idle task's task_t allocation on NUMA machines

[Samuel Thibault <samuel.thibault@ens-lyon.org>]

Hi,

Currently, the task_t structure of the idle task is always allocated
on CPU0, hence on node 0: while booting, for each CPU, CPU 0 calls
fork_idle(), hence copy_process(), hence dup_task_struct(), hence
alloc_task_struct(), hence kmem_cache_alloc(), which picks up memory
from the allocation cache of the current CPU, i.e. on node 0.

This is a bad idea: every write needs be written back to node 0 at some
time, so that node 0 can get a small bit busy especially when other
nodes are idle.

A solution would be to add to copy_process(), dup_task_struct(),
alloc_task_struct() and kmem_cache_alloc() the node number on which
allocation should be performed. This might also be useful if performing
node load balancing at fork(): one could then allocate task_t directly
on the new node. It might also be useful when allocating data for
another node.

Regards,
Samuel