From: "Paul E. McKenney" <paulmck@kernel.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
LKML <linux-kernel@vger.kernel.org>,
Ingo Molnar <mingo@kernel.org>,
"Joel Fernandes (Google)" <joel@joelfernandes.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
"Gustavo A. R. Silva" <gustavo@embeddedor.com>,
Thomas Gleixner <tglx@linutronix.de>,
Josh Triplett <josh@joshtriplett.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Lai Jiangshan <jiangshanlai@gmail.com>
Subject: Re: [PATCH] tracing/perf: Move rcu_irq_enter/exit_irqson() to perf trace point hook
Date: Tue, 11 Feb 2020 07:06:15 -0800 [thread overview]
Message-ID: <20200211150615.GK2935@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20200211090503.68c0d70f@gandalf.local.home>
On Tue, Feb 11, 2020 at 09:05:03AM -0500, Steven Rostedt wrote:
> On Tue, 11 Feb 2020 12:49:54 +0100
> Peter Zijlstra <peterz@infradead.org> wrote:
>
> > On Mon, Feb 10, 2020 at 05:06:43PM -0500, Steven Rostedt wrote:
> > > + if (!rcu_watching) { \
> > > + /* Can not use RCU if rcu is not watching and in NMI */ \
> > > + if (in_nmi()) \
> > > + return; \
> > > + rcu_irq_enter_irqson(); \
> > > + } \
> >
> > I saw the same weirdness in __trace_stack(), and I'm confused by it.
> >
> > How can we ever get to: in_nmi() && !rcu_watching() ? That should be a
> > BUG. In particular, nmi_enter() has rcu_nmi_enter().
> >
> > Paul, can that really happen?
>
> The stack tracer connects to the function tracer and is called at all
> the places that function tracing can be called from. As I like being
> able to trace RCU internal functions (especially as they are complex),
> I don't want to set them all to notrace. But, for callbacks that
> require RCU to be watching, we need this check, because there's some
> internal state that we can be in an NMI and RCU is not watching (as
> there's some places in nmi_enter that can be traced!).
>
> And if we are tracing preempt_enable and preempt_disable (as Joel added
> trace events there), it may be the case for trace events too.
Ah, thank you for the reminder!
Should Documentation/RCU/Design/Requirements/Requirements.rst be
updated to include this?
And I have to ask... What happens if we are very early in from-idle
NMI entry (or very late in NMI exit), such that both in_nmi() and
rcu_is_watching() are returning false? Or did I miss a turn somewhere?
Thanx, Paul
next prev parent reply other threads:[~2020-02-11 15:06 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-10 22:06 [PATCH] tracing/perf: Move rcu_irq_enter/exit_irqson() to perf trace point hook Steven Rostedt
2020-02-11 0:30 ` Mathieu Desnoyers
2020-02-11 2:22 ` Steven Rostedt
2020-02-11 2:32 ` joel
2020-02-11 15:19 ` Mathieu Desnoyers
2020-02-11 12:00 ` Peter Zijlstra
2020-02-11 13:03 ` Paul E. McKenney
2020-02-11 13:16 ` Peter Zijlstra
2020-02-11 13:23 ` Paul E. McKenney
2020-02-11 14:10 ` Steven Rostedt
2020-02-11 11:49 ` Peter Zijlstra
2020-02-11 12:59 ` Paul E. McKenney
2020-02-11 13:10 ` Peter Zijlstra
2020-02-11 13:20 ` Paul E. McKenney
2020-02-11 14:05 ` Steven Rostedt
2020-02-11 15:05 ` Peter Zijlstra
2020-02-11 15:29 ` Peter Zijlstra
2020-02-11 16:16 ` Steven Rostedt
2020-02-11 15:06 ` Paul E. McKenney [this message]
2020-02-11 15:31 ` Peter Zijlstra
2020-02-11 15:40 ` Paul E. McKenney
2020-02-11 12:21 ` Peter Zijlstra
2020-02-11 14:10 ` Steven Rostedt
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