linux-kernel - Re: [PATCH] ring-buffer: Use a housekeeping CPU to wake up waiters

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Message-ID: <20260107111709.0d115cd8@gandalf.local.home>
Date: Wed, 7 Jan 2026 11:17:09 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Petr Tesarik <ptesarik@...e.com>
Cc: Masami Hiramatsu <mhiramat@...nel.org>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, Sebastian Andrzej Siewior
 <bigeasy@...utronix.de>, Clark Williams <clrkwllms@...nel.org>,
 linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
 linux-rt-devel@...ts.linux.dev
Subject: Re: [PATCH] ring-buffer: Use a housekeeping CPU to wake up waiters

On Wed, 7 Jan 2026 10:51:37 +0100
Petr Tesarik <ptesarik@...e.com> wrote:

> Erm. It's actually good I had a look. :-(
> 
> A helpful comment in irq_work_queue_on() explains that "arch remote IPI
> send/receive backend aren't NMI safe". That's something I wasn't aware
> of, and I'm afraid it's the end of story. The comment is followed by a
> WARN_ON_ONCE(in_nmi()), and I can easily trigger it with "perf top"
> while nmi:nmi_handler is traced.
> 
> Please, remove the patch again. I'm sorry.

Or we simply change it to:

static inline void
rb_irq_work_queue(struct rb_irq_work *irq_work)
{
	int cpu;

	/* irq_work_queue_on() is not allowed in NMI context */
	if (in_nmi()) {
		irq_work_queue(&irq_work->work, cpu);
		return;
	}

	cpu = housekeeping_any_cpu(HK_TYPE_KERNEL_NOISE);
	/*
	 * If CPU isolation is not active, cpu is always the current
	 * CPU, and the following is equivallent to irq_work_queue().
	 */
	irq_work_queue_on(&irq_work->work, cpu);
}


-- Steve