linux-kernel - Re: Can we switch the tracepoints from preempt protection to rcu_read

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Message-ID: <20250117122829.615753e8@gandalf.local.home>
Date: Fri, 17 Jan 2025 12:28:29 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: LKML <linux-kernel@...r.kernel.org>, Sebastian Andrzej Siewior
 <bigeasy@...utronix.de>, linux-rt-users <linux-rt-users@...r.kernel.org>
Subject: Re: Can we switch the tracepoints from preempt protection to
 rcu_read_lock?

On Tue, 10 Dec 2024 14:03:38 -0500
Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:

> > I'm not asking for you to do this work, but can you remind me what you saw
> > when you created the faultable tracepoints?  
> 
> I saw the future! ;-)

Well, I actually meant what you saw in the tracing code that would have an
issue with removing preempt_disable from tracepoints ;-)

Anyway. Sebastian,

Doing a quick scan, one issue is your code:

static inline unsigned int tracing_gen_ctx_dec(void)
{
	unsigned int trace_ctx;

	trace_ctx = tracing_gen_ctx();
	/*
	 * Subtract one from the preemption counter if preemption is enabled,
	 * see trace_event_buffer_reserve()for details.
	 */
	if (IS_ENABLED(CONFIG_PREEMPTION))
		trace_ctx--;
	return trace_ctx;
}

Looks like that could be removed if we remove preemption from the tracepoints.

-- Steve