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

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Date:   Mon, 3 Oct 2022 19:29:36 -0700
From:   John Stultz <jstultz@...gle.com>
To:     Hillf Danton <hdanton@...a.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        "Connor O'Brien" <connoro@...gle.com>,
        Qais Yousef <qais.yousef@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [RFC PATCH v4 2/3] sched: Avoid placing RT threads on cores
 handling long softirqs

On Mon, Oct 3, 2022 at 6:56 PM Hillf Danton <hdanton@...a.com> wrote:
> On 3 Oct 2022 23:20:32 +0000 John Stultz <jstultz@...gle.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.

Apologies, I'm not sure I'm following you here. Why would ksoftirqd
preempt the rt task?

thanks
-john