linux-kernel - Re: [PATCH 2/3] rcu: Defer RCU kthreads wakeup when CPU is dying

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Date: Tue, 19 Dec 2023 12:42:47 +0800
From: Hillf Danton <hdanton@...a.com>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Neeraj Upadhyay <neeraj.upadhyay@....com>,
	Uladzislau Rezki <urezki@...il.com>,
	"Paul E . McKenney" <paulmck@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 2/3] rcu: Defer RCU kthreads wakeup when CPU is dying

On Tue, 19 Dec 2023 00:19:15 +0100 Frederic Weisbecker <frederic@...nel.org>
> +static void swake_up_one_online(struct swait_queue_head *wqh)
> +{
> +	int cpu = get_cpu();
> +
> +	/*
> +	 * If called from rcutree_report_cpu_starting(), wake up
> +	 * is dangerous that late in the CPU-down hotplug process. The
> +	 * scheduler might queue an ignored hrtimer. Defer the wake up
> +	 * to an online CPU instead.
> +	 */

But why is scheduler having any interest selecting a dying CPU for
adding a hrtimer on at the first place?

> +	if (unlikely(cpu_is_offline(cpu))) {
> +		int target;
> +
> +		target = cpumask_any_and(housekeeping_cpumask(HK_TYPE_RCU),
> +					 cpu_online_mask);
> +
> +		smp_call_function_single(target, swake_up_one_online_ipi,
> +					 wqh, 0);
> +		put_cpu();
> +	} else {
> +		put_cpu();
> +		swake_up_one(wqh);
> +	}
> +}