Messages in this thread | | | Date | Tue, 10 Mar 2020 09:49:27 -0700 | From | "Paul E. McKenney" <> | Subject | Re: Instrumentation and RCU |
| |
On Tue, Mar 10, 2020 at 11:13:27AM -0400, Mathieu Desnoyers wrote: > > > ----- 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 ?
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
| |