sched/mmcid: Introduce per task/CPU ownership infrastructure

The MM CID management has two fundamental requirements:

  1) It has to guarantee that at no given point in time the same CID is
     used by concurrent tasks in userspace.

  2) The CID space must not exceed the number of possible CPUs in a
     system. While most allocators (glibc, tcmalloc, jemalloc) do not care
     about that, there seems to be at least librseq depending on it.

The CID space compaction itself is not a functional correctness
requirement, it is only a useful optimization mechanism to reduce the
memory foot print in unused user space pools.

The optimal CID space is:

    min(nr_tasks, nr_cpus_allowed);

Where @nr_tasks is the number of actual user space threads associated to
the mm and @nr_cpus_allowed is the superset of all task affinities. It is
growth only as it would be insane to take a racy snapshot of all task
affinities when the affinity of one task changes just do redo it 2
milliseconds later when the next task changes its affinity.

That means that as long as the number of tasks is lower or equal than the
number of CPUs allowed, each task owns a CID. If the number of tasks
exceeds the number of CPUs allowed it switches to per CPU mode, where the
CPUs own the CIDs and the tasks borrow them as long as they are scheduled
in.

For transition periods CIDs can go beyond the optimal space as long as they
don't go beyond the number of possible CPUs.

The current upstream implementation adds overhead into task migration to
keep the CID with the task. It also has to do the CID space consolidation
work from a task work in the exit to user space path. As that work is
assigned to a random task related to a MM this can inflict unwanted exit
latencies.

This can be done differently by implementing a strict CID ownership
mechanism. Either the CIDs are owned by the tasks or by the CPUs. The
latter provides less locality when tasks are heavily migrating, but there
is no justification to optimize for overcommit scenarios and thereby
penalizing everyone else.

Provide the basic infrastructure to implement this:

  - Change the UNSET marker to BIT(31) from ~0U
  - Add the ONCPU marker as BIT(30)
  - Add the TRANSIT marker as BIT(29)

That allows to check for ownership trivially and provides a simple check for
UNSET as well. The TRANSIT marker is required to prevent CID space
exhaustion when switching from per CPU to per task mode.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://patch.msgid.link/20251119172549.960252358@linutronix.de

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