[PATCH v3 0/5] mm/memcg: Address PREEMPT_RT problems instead of disabling it.

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

Hi,

this series aims to address the memcg related problem on PREEMPT_RT.

I tested them on CONFIG_PREEMPT and CONFIG_PREEMPT_RT with the
tools/testing/selftests/cgroup/* tests and I haven't observed any
regressions (other than the lockdep report that is already there).

Changes since v2:
- rebased on top of v5.17-rc4-mmots-2022-02-15-20-39.

- Added memcg_stats_lock() in 3/5 so it a little more obvious and
  hopefully easiert to maintain.

- Opencoded obj_cgroup_uncharge_pages() in drain_obj_stock(). The
  __locked suffix was confusing.

v2: https://lore.kernel.org/all/20220211223537.2175879-1-bigeasy@linutronix.de/

Changes since v1:
- Made a full patch from Michal Hocko's diff to disable the from-IRQ vs
  from-task optimisation

- Disabling threshold event handlers is using now IS_ENABLED(PREEMPT_RT)
  instead of #ifdef. The outcome is the same but there is no need to
  shuffle the code around.

v1: https://lore.kernel.org/all/20220125164337.2071854-1-bigeasy@linutronix.de/

Changes since the RFC:
- cgroup.event_control / memory.soft_limit_in_bytes is disabled on
  PREEMPT_RT. It is a deprecated v1 feature. Fixing the signal path is
  not worth it.

- The updates to per-CPU counters are usually synchronised by disabling
  interrupts. There are a few spots where assumption about disabled
  interrupts are not true on PREEMPT_RT and therefore preemption is
  disabled. This is okay since the counter are never written from
  in_irq() context.

RFC: https://lore.kernel.org/all/20211222114111.2206248-1-bigeasy@linutronix.de/

Sebastian