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

[Namhyung Kim]

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.

Thanks,
Namhyung

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