linux-kernel - Re: [RFC 0/3] Revert SRCU from tracepoint infrastructure

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Date:   Mon, 10 Feb 2020 14:44:32 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Joel Fernandes, Google" <joel@...lfernandes.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Ingo Molnar <mingo@...hat.com>,
        Richard Fontana <rfontana@...hat.com>,
        rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
Subject: Re: [RFC 0/3] Revert SRCU from tracepoint infrastructure

On Mon, Feb 10, 2020 at 05:36:52AM -0800, Paul E. McKenney wrote:

> > Furthermore, using srcu would be detrimental, because of how it has
> > smp_mb() in the read side primitives.
> 
> Note that rcu_irq_enter() and rcu_irq_exit() also contain value-returning
> atomics, which imply full memory barriers.

There is a whole lot of perf that doesn't go through tracepoints. It
makes absolutely no sense to make all that more expensive just because
tracepoints are getting 'funny'.

> > The best we can do is move that rcu_irq_enter/exit_*() crud into the
> > perf tracepoint glue I suppose.
> 
> One approach would be to define a synchronize_preempt_disable() that
> waits only for pre-existing disabled-preemption regions (including
> of course diabled-irq and NMI-handler regions.  Something like Steve
> Rostedt's workqueue-baed schedule_on_each_cpu(ftrace_sync) implementation
> might work.
> 
> There are of course some plusses and minuses:
> 
> +	Works on preempt-disable regions in idle-loop code without
> 	the need to invoke rcu_idle_exit() and rcu_idle_enter()..
> 
> +	Straightforward implementation.
> 
> -	Does not work on preempt-disable regions on offline CPUs.
> 	(I have no idea if this really matters.)

I'd hope not ;-)

> -	Schedules on idle CPUs, so usage needs to be restricted to
> 	avoid messing up energy-efficient systems.  (It should be
> 	just fine to use this for tracing.)

Unless you're tracing energy usage -- weird some people actually do that
:-)