FROMLIST: mm: disable rcu safe vma freeing for single threaded user space

Performance tuning: as single threaded userspace does not use
speculative page faults, it does not require rcu safe vma freeing.
Turn this off to avoid the related (small) extra overheads.

For multi threaded userspace, we often see a performance benefit from
the rcu safe vma freeing - even in tests that do not have any frequent
concurrent page faults ! This is because rcu safe vma freeing prevents
recently released vmas from being immediately reused in a new thread.

Signed-off-by: Michel Lespinasse <michel@lespinasse.org>
Link: https://lore.kernel.org/all/20220128131006.67712-30-michel@lespinasse.org/


Bug: 161210518
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: I81ef7ab43e2757f268c567d5bfe6ab02f1e43a1c