mm/memcg: disable threshold event handlers on PREEMPT_RT

During the integration of PREEMPT_RT support, the code flow around
memcg_check_events() resulted in `twisted code'.  Moving the code around
and avoiding then would then lead to an additional local-irq-save
section within memcg_check_events().  While looking better, it adds a
local-irq-save section to code flow which is usually within an
local-irq-off block on non-PREEMPT_RT configurations.

The threshold event handler is a deprecated memcg v1 feature.  Instead
of trying to get it to work under PREEMPT_RT just disable it.  There
should be no users on PREEMPT_RT.  From that perspective it makes even
less sense to get it to work under PREEMPT_RT while having zero users.

Make memory.soft_limit_in_bytes and cgroup.event_control return
-EOPNOTSUPP on PREEMPT_RT.  Make an empty memcg_check_events() and
memcg_write_event_control() which return only -EOPNOTSUPP on PREEMPT_RT.
Document that the two knobs are disabled on PREEMPT_RT.

Link: https://lkml.kernel.org/r/[email protected]
Suggested-by: Michal Hocko <[email protected]>
Suggested-by: Michal Koutný <[email protected]>
Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
Acked-by: Roman Gushchin <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Reviewed-by: Shakeel Butt <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: kernel test robot <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Cc: Waiman Long <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

0 files changed, 0 insertions, 0 deletions