linux-kernel - Re: [PATCH] sched/rt: Use cpu_active_mask to prevent rto_push_irq

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Date:   Thu, 17 Nov 2022 17:00:28 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Xuewen Yan <xuewen.yan@...soc.com>
Cc:     <peterz@...radead.org>, <mingo@...hat.com>,
        <juri.lelli@...hat.com>, <vincent.guittot@...aro.org>,
        <dietmar.eggemann@....com>, <bsegall@...gle.com>,
        <mgorman@...e.de>, <bristot@...hat.com>, <vschneid@...hat.com>,
        <ke.wang@...soc.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched/rt: Use cpu_active_mask to prevent
 rto_push_irq_work's dead loop

On Mon, 14 Nov 2022 20:04:53 +0800
Xuewen Yan <xuewen.yan@...soc.com> wrote:

> +++ b/kernel/sched/rt.c
> @@ -2219,6 +2219,7 @@ static int rto_next_cpu(struct root_domain *rd)
>  {
>  	int next;
>  	int cpu;
> +	struct cpumask tmp_cpumask;

If you have a machine with thousands of CPUs, this will likely kill the
stack.

>  
>  	/*
>  	 * When starting the IPI RT pushing, the rto_cpu is set to -1,
> @@ -2238,6 +2239,11 @@ static int rto_next_cpu(struct root_domain *rd)
>  		/* When rto_cpu is -1 this acts like cpumask_first() */
>  		cpu = cpumask_next(rd->rto_cpu, rd->rto_mask);
>  
> +		cpumask_and(&tmp_cpumask, rd->rto_mask, cpu_active_mask);
> +		if (rd->rto_cpu == -1 && cpumask_weight(&tmp_cpumask) == 1 &&
> +		    cpumask_test_cpu(smp_processor_id(), &tmp_cpumask))
> +			break;
> +

Kill the above.

>  		rd->rto_cpu = cpu;
>  
>  		if (cpu < nr_cpu_ids) {

Why not just add here:

			if (!cpumask_test_cpu(cpu, cpu_active_mask))
				continue;
			return cpu;
		}

?

-- Steve