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

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Message-ID: <Y9D4PIZacXgWapWX@hirez.programming.kicks-ass.net>
Date:   Wed, 25 Jan 2023 10:37:00 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Steven Rostedt <rostedt@...dmis.org>, mingo@...nel.org,
        will@...nel.org, boqun.feng@...il.com, tglx@...utronix.de,
        bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org,
        hpa@...or.com, seanjc@...gle.com, pbonzini@...hat.com,
        jgross@...e.com, srivatsa@...il.mit.edu, amakhalov@...are.com,
        pv-drivers@...are.com, mhiramat@...nel.org, wanpengli@...cent.com,
        vkuznets@...hat.com, boris.ostrovsky@...cle.com, rafael@...nel.org,
        daniel.lezcano@...aro.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com,
        vschneid@...hat.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        linux-trace-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH 3/6] ftrace/x86: Warn and ignore graph tracing when RCU
 is disabled

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.