kernek/fork.c: allocate idle task for a CPU always on its local node

Linux preallocates the task structs of the idle tasks for all possible
CPUs.  This currently means they all end up on node 0.  This also
implies that the cache line of MWAIT, which is around the flags field in
the task struct, are all located in node 0.

We see a noticeable performance improvement on Knights Landing CPUs when
the cache lines used for MWAIT are located in the local nodes of the
CPUs using them.  I would expect this to give a (likely slight)
improvement on other systems too.

The patch implements placing the idle task in the node of its CPUs, by
passing the right target node to copy_process()

[akpm@linux-foundation.org: use NUMA_NO_NODE, not a bare -1]
Link: http://lkml.kernel.org/r/1463492694-15833-1-git-send-email-andi@firstfloor.org


Signed-off-by: Andi Kleen <ak@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>