Re: [PATCH 1/1] mm/vmstat: Protect per cpu variables with preempt disable on RT

[Andrew Morton <akpm@linux-foundation.org>]

On Thu,  5 Aug 2021 17:00:19 +0100 Mel Gorman <mgorman@techsingularity.net> wrote:

> From: Ingo Molnar <mingo@elte.hu>
> 
> 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();
}

?