Re: Instrumentation and RCU

[Mathieu Desnoyers]

----- On Mar 9, 2020, at 4:47 PM, paulmck paulmck@xxxxxxxxxx wrote:
[...]

> 
> 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...

Hi Paul,

I think that before introducing yet another RCU flavor, it's important
to take a step back and look at the tracer requirements first. If those
end up being covered by currently available RCU flavors, then why add
another ?

I can start with a few use-cases I have in mind. Others should feel free
to pitch in:

Tracing callsite context:

1) Thread context

   1.1) Preemption enabled

   One tracepoint in this category is syscall enter/exit. We should introduce
   a variant of tracepoints relying on SRCU for this use-case so we can take
   page faults when fetching userspace data.

   1.2) Preemption disabled

   Tree-RCU works fine.

   1.3) IRQs disabled

   Tree-RCU works fine.

2) IRQ handler context

   Tree-RCU works fine.

3) NMI context

   Tree-RCU works fine.

4) cpuidle context (!rcu_is_watching())

   - By all means, we should not have tracepoints requiring to temporarily enable
     RCU in frequent code-paths. It appears that we should be able to remove the few
     offenders we currently have (e.g. enter from usermode),
   - For tracepoints which are infrequently called from !rcu_is_watching context, checking
     whether RCU is watching and only enabling when needed should be fast enough.

Are there other use-cases am I missing that would justify adding another flavor of RCU ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com