Re: [PATCH RFC ftrace] Chose RCU Tasks based on TASKS_RCU rather than PREEMPTION

[Steven Rostedt]

On Wed, 28 Feb 2024 11:38:29 -0800
"Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote:

> The advent of CONFIG_PREEMPT_AUTO, AKA lazy preemption, will mean that
> even kernels built with CONFIG_PREEMPT_NONE or CONFIG_PREEMPT_VOLUNTARY
> might see the occasional preemption, and that this preemption just might
> happen within a trampoline.
> 
> Therefore, update ftrace_shutdown() to invoke synchronize_rcu_tasks()
> based on CONFIG_TASKS_RCU instead of CONFIG_PREEMPTION.
> 
> Only build tested.
> 
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: <linux-trace-kernel@xxxxxxxxxxxxxxx>
> 
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 2da4eaa2777d6..c9e6c69cf3446 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -3156,7 +3156,7 @@ int ftrace_shutdown(struct ftrace_ops *ops, int command)
>  		 * synchronize_rcu_tasks() will wait for those tasks to
>  		 * execute and either schedule voluntarily or enter user space.
>  		 */
> -		if (IS_ENABLED(CONFIG_PREEMPTION))
> +		if (IS_ENABLED(CONFIG_TASKS_RCU))
>  			synchronize_rcu_tasks();

What happens if CONFIG_TASKS_RCU is not enabled? Does
synchronize_rcu_tasks() do anything? Or is it just a synchronize_rcu()?

If that's the case, perhaps just remove the if statement and make it:

	synchronize_rcu_tasks();

Not sure an extra synchronize_rcu() will hurt (especially after doing a
synchronize_rcu_tasks_rude() just before hand!

-- Steve