Re: [PATCH sched/core] sched/rt: Fix RT_PUSH_IPI soft lockup loop

[Tejun Heo]

Hello, Steven.

On Thu, May 14, 2026 at 10:03:00AM -0400, Steven Rostedt wrote:
> I was thinking about this more and does disabling the RT_PUSH_IPI cause any
> problems for you?
> 
>   # echo NO_RT_PUSH_IPI > /sys/kernel/debug/sched/features

Not at all. This is actually the mitigation that we deployed across the
affected machines.

...
> -/* RT IPI pull logic requires IRQ_WORK */
> -#if defined(CONFIG_IRQ_WORK) && defined(CONFIG_SMP)
> +/*
> + * RT IPI pull logic requires IRQ_WORK and doesn't make sense for uniprocessors.
> + * If CONFIG_IRQ_FORCED_THREADING isn't set, then softirqs do not run as threads
> + * and can cause latency larger than what RT_PUSH_IPI can save, killing the
> + * effect of it.
> + */
> +#if defined(CONFIG_IRQ_WORK) && defined(CONFIG_SMP) &&	\
> +	defined(CONFIG_IRQ_FORCED_THREADING)
>  # define HAVE_RT_PUSH_IPI
>  #endif

Maybe it should trigger on force_irqthreads so that it's active only when
irq threads are actully enabled.

Whichever way it's done tho, wouldn't this still leave machines in that
config susceptible to IPI storms? It took a combination of factors to
trigger - mpi3mr's threaded irq, psimon activated by systemd, and sustained
network load - but those factors are not that exotic.

Thanks.

-- 
tejun