linux-kernel - Re: [PATCH v2] sched/rt: Skip currently executing CPU in rto_next

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <20251125112605.2c73227a@gandalf.local.home>
Date: Tue, 25 Nov 2025 11:26:05 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Chen Jinghuang <chenjinghuang2@...wei.com>
Cc: <bsegall@...gle.com>, <dietmar.eggemann@....com>,
 <juri.lelli@...hat.com>, <linux-kernel@...r.kernel.org>, <mgorman@...e.de>,
 <mingo@...hat.com>, <peterz@...radead.org>, <vincent.guittot@...aro.org>,
 <vschneid@...hat.com>
Subject: Re: [PATCH v2] sched/rt: Skip currently executing CPU in
 rto_next_cpu()

On Tue, 25 Nov 2025 08:36:49 +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.
> 
> 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 by filtering the initiating CPU in rto_next_cpu().
> 
> Fixes: 4bdced5c9a292 ("sched/rt: Simplify the IPI based RT balancing logic")
> Suggested-by: Steven Rostedt <rostedt@...dmis.org>
> Signed-off-by: Chen Jinghuang <chenjinghuang2@...wei.com>
> ---
> v1-->v2:
> 	- Replace the original "check NEED_RESCHED on target CPU"
> 	  logic with "skip the currently executing CPU"
> 	- This modification eliminates self-IPIS
> ---
> ---
>  kernel/sched/rt.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 7936d4333731..dc0d583aa59a 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;
>  
> @@ -2118,14 +2119,17 @@ 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);
> -
> -		rd->rto_cpu = cpu;
> +		do {
> +			/* When rto_cpu is -1 this acts like cpumask_first() */
> +			cpu = cpumask_next(rd->rto_cpu, rd->rto_mask);
> +			rd->rto_cpu = cpu;
> +			/* Do not send IPI to self */
> +		} while (cpu == this_cpu);

So this fixes the issue you see too! Great!

>  
>  		if (cpu < nr_cpu_ids)
>  			return cpu;
>  
> +

Unneeded extra whitespace.

Other than that:

Reviewed-by: Steven Rostedt (Google) <rostedt@...dmis.org>

-- Steve

>  		rd->rto_cpu = -1;
>  
>  		/*