Re: Copying TLS/user register data per perf-sample?

[Beau Belgrave]

On Tue, Apr 09, 2024 at 04:32:46PM -0700, Namhyung Kim wrote:
> Hello,
> 
> On Thu, Apr 4, 2024 at 12:26 PM Beau Belgrave <beaub@xxxxxxxxxxxxxxxxxxx> wrote:
> >
> > Hello,
> >
> > I'm looking into the possibility of capturing user data that is pointed
> > to by a user register (IE: fs/gs for TLS on x86/64) for each sample via
> > perf_events.
> >
> > I was hoping to find a way to do this similar to PERF_SAMPLE_STACK_USER.
> > I think it could even use roughly the same ABI in the perf ring buffer.
> > Or it may be possible by some kprobe linked to the perf sample function.
> >
> > This would allow a profiler to collect TLS (or other values) on x64. In
> > the Open Telemetry profiling SIG [1], we are trying to find a fast way
> > to grab a tracing association quickly on a per-thread basis. The team
> > at Elastic has a bespoke way to do this [2], however, I'd like to see a
> > more general way to achieve this. The folks I've been talking with seem
> > open to the idea of just having a TLS value for this we could capture
> > upon each sample. We could then just state, Open Telemetry SDKs should
> > have a TLS value for span correlation. However, we need a way to sample
> > the TLS value(s) when a sampling event is generated.
> >
> > Is this already possible via some other means? It'd be great to be able
> > to do this directly at the perf_event sample via the ABI or a probe.
> 
> I don't think the current perf ABI allows capturing %fs/%gs + offset.
> IIRC kprobes/uprobes don't have that too but I could be wrong.
> 

Yeah, I didn't see it either. I have some patches that I will submit in
a bit as RFC that enable this functionality. I was hoping there was
already an easy way to do this.

Thanks,
-Beau

> Thanks,
> Namhyung
> 
> >
> > 1. https://opentelemetry.io/blog/2024/profiling/
> > 2. https://www.elastic.co/blog/continuous-profiling-distributed-tracing-correlation