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    SubjectRe: [PATCH 2/2] cpuset: Call set_cpus_allowed_ptr() with appropriate mask for task
    On Tue, Jan 31, 2023 at 09:22:44PM -0500, Waiman Long wrote:
    > On 1/31/23 17:17, 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.
    >
    > Now I see how the sched_setaffinity() change is impacting arm64. Instead of
    > putting in the bandage in cpuset. I would suggest doing another cpu masking
    > in __set_cpus_allowed_ptr() similar to what is now done for user_cpus_ptr.

    NO! cpuset is *BROKEN* it has been for a while, it needs to get fixed.

    Masking the offline CPUs is *WRONG*.

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