linux-kernel - Re: [PATCH v2 3/9] rcu,tracing: Create trace_rcu

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Message-ID: <20200213082716.GI14897@hirez.programming.kicks-ass.net>
Date:   Thu, 13 Feb 2020 09:27:16 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        rostedt@...dmis.org, mingo@...nel.org, gregkh@...uxfoundation.org,
        gustavo@...eddedor.com, tglx@...utronix.de, paulmck@...nel.org,
        josh@...htriplett.org, mathieu.desnoyers@...icios.com,
        jiangshanlai@...il.com
Subject: Re: [PATCH v2 3/9] rcu,tracing: Create trace_rcu_{enter,exit}()

On Wed, Feb 12, 2020 at 06:20:05PM -0500, Joel Fernandes wrote:
> On Wed, Feb 12, 2020 at 10:01:42PM +0100, Peter Zijlstra wrote:

> > +#define trace_rcu_enter()					\
> > +({								\
> > +	unsigned long state = 0;				\
> > +	if (!rcu_is_watching())	{				\
> > +		if (in_nmi()) {					\
> > +			state = __TR_NMI;			\
> > +			rcu_nmi_enter();			\
> > +		} else {					\
> > +			state = __TR_IRQ;			\
> > +			rcu_irq_enter_irqsave();		\
> 
> I think this can be simplified. You don't need to rely on in_nmi() here. I
> believe for NMI's, you can just call rcu_irq_enter_irqsave() and that should
> be sufficient to get RCU watching. Paul can correct me if I'm wrong, but I am
> pretty sure that would work.
> 
> In fact, I think a better naming for rcu_irq_enter_irqsave() pair could be
> (in the first patch):
> 
> rcu_ensure_watching_begin();
> rcu_ensure_watching_end();

So I hadn't looked deeply into rcu_irq_enter(), it seems to call
rcu_nmi_enter_common(), but with @irq=true.

What exactly is the purpose of that @irq argument, and how much will it
hurt to lie there? Will it come apart if we have @irq != !in_nmi()
for example?

There is a comment in there that says ->dynticks_nmi_nesting ought to be
odd only if we're in NMI. The only place that seems to care is
rcu_nmi_exit_common(), and that does indeed do something different for
IRQs vs NMIs.

So I don't think we can blindly unify this. But perhaps Paul sees a way?