Messages in this thread | | | Date | Tue, 10 Mar 2020 00:52:11 +0100 | From | Frederic Weisbecker <> | Subject | Re: Instrumentation and RCU |
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On Mon, Mar 09, 2020 at 01:47:10PM -0700, Paul E. McKenney wrote: > On Mon, Mar 09, 2020 at 06:02:32PM +0100, Thomas Gleixner 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. > > 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... >
I was thinking about having a tracing-specific implementation of RCU. Last week Steve told me that the tracing ring buffer has its own ad-hoc RCU implementation which schedule a thread on each CPU to complete a grace period (did I understand it right?). Of course such a flavour of RCU wouldn't be nice to nohz_full but surely we can arrange some tweaks for those who require strong isolation. I'm sure you're having a much better idea though.
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