From: "Paul E. McKenney" <paulmck@kernel.org>
To: Frederic Weisbecker <frederic@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: Mon, 9 Mar 2020 19:26:49 -0700 [thread overview]
Message-ID: <20200310022649.GW2935@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20200309235210.GB20868@lenoir>
On Tue, Mar 10, 2020 at 12:52:11AM +0100, Frederic Weisbecker wrote:
> 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.
Well, that too. Please see CONFIG_TASKS_RCU_RUDE in current
"dev" on -rcu. But yes, another is on its way...
Hey, it compiled, so it much be perfect, right? :-/
Thanx, Paul
next prev parent reply other threads:[~2020-03-10 2:26 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-09 17:02 Instrumentation and RCU 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
2020-03-10 2:26 ` Paul E. McKenney [this message]
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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