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Date: Wed, 25 Jan 2023 11:32:04 +0000
From: Mark Rutland <mark.rutland@....com>
To: Peter Zijlstra <peterz@...radead.org>
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,
Paul McKenney <paulmck@...nel.org>,
Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 3/6] ftrace/x86: Warn and ignore graph tracing when RCU
is disabled
On Wed, Jan 25, 2023 at 11:47:44AM +0100, Peter Zijlstra wrote:
> 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().
> >
> > > 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?
>
> OK, the below seems to work nice for me -- although I'm still on a
> hacked up printk, but the recursive RCU not watching fail seems to be
> tamed.
FWIW, I gave that a spin on arm64 with the ftrace selftests, and I see no
splats, so it looks good on that front.
Currently arm64's BUG/WARN exception handling does the usual
lockdep/rcu/whatever stuff before getting to report_bug(), so that bit should
be redundant (and any WARN() or BUG() early in the entry code is likely to lead
to a stack overflow and kill the kernel), but it shouldn't be harmful.
> Ofc. Paul might have an opinion on this glorious bodge ;-)
I'll leave that to Paul. ;)
Thanks,
Mark.
>
> ---
>
> diff --git a/include/linux/trace_recursion.h b/include/linux/trace_recursion.h
> index c303f7a114e9..d48cd92d2364 100644
> --- a/include/linux/trace_recursion.h
> +++ b/include/linux/trace_recursion.h
> @@ -135,6 +135,21 @@ extern void ftrace_record_recursion(unsigned long ip, unsigned long parent_ip);
> # define do_ftrace_record_recursion(ip, pip) do { } while (0)
> #endif
>
> +#ifdef CONFIG_ARCH_WANTS_NO_INSTR
> +# define trace_warn_on_no_rcu(ip) \
> + ({ \
> + bool __ret = !rcu_is_watching(); \
> + if (__ret && !trace_recursion_test(TRACE_RECORD_RECURSION_BIT)) { \
> + trace_recursion_set(TRACE_RECORD_RECURSION_BIT); \
> + WARN_ONCE(true, "RCU not on for: %pS\n", (void *)ip); \
> + trace_recursion_clear(TRACE_RECORD_RECURSION_BIT); \
> + } \
> + __ret; \
> + })
> +#else
> +# define trace_warn_on_no_rcu(ip) false
> +#endif
> +
> /*
> * Preemption is promised to be disabled when return bit >= 0.
> */
> @@ -144,6 +159,9 @@ static __always_inline int trace_test_and_set_recursion(unsigned long ip, unsign
> unsigned int val = READ_ONCE(current->trace_recursion);
> int bit;
>
> + if (trace_warn_on_no_rcu(ip))
> + return -1;
> +
> bit = trace_get_context_bit() + start;
> if (unlikely(val & (1 << bit))) {
> /*
> diff --git a/lib/bug.c b/lib/bug.c
> index c223a2575b72..0a10643ea168 100644
> --- a/lib/bug.c
> +++ b/lib/bug.c
> @@ -47,6 +47,7 @@
> #include <linux/sched.h>
> #include <linux/rculist.h>
> #include <linux/ftrace.h>
> +#include <linux/context_tracking.h>
>
> extern struct bug_entry __start___bug_table[], __stop___bug_table[];
>
> @@ -153,7 +154,7 @@ struct bug_entry *find_bug(unsigned long bugaddr)
> return module_find_bug(bugaddr);
> }
>
> -enum bug_trap_type report_bug(unsigned long bugaddr, struct pt_regs *regs)
> +static enum bug_trap_type __report_bug(unsigned long bugaddr, struct pt_regs *regs)
> {
> struct bug_entry *bug;
> const char *file;
> @@ -209,6 +210,30 @@ enum bug_trap_type report_bug(unsigned long bugaddr, struct pt_regs *regs)
> return BUG_TRAP_TYPE_BUG;
> }
>
> +enum bug_trap_type report_bug(unsigned long bugaddr, struct pt_regs *regs)
> +{
> + enum bug_trap_type ret;
> + bool rcu = false;
> +
> +#ifdef CONFIG_CONTEXT_TRACKING_IDLE
> + /*
> + * Horrible hack to shut up recursive RCU isn't watching fail since
> + * lots of the actual reporting also relies on RCU.
> + */
> + if (!rcu_is_watching()) {
> + rcu = true;
> + ct_state_inc(RCU_DYNTICKS_IDX);
> + }
> +#endif
> +
> + ret = __report_bug(bugaddr, regs);
> +
> + if (rcu)
> + ct_state_inc(RCU_DYNTICKS_IDX);
> +
> + return ret;
> +}
> +
> static void clear_once_table(struct bug_entry *start, struct bug_entry *end)
> {
> struct bug_entry *bug;
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