linux-kernel - Re: [RESEND] sched/rt: Skip currently executing CPU in rto_next

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Message-ID: <20260105162503.36377c7e@gandalf.local.home>
Date: Mon, 5 Jan 2026 16:25:03 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Chen Jinghuang <chenjinghuang2@...wei.com>
Cc: <mingo@...hat.com>, <peterz@...radead.org>, <juri.lelli@...hat.com>,
 <vincent.guittot@...aro.org>, <dietmar.eggemann@....com>,
 <bsegall@...gle.com>, <mgorman@...e.de>, <vschneid@...hat.com>,
 <linux-kernel@...r.kernel.org>
Subject: Re: [RESEND] sched/rt: Skip currently executing CPU in
 rto_next_cpu()


Peter,

Are you OK with this change? If so, care to take it?

-- Steve


On Mon, 5 Jan 2026 03:40:12 +0000
Chen Jinghuang <chenjinghuang2@...wei.com> wrote:

> CPU0 becomes overloaded when hosting a CPU-bound RT task, a non-CPU-bound
> RT task, and a CFS task stuck in kernel space. When other CPUs switch from
> RT to non-RT tasks, RT load balancing (LB) is triggered; with
> HAVE_RT_PUSH_IPI enabled, they send IPIs to CPU0 to drive the execution
> of rto_push_irq_work_func. During push_rt_task on CPU0,
> if next_task->prio < rq->donor->prio, resched_curr() sets NEED_RESCHED
> and after the push operation completes, CPU0 calls rto_next_cpu().
> Since only CPU0 is overloaded in this scenario, rto_next_cpu() should
> ideally return -1 (no further IPI needed).
> 
> However, multiple CPUs invoking tell_cpu_to_push() during LB increments
> rd->rto_loop_next. Even when rd->rto_cpu is set to -1, the mismatch between
> rd->rto_loop and rd->rto_loop_next forces rto_next_cpu() to restart its
> search from -1. With CPU0 remaining overloaded (satisfying rt_nr_migratory
> && rt_nr_total > 1), it gets reselected, causing CPU0 to queue irq_work to
> itself and send self-IPIs repeatedly. As long as CPU0 stays overloaded and
> other CPUs run pull_rt_tasks(), it falls into an infinite self-IPI loop,
> which triggers a CPU hardlockup due to continuous self-interrupts.
> 
> The trigging scenario is as follows:
> 
>          cpu0                      cpu1                    cpu2
>                                 pull_rt_task
>                               tell_cpu_to_push
>                  <------------irq_work_queue_on
> rto_push_irq_work_func
>        push_rt_task
>     resched_curr(rq)                                   pull_rt_task
>     rto_next_cpu                                     tell_cpu_to_push
>                       <-------------------------- atomic_inc(rto_loop_next)
> rd->rto_loop != next
>      rto_next_cpu
>    irq_work_queue_on
> rto_push_irq_work_func
> 
> Fix redundant self-IPI by filtering the initiating CPU in rto_next_cpu().
> This solution has been verified to effectively eliminate spurious self-IPIs
> and prevent CPU hardlockup scenarios.
> 
> Fixes: 4bdced5c9a29 ("sched/rt: Simplify the IPI based RT balancing logic")
> Suggested-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> Signed-off-by: Chen Jinghuang <chenjinghuang2@...wei.com>
> Reviewed-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> 
> ---
> Changes since v2: https://lore.kernel.org/all/20251125083649.1814558-1-chenjinghuang2@huawei.com/
> - Replace the original "check NEED_RESCHED on target CPU"
>   logic with "skip the currently executing CPU"
> - This modification eliminates self-IPIS
> 
> Changes since v1: https://lore.kernel.org/all/20251121014004.564508-1-chenjinghuang2@huawei.com/
> - Remove unneeded extra whitespace
> - Add Reviewed-by tag from Steven Rostedt
> ---
>  kernel/sched/rt.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index f1867fe8e5c5..ec10ed7bb75d 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -2100,6 +2100,7 @@ static void push_rt_tasks(struct rq *rq)
>   */
>  static int rto_next_cpu(struct root_domain *rd)
>  {
> +	int this_cpu = smp_processor_id();
>  	int next;
>  	int cpu;
>  
> @@ -2118,10 +2119,13 @@ static int rto_next_cpu(struct root_domain *rd)
>  	 */
>  	for (;;) {
>  
> -		/* When rto_cpu is -1 this acts like cpumask_first() */
> -		cpu = cpumask_next(rd->rto_cpu, rd->rto_mask);
> +		do {
> +			/* When rto_cpu is -1 this acts like cpumask_first() */
> +			cpu = cpumask_next(rd->rto_cpu, rd->rto_mask);
>  
> -		rd->rto_cpu = cpu;
> +			rd->rto_cpu = cpu;
> +			/* Do not send IPI to self */
> +		} while (cpu == this_cpu);
>  
>  		if (cpu < nr_cpu_ids)
>  			return cpu;