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

[Peter Zijlstra]

On Tue, Jan 24, 2023 at 05:12:14PM +0000, Mark Rutland wrote:
> 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().

Aye.

> > 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?

Yeah, I'm currently running with a hacked up printk that redirects
everything into early_printk() but it still trips up lots.

I was just about to go stick on RCU magic into WARN itself, this isn't
going to be the only site triggering this fail-cascade.