Messages in this thread | | | Date | Tue, 10 Mar 2020 11:13:27 -0400 (EDT) | From | Mathieu Desnoyers <> | Subject | Re: Instrumentation and RCU |
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----- On Mar 9, 2020, at 4:47 PM, paulmck paulmck@kernel.org 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
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