On Thu, 5 Aug 2021 17:00:19 +0100 Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> wrote:
> From: Ingo Molnar <mingo@xxxxxxx>
>
> Disable preemption on -RT for the vmstat code. On vanila the code runs
> in IRQ-off regions while on -RT it may not when stats are updated under
> a local_lock. "preempt_disable" ensures that the same resources is not
> updated in parallel due to preemption.
>
> This patch differs from the preempt-rt version where __count_vm_event and
> __count_vm_events are also protected. The counters are explicitly "allowed
> to be to be racy" so there is no need to protect them from preemption. Only
> the accurate page stats that are updated by a read-modify-write need
> protection. This patch also differs in that a preempt_[en|dis]able_rt
> helper is not used. As vmstat is the only user of the helper, it was
> suggested that it be open-coded in vmstat.c instead of risking the helper
> being used in unnecessary contexts.
>
> ...
>
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -319,6 +319,16 @@ void __mod_zone_page_state(struct zone *zone, enum zone_stat_item item,
> long x;
> long t;
>
> + /*
> + * Accurate vmstat updates require a RMW. On !PREEMPT_RT kernels,
> + * atomicity is provided by IRQs being disabled -- either explicitly
> + * or via local_lock_irq. On PREEMPT_RT, local_lock_irq only disables
> + * CPU migrations and preemption potentially corrupts a counter so
> + * disable preemption.
> + */
> + if (IS_ENABLED(CONFIG_PREEMPT_RT))
> + preempt_disable();
This is so obvious I expect it has been discussed, but... why not
static inline void preempt_disable_if_rt(void)
{
if (IS_ENABLED(CONFIG_PREEMPT_RT))
preempt_disable();
}
?
