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From: Frederic Weisbecker <frederic@kernel.org>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	LKML <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Joel Fernandes <joel@joelfernandes.org>
Subject: Re: Instrumentation and RCU
Date: Tue, 10 Mar 2020 00:52:11 +0100
Message-ID: <20200309235210.GB20868@lenoir> (raw)
In-Reply-To: <20200309204710.GU2935@paulmck-ThinkPad-P72>

On Mon, Mar 09, 2020 at 01:47:10PM -0700, Paul E. McKenney wrote:
> On Mon, Mar 09, 2020 at 06:02:32PM +0100, Thomas Gleixner wrote:
> > #3) RCU idle
> > 
> >     Being able to trace code inside RCU idle sections is very similar to
> >     the question raised in #1.
> > 
> >     Assume all of the instrumentation would be doing conditional RCU
> >     schemes, i.e.:
> > 
> >     if (rcuidle)
> >     	....
> >     else
> >         rcu_read_lock_sched()
> > 
> >     before invoking the actual instrumentation functions and of course
> >     undoing that right after it, that really begs the question whether
> >     it's worth it.
> > 
> >     Especially constructs like:
> > 
> >     trace_hardirqs_off()
> >        idx = srcu_read_lock()
> >        rcu_irq_enter_irqson();
> >        ...
> >        rcu_irq_exit_irqson();
> >        srcu_read_unlock(idx);
> > 
> >     if (user_mode)
> >        user_exit_irqsoff();
> >     else
> >        rcu_irq_enter();
> > 
> >     are really more than questionable. For 99.9999% of instrumentation
> >     users it's absolutely irrelevant whether this traces the interrupt
> >     disabled time of user_exit_irqsoff() or rcu_irq_enter() or not.
> > 
> >     But what's relevant is the tracer overhead which is e.g. inflicted
> >     with todays trace_hardirqs_off/on() implementation because that
> >     unconditionally uses the rcuidle variant with the scru/rcu_irq dance
> >     around every tracepoint.
> > 
> >     Even if the tracepoint sits in the ASM code it just covers about ~20
> >     low level ASM instructions more. The tracer invocation, which is
> >     even done twice when coming from user space on x86 (the second call
> >     is optimized in the tracer C-code), costs definitely way more
> >     cycles. When you take the scru/rcu_irq dance into account it's a
> >     complete disaster performance wise.
> 
> Suppose that we had a variant of RCU that had about the same read-side
> overhead as Preempt-RCU, but which could be used from idle as well as
> from CPUs in the process of coming online or going offline?  I have not
> thought through the irq/NMI/exception entry/exit cases, but I don't see
> why that would be problem.
> 
> This would have explicit critical-section entry/exit code, so it would
> not be any help for trampolines.
> 
> Would such a variant of RCU help?
> 
> Yeah, I know.  Just what the kernel doesn't need, yet another variant
> of RCU...
> 

I was thinking about having a tracing-specific implementation of RCU.
Last week Steve told me that the tracing ring buffer has its own ad-hoc
RCU implementation which schedule a thread on each CPU to complete a grace
period (did I understand it right?). Of course such a flavour of RCU wouldn't
be nice to nohz_full but surely we can arrange some tweaks for those who
require strong isolation. I'm sure you're having a much better idea though.

  parent reply index

Thread overview: 44+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-09 17:02 Thomas Gleixner
2020-03-09 18:15 ` Steven Rostedt
2020-03-09 18:42   ` Joel Fernandes
2020-03-09 19:07     ` Steven Rostedt
2020-03-09 19:20       ` Mathieu Desnoyers
2020-03-16 15:02       ` Joel Fernandes
2020-03-09 18:59   ` Thomas Gleixner
2020-03-10  8:09     ` Masami Hiramatsu
2020-03-10 11:43       ` Thomas Gleixner
2020-03-10 15:31         ` Mathieu Desnoyers
2020-03-10 15:46           ` Steven Rostedt
2020-03-10 16:21             ` Mathieu Desnoyers
2020-03-11  0:18               ` Masami Hiramatsu
2020-03-11  0:37                 ` Mathieu Desnoyers
2020-03-11  7:48                   ` Masami Hiramatsu
2020-03-10 16:06         ` Masami Hiramatsu
2020-03-12 13:53         ` Peter Zijlstra
2020-03-10 15:24       ` Mathieu Desnoyers
2020-03-10 17:05       ` Daniel Thompson
2020-03-09 18:37 ` Mathieu Desnoyers
2020-03-09 18:44   ` Steven Rostedt
2020-03-09 18:52     ` Mathieu Desnoyers
2020-03-09 19:09       ` Steven Rostedt
2020-03-09 19:25         ` Mathieu Desnoyers
2020-03-09 19:52   ` Thomas Gleixner
2020-03-10 15:03     ` Mathieu Desnoyers
2020-03-10 16:48       ` Thomas Gleixner
2020-03-10 17:40         ` Mathieu Desnoyers
2020-03-10 18:31           ` Thomas Gleixner
2020-03-10 18:37             ` Mathieu Desnoyers
2020-03-10  1:40   ` Alexei Starovoitov
2020-03-10  8:02     ` Thomas Gleixner
2020-03-10 16:54     ` Paul E. McKenney
2020-03-17 17:56     ` Joel Fernandes
2020-03-09 20:18 ` Peter Zijlstra
2020-03-09 20:47 ` Paul E. McKenney
2020-03-09 20:58   ` Steven Rostedt
2020-03-09 21:25     ` Paul E. McKenney
2020-03-09 23:52   ` Frederic Weisbecker [this message]
2020-03-10  2:26     ` Paul E. McKenney
2020-03-10 15:13   ` Mathieu Desnoyers
2020-03-10 16:49     ` Paul E. McKenney
2020-03-10 17:22       ` Mathieu Desnoyers
2020-03-10 17:26         ` Paul E. McKenney

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