Re: [PATCH 3/6] ftrace/x86: Warn and ignore graph tracing when RCU is disabled

[Mark Rutland]

On Tue, Jan 24, 2023 at 03:44:35PM +0100, Peter Zijlstra wrote:
> On Mon, Jan 23, 2023 at 05:07:53PM -0500, Steven Rostedt wrote:
> 
> > Actually, perhaps we can just add this, and all you need to do is create
> > and set CONFIG_NO_RCU_TRACING (or some other name).
> 
> Elsewhere I've used CONFIG_ARCH_WANTS_NO_INSTR for this.

Yes please; if we use CONFIG_ARCH_WANTS_NO_INSTR then arm64 will get this "for
free" once we add the missing checks (which I assume we need) in our ftrace_prepare_return().

> Anyway, I took it for a spin and it .... doesn't seems to do the job.
> 
> With my patch the first splat is
> 
>   "RCU not on for: cpuidle_poll_time+0x0/0x70"
> 
> While with yours I seems to get the endless:
> 
>   "WARNING: suspicious RCU usage"
> 
> thing. Let me see if I can figure out where it goes side-ways.

Hmmm... for WARN_ONCE() don't we need to wake RCU first also? I thought we
needed that at least for the printk machinery?

Thanks,
Mark.