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

[Paul E. McKenney]

On Mon, Feb 10, 2020 at 02:44:32PM +0100, Peter Zijlstra wrote:
> 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'.

OK.

> > > 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 ;-)

Me too, but I have harbored a great many hopes over the decades.  ;-)

> > -	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
> :-)

Would they be changing tracing of their energy usage often in production?

							Thanx, Paul