask the 'effective_cpus' mask with the possible mask for
> each task being updated.
> 
> Fixes: 431c69fac05b ("cpuset: Honour task_cpu_possible_mask() in guarantee_online_cpus()")
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
> 
> Note: We wondered whether it was worth calling guarantee_online_cpus()
> if the cpumask_and() returns 0 in update_tasks_cpumask(), but given that
> this path is only called when the effective mask changes, it didn't
> seem appropriate. Ultimately, if you have 32-bit tasks attached to a
> cpuset containing only 64-bit cpus, then the affinity is going to be
> forced.

Right, so the case above where the cpuset is shrunk to 0-1, then the
intersection between the cpuset and task_cpu_possible_mask() is empty
and it currently simply fails to update mask.

I argued it was probably desired to walk up the tree to find a wider
parent until the intersection of {cpuset, task_cpu_possible_mask,
online} becomes non-empty.

But I suppose that can wait until we have more time.