timers/migration: Fix imbalanced NUMA trees

timers/migration: Fix imbalanced NUMA trees

When a CPU from a new node boots, the old root may happen to be
connected to the new root even if their node mismatch, as depicted in
the following scenario:

1) CPU 0 boots and creates the first group for node 0.

   [GRP0:0]
    node 0
      |
    CPU 0

2) CPU 1 from node 1 boots and creates a new top that corresponds to
   node 1, but it also connects the old root from node 0 to the new root
   from node 1 by mistake.

             [GRP1:0]
              node 1
            /        \
           /          \
   [GRP0:0]             [GRP0:1]
    node 0               node 1
      |                    |
    CPU 0                CPU 1

3) This eventually leads to an imbalanced tree where some node 0 CPUs
   migrate node 1 timers (and vice versa) way before reaching the
   crossnode groups, resulting in more frequent remote memory accesses
   than expected.

                      [GRP2:0]
                      NUMA_NO_NODE
                     /             \
             [GRP1:0]              [GRP1:1]
              node 1               node 0
            /        \                |
           /          \             [...]
   [GRP0:0]             [GRP0:1]
    node 0               node 1
      |                    |
    CPU 0...              CPU 1...

A balanced tree should only contain groups having children that belong
to the same node:

                      [GRP2:0]
                      NUMA_NO_NODE
                     /             \
             [GRP1:0]              [GRP1:0]
              node 0               node 1
            /        \             /      \
           /          \           /        \
   [GRP0:0]          [...]      [...]    [GRP0:1]
    node 0                                node 1
      |                                     |
    CPU 0...                              CPU 1...

In order to fix this, the hierarchy must be unfolded up to the crossnode
level as soon as a node mismatch is detected. For example the stage 2
above should lead to this layout:

                      [GRP2:0]
                      NUMA_NO_NODE
                     /             \
             [GRP1:0]              [GRP1:1]
              node 0               node 1
              /                         \
             /                           \
        [GRP0:0]                        [GRP0:1]
        node 0                           node 1
          |                                |
       CPU 0                             CPU 1

This means that not only GRP1:0 must be created but also GRP1:1 and
GRP2:0 in order to prepare a balanced tree for next CPUs to boot.

Fixes: 7ee988770326 ("timers: Implement the hierarchical pull model")
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://patch.msgid.link/20251024132536.39841-4-frederic@kernel.org