linux-kernel - Re: [PATCH] sched/rt: rto_next_cpu: Skip CPUs with NEED

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Message-ID: <20251121123811.3d34b10b@gandalf.local.home>
Date: Fri, 21 Nov 2025 12:38:11 -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: [PATCH] sched/rt: rto_next_cpu: Skip CPUs with NEED_RESCHED

On Fri, 21 Nov 2025 01:40:04 +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,
> wasting CPU cycles on unnecessary interrupt handling.

Is it truly "infinite", or just wasted due to other CPUs requesting a pull?

Also, it appears the issue here is that it's sending to itself.

> 
> The triggering 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/cross-CPU IPI when target CPU already has a pending
> reschedule, making the IPI unnecessary.
> 
> Signed-off-by: Chen Jinghuang <chenjinghuang2@...wei.com>
> ---
>  kernel/sched/rt.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 7936d4333731..29ce1af9f121 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -2123,8 +2123,20 @@ static int rto_next_cpu(struct root_domain *rd)
>  
>  		rd->rto_cpu = cpu;
>  
> -		if (cpu < nr_cpu_ids)
> +		if (cpu < nr_cpu_ids) {
> +			struct task_struct *t;
> +			struct rq *rq = cpu_rq(cpu);
> +
> +			rcu_read_lock();
> +			t = rcu_dereference(rq->curr);
> +			if (test_tsk_need_resched(t)) {
> +				rcu_read_unlock();
> +				continue;
> +			}
> +			rcu_read_unlock();
> +
>  			return cpu;
> +		}
>  
>  		rd->rto_cpu = -1;
>  

Instead of skipping need resched, would skipping the current CPU work too?

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 7936d4333731..cacd8912cd31 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;

-- Steve