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

[Paul E. McKenney]

On Wed, Feb 28, 2024 at 03:22:36PM -0500, Steven Rostedt wrote:
> 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()?

It is 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!

That would work for me.  If there are no objections, I will make this
change.

							Thanx, Paul