Re: Instrumentation and RCU

[Paul E. McKenney]

On Tue, Mar 10, 2020 at 11:13:27AM -0400, Mathieu Desnoyers wrote:
> 
> 
> ----- 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 ?

Well, we have BPF requirements as well.

> 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.

Agreed, SRCU works fine for the page-fault case, as the read-side memory
barriers are in the noise compared to page-fault overhead.  Back in
the day, there were light-weight system calls.  Are all of these now
converted to VDSO or similar?

>    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 ?

BPF programs that might sometimes sleep, but are usually lightweight.

I will be double-checking this, of course.

							Thanx, Paul