Re: Instrumentation and RCU

[Paul E. McKenney]

On Mon, Mar 09, 2020 at 06:02:32PM +0100, Thomas Gleixner wrote:
> Folks,
> 
> I'm starting a new conversation because there are about 20 different
> threads which look at that problem in various ways and the information
> is so scattered that creating a coherent picture is pretty much
> impossible.
> 
> There are several problems to solve:
> 
>    1) Fragile low level entry code
> 
>    2) Breakpoint utilization
> 
>    3) RCU idle
> 
>    4) Callchain protection
> 
> #1 Fragile low level entry code
> 
>    While I understand the desire of instrumentation to observe
>    everything we really have to ask the question whether it is worth the
>    trouble especially with entry trainwrecks like x86, PTI and other
>    horrors in that area.
> 
>    I don't think so and we really should just bite the bullet and forbid
>    any instrumentation in that code unless it is explicitly designed
>    for that case, makes sense and has a real value from an observation
>    perspective.
> 
>    This is very much related to #3..
> 
> #2) Breakpoint utilization
> 
>     As recent findings have shown, breakpoint utilization needs to be
>     extremly careful about not creating infinite breakpoint recursions.
> 
>     I think that's pretty much obvious, but falls into the overall
>     question of how to protect callchains.
> 
> #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...

							Thanx, Paul

> #4 Protecting call chains
> 
>    Our current approach of annotating functions with notrace/noprobe is
>    pretty much broken.
> 
>    Functions which are marked NOPROBE or notrace call out into functions
>    which are not marked and while this might be ok, there are enough
>    places where it is not. But we have no way to verify that.
> 
>    That's just a recipe for disaster. We really cannot request from
>    sysadmins who want to use instrumentation to stare at the code first
>    whether they can place/enable an instrumentation point somewhere.
>    That'd be just a bad joke.
> 
>    I really think we need to have proper text sections which are off
>    limit for any form of instrumentation and have tooling to analyze the
>    calls into other sections. These calls need to be annotated as safe
>    and intentional.
> 
> Thoughts?
> 
> Thanks,
> 
>         tglx
> 
> 
> 
> 
> 
> 
>    
>