linux-kernel - Re: [PATCH v2] tracing/perf: Move rcu_irq_enter/exit

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Message-ID: <698566505.617724.1581438456170.JavaMail.zimbra@efficios.com>
Date:   Tue, 11 Feb 2020 11:27:36 -0500 (EST)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     rostedt <rostedt@...dmis.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Joel Fernandes, Google" <joel@...lfernandes.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        paulmck <paulmck@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [PATCH v2] tracing/perf: Move rcu_irq_enter/exit_irqson() to
 perf trace point hook

----- On Feb 11, 2020, at 11:18 AM, rostedt rostedt@...dmis.org wrote:

> On Tue, 11 Feb 2020 16:34:52 +0100
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
>> > +		if (unlikely(in_nmi()))
>> > +			goto out;
>> 
>> unless I'm mistaken, we can simply do rcu_nmi_enter() in this case, and
>> rcu_nmi_exit() on the other end.
>> 
>> > +		rcu_irq_enter_irqson();
> 
> The thing is, I don't think this can ever happen. We've had in the
> tracepoint.h:
> 
>		/* srcu can't be used from NMI */			\
>		WARN_ON_ONCE(rcuidle && in_nmi());			\
> 
> And this has yet to trigger.

But that "rcuidle" state is defined on a per-tracepoint basis, whereas
"!rcu_is_watching()" is a state which depends on the current execution
context. I don't follow how the fact that this WARN_ON_ONCE() never
triggered allows us to infer anything about (!rcu_is_watching() && in_nmi()).

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com