Re: Instrumentation and RCU

[Joel Fernandes]

On Mon, Mar 9, 2020 at 11:15 AM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> On Mon, 09 Mar 2020 18:02:32 +0100
> Thomas Gleixner <tglx@xxxxxxxxxxxxx> 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.
>
> Is this specifically to do with the kernel/trace/trace_preemptirqs.c code
> that was added by Joel?

Just started a vacation here and will be back on January 12th. Will
take a detailed look at Thomas's email at that time.

Adding some more folks (Daniel, Valentin) who have used the
preempt/irq tracepoints.

I agree we should reorder things and avoid these circular
dependencies, it bothers me too. I am happy to help with any clean ups
related to it. Let us definitely discuss more and fix it. Thanks.

- Joel