Re: [RFC PATCH 1/3] mm/memcg: Protect per-CPU counter by disabling preemption on PREEMPT_RT

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

On 2022-01-05 15:16:53 [+0100], Michal Koutný wrote:
> On Wed, Dec 22, 2021 at 12:41:09PM +0100, Sebastian Andrzej Siewior <bigeasy@linutronix.de> wrote:
> > The sections with disabled preemption must exclude
> > memcg_check_events() so that spinlock_t locks can still be acquired
> > (for instance in eventfd_signal()).
> 
> The resulting construct in uncharge_batch() raises eybrows. If you can decouple
> per-cpu updates from memcg_check_events() on PREEMPT_RT, why not tackle
> it the same way on !PREEMPT_RT too (and have just one variant of the
> block)?

That would mean that mem_cgroup_event_ratelimit() needs a
local_irq_save(). If that is okay then sure I can move it that way.

> (Actually, it doesn't seem to me that memcg_check_events() can be
> extracted like this from the preempt disabled block since
> mem_cgroup_event_ratelimit() relies on similar RMW pattern.
> Things would be simpler if PREEMPT_RT didn't allow the threshold event
> handlers (akin to Michal Hocko's suggestion of rejecting soft limit).)

I added a preempt-disable() section restricted to RT to
mem_cgroup_event_ratelimit(). I had to exclude memcg_check_events() from
the block because of the spinlock_t locks involved down the road
(eventfd_signal() for instance).

I remember Michal (Hocko) suggested excluding/ rejecting soft limit but
I didn't know where exactly and its implications. In this block here I
just followed the replacement of irq-off with preempt-off for RT.

> Thanks,
> Michal

Sebastian