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

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

On 2021-12-22 21:31:36 [-0500], Waiman Long wrote:
> On 12/22/21 06:41, Sebastian Andrzej Siewior wrote:
> > The per-CPU counter are modified with the non-atomic modifier. The
> > consistency is ensure by disabling interrupts for the update.
> > This breaks on PREEMPT_RT because some sections additionally
> > acquire a spinlock_t lock (which becomes sleeping and must not be
> > acquired with disabled interrupts). Another problem is that
> > mem_cgroup_swapout() expects to be invoked with disabled interrupts
> > because the caller has to acquire a spinlock_t which is acquired with
> > disabled interrupts. Since spinlock_t never disables interrupts on
> > PREEMPT_RT the interrupts are never disabled at this point.
> > 
> > The code is never called from in_irq() context on PREEMPT_RT therefore
> 
> How do you guarantee that these percpu update functions won't be called in
> in_irq() context for PREEMPT_RT? Do you think we should add a
> WARN_ON_ONCE(in_irq()) just to be sure?

There are no invocations to the memory allocator (neither malloc() nor
free()) on RT and the memory allocator itself (SLUB and the
page-allocator so both) has sleeping locks. That means invocations
in_atomic() are bad. All interrupt handler are force-threaded. Those
which are not (like timer, per-CPU interrupts or those which explicitly
asked not to be force threaded) are limited in their doing as they can't
invoke anything that has a sleeping lock. Lockdep or
CONFIG_DEBUG_ATOMIC_SLEEP will yell here.
The other counter are protected the same way, see
  c68ed7945701a ("mm/vmstat: protect per cpu variables with preempt disable on RT")

> Cheers,
> Longman

Sebastian