Messages in this thread | | | Date | Tue, 10 Mar 2020 10:26:42 -0700 | From | "Paul E. McKenney" <> | Subject | Re: Instrumentation and RCU |
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On Tue, Mar 10, 2020 at 01:22:45PM -0400, Mathieu Desnoyers wrote: > ----- On Mar 10, 2020, at 12:49 PM, paulmck paulmck@kernel.org wrote: > > > 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? > > There is a big difference between allowing page faults to happen, and expecting > page faults to happen every time. I suspect many use-cases will end up having > a fast-path which touches user-space data which is in the page cache, but > may end up triggering page faults in rare occasions. > > Therefore, this might justify an SRCU which has low-overhead read-side.
OK, good to know, thank you!
Thanx, Paul
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