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?

Cheers,
Longman