From: "Paul E. McKenney" <paulmck@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Joel Fernandes <joel@joelfernandes.org>,
linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
rostedt@goodmis.org, mingo@kernel.org,
gregkh@linuxfoundation.org, gustavo@embeddedor.com,
tglx@linutronix.de, josh@joshtriplett.org,
mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com
Subject: Re: [PATCH v2 3/9] rcu,tracing: Create trace_rcu_{enter,exit}()
Date: Thu, 13 Feb 2020 05:51:38 -0800 [thread overview]
Message-ID: <20200213135138.GB2935@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20200213082716.GI14897@hirez.programming.kicks-ass.net>
On Thu, Feb 13, 2020 at 09:27:16AM +0100, Peter Zijlstra wrote:
> 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?
The reason for the irq argument is to avoid invoking
rcu_prepare_for_idle() and rcu_dynticks_task_enter() from NMI context
from rcu_nmi_exit_common(). Similarly, we need to avoid invoking
rcu_dynticks_task_exit() and rcu_cleanup_after_idle() from NMI context
from rcu_nmi_enter_common().
It might well be that I could make these functions be NMI-safe, but
rcu_prepare_for_idle() in particular would be a bit ugly at best.
So, before looking into that, I have a question. Given these proposed
changes, will rcu_nmi_exit_common() and rcu_nmi_enter_common() be able
to just use in_nmi()?
Thanx, Paul
next prev parent reply other threads:[~2020-02-13 13:51 UTC|newest]
Thread overview: 82+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-12 21:01 [PATCH v2 0/9] tracing vs rcu vs nmi Peter Zijlstra
2020-02-12 21:01 ` [PATCH v2 1/9] rcu: Rename rcu_irq_{enter,exit}_irqson() Peter Zijlstra
2020-02-12 21:01 ` Peter Zijlstra
2020-02-12 22:38 ` Joel Fernandes
2020-02-12 22:38 ` Joel Fernandes
2020-02-12 21:01 ` [PATCH v2 2/9] rcu: Mark rcu_dynticks_curr_cpu_in_eqs() inline Peter Zijlstra
2020-02-12 21:01 ` Peter Zijlstra
2020-02-12 22:38 ` Joel Fernandes
2020-02-13 1:41 ` Steven Rostedt
2020-02-13 14:25 ` Joel Fernandes
2020-02-12 21:01 ` [PATCH v2 3/9] rcu,tracing: Create trace_rcu_{enter,exit}() Peter Zijlstra
2020-02-12 21:01 ` Peter Zijlstra
2020-02-12 23:20 ` Joel Fernandes
2020-02-13 8:27 ` Peter Zijlstra
2020-02-13 13:31 ` Joel Fernandes
2020-02-13 13:51 ` Paul E. McKenney [this message]
2020-02-13 16:40 ` Peter Zijlstra
2020-02-13 18:56 ` Paul E. McKenney
2020-02-13 20:44 ` Joel Fernandes
2020-02-13 20:44 ` Joel Fernandes
2020-02-13 20:54 ` Paul E. McKenney
2020-02-13 21:19 ` Joel Fernandes
2020-02-13 21:38 ` Steven Rostedt
2020-02-13 21:38 ` Steven Rostedt
2020-02-13 21:50 ` Paul E. McKenney
2020-02-13 21:50 ` Paul E. McKenney
2020-02-13 22:04 ` Steven Rostedt
2020-02-13 22:39 ` Paul E. McKenney
2020-02-14 6:19 ` Masami Hiramatsu
2020-02-15 14:59 ` Paul E. McKenney
2020-02-15 14:59 ` Paul E. McKenney
2020-02-17 8:55 ` Masami Hiramatsu
2020-02-17 16:31 ` Paul E. McKenney
2020-02-18 4:33 ` Masami Hiramatsu
2020-02-18 16:12 ` Paul E. McKenney
2020-02-18 16:15 ` Mathieu Desnoyers
2020-02-18 16:35 ` Steven Rostedt
2020-02-18 17:46 ` Steven Rostedt
2020-02-18 17:46 ` Steven Rostedt
2020-02-18 20:18 ` Paul E. McKenney
2020-02-19 2:45 ` Masami Hiramatsu
2020-03-06 18:01 ` Masami Hiramatsu
2020-03-06 18:01 ` Masami Hiramatsu
2020-03-06 18:47 ` Joel Fernandes
2020-03-06 18:47 ` Joel Fernandes
2020-03-06 19:11 ` Joel Fernandes
2020-03-07 1:58 ` Masami Hiramatsu
2020-03-07 1:58 ` Masami Hiramatsu
2020-03-06 0:42 ` Thomas Gleixner
2020-02-13 21:48 ` Paul E. McKenney
2020-02-13 22:58 ` Joel Fernandes
2020-02-13 22:58 ` Joel Fernandes
2020-02-13 23:55 ` Steven Rostedt
2020-02-18 19:58 ` Peter Zijlstra
2020-02-18 19:58 ` Peter Zijlstra
2020-02-18 20:17 ` Paul E. McKenney
2020-02-18 20:40 ` Peter Zijlstra
2020-02-18 21:39 ` Paul E. McKenney
2020-02-19 9:57 ` Peter Zijlstra
2020-02-19 12:46 ` Paul E. McKenney
2020-02-12 23:27 ` Joel Fernandes
2020-02-12 23:27 ` Joel Fernandes
2020-02-13 8:28 ` Peter Zijlstra
2020-02-13 18:39 ` Joel Fernandes
2020-02-12 21:01 ` [PATCH v2 4/9] sched,rcu,tracing: Avoid tracing before in_nmi() is correct Peter Zijlstra
2020-02-12 21:01 ` Peter Zijlstra
2020-02-12 21:01 ` [PATCH v2 5/9] x86,tracing: Add comments to do_nmi() Peter Zijlstra
2020-02-12 21:01 ` [PATCH v2 6/9] perf,tracing: Prepare the perf-trace interface for RCU changes Peter Zijlstra
2020-02-12 21:01 ` Peter Zijlstra
2020-02-12 23:28 ` Joel Fernandes
2020-02-13 8:29 ` Peter Zijlstra
2020-02-13 18:38 ` Joel Fernandes
2020-02-12 21:01 ` [PATCH v2 7/9] tracing: Employ trace_rcu_{enter,exit}() Peter Zijlstra
2020-02-12 21:01 ` Peter Zijlstra
2020-02-12 21:01 ` [PATCH v2 8/9] tracing: Remove regular RCU context for _rcuidle tracepoints (again) Peter Zijlstra
2020-02-12 21:01 ` [PATCH v2 9/9] perf,tracing: Allow function tracing when !RCU Peter Zijlstra
2020-02-14 2:28 ` Sergey Senozhatsky
2020-02-14 2:42 ` Sergey Senozhatsky
2020-02-14 3:32 ` Steven Rostedt
2020-02-14 3:32 ` Steven Rostedt
2020-02-14 20:38 ` Kim Phillips
2020-02-14 22:48 ` Steven Rostedt
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