Re: [RFC PATCH v4 2/3] sched: Avoid placing RT threads on cores handling long softirqs

[Hillf Danton <hdanton@sina.com>]

On 3 Oct 2022 23:20:32 +0000 John Stultz <jstultz@google.com>
> +#ifdef CONFIG_RT_SOFTIRQ_OPTIMIZATION
> +#define __use_softirq_opt 1
> +/*
> + * Return whether the given cpu is currently non-preemptible
> + * while handling a potentially long softirq, or if the current
> + * task is likely to block preemptions soon because it is a
> + * ksoftirq thread that is handling slow softirq.
> + */
> +static bool cpu_busy_with_softirqs(int cpu)
> +{
> +	u32 softirqs = per_cpu(active_softirqs, cpu) |
> +		       __cpu_softirq_pending(cpu);
> +	struct task_struct *cpu_ksoftirqd = per_cpu(ksoftirqd, cpu);
> +	struct task_struct *curr;
> +	struct rq *rq = cpu_rq(cpu);
> +	int ret;
> +
> +	rcu_read_lock();
> +	curr = READ_ONCE(rq->curr); /* unlocked access */
> +	ret = (softirqs & LONG_SOFTIRQ_MASK) &&
> +		 (curr == cpu_ksoftirqd ||
> +		  preempt_count() & SOFTIRQ_MASK);
> +	rcu_read_unlock();
> +	return ret;
> +}
> +#else
> +#define __use_softirq_opt 0
> +static bool cpu_busy_with_softirqs(int cpu)
> +{
> +	return false;
> +}
> +#endif /* CONFIG_RT_SOFTIRQ_OPTIMIZATION */
> +
> +static bool rt_task_fits_cpu(struct task_struct *p, int cpu)
> +{
> +	return !cpu_busy_with_softirqs(cpu) && rt_task_fits_capacity(p, cpu);
> +}

On one hand, RT task is not layency sensitive enough if it fails to preempt
ksoftirqd. On the other, deferring softirq to ksoftirqd barely makes sense
in 3/3 if it preempts the current RT task.