Messages in this thread | | | Date | Tue, 10 Mar 2020 13:22:45 -0400 (EDT) | From | Mathieu Desnoyers <> | Subject | Re: Instrumentation and RCU |
| |
----- 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.
Thanks,
Mathieu
-- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
| |