Re: Instrumentation and RCU

[Mathieu Desnoyers]

----- On Mar 10, 2020, at 12:49 PM, paulmck paulmck@xxxxxxxxxx wrote:

> On Tue, Mar 10, 2020 at 11:13:27AM -0400, Mathieu Desnoyers wrote:
>> 
>> 
>> ----- On Mar 9, 2020, at 4:47 PM, paulmck paulmck@xxxxxxxxxx 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