cgroups: add a task counter subsystem

cgroups: add a task counter subsystem

Add a new subsystem to limit the number of running tasks, similar to the
NR_PROC rlimit but in the scope of a cgroup.

The user can set an upper bound limit that is checked every time a task
forks in a cgroup or is moved into a cgroup with that subsystem binded.

The primary goal is to protect against forkbombs that explode inside a
container.  The traditional NR_PROC rlimit is not efficient in that case
because if we run containers in parallel under the same user, one of these
could starve all the others by spawning a high number of tasks close to
the user wide limit.

This is a prevention against forkbombs, so it's not deemed to cure the
effects of a forkbomb when the system is in a state where it's not
responsive.  It's aimed at preventing from ever reaching that state and
stop the spreading of tasks early.  While defining the limit on the
allowed number of tasks, it's up to the user to find the right balance
between the resource its containers may need and what it can afford to
provide.

As it's totally dissociated from the rlimit NR_PROC, both can be
complementary: the cgroup task counter can set an upper bound per
container and the rlmit can be an upper bound on the overall set of
containers.

Also this subsystem can be used to kill all the tasks in a cgroup without
races against concurrent forks, by setting the limit of tasks to 0, any
further forks can be rejected.  This is a good way to kill a forkbomb in a
container, or simply kill any container without the need to retry an
unbound number of times.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Paul Menage <paul@paulmenage.org>
Reviewed-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Aditya Kali <adityakali@google.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Cc: Tim Hockin <thockin@hockin.org>
Cc: Tejun Heo <htejun@gmail.com>
Acked-by: Kirill A. Shutemov <kirill@shutemov.name>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>