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

[Beau Belgrave]

On Wed, Apr 10, 2024 at 10:06:28PM +0900, Masami Hiramatsu wrote:
> On Thu, 4 Apr 2024 12:26:41 -0700
> 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.
> > 
> 
> Have you tried to use uprobes? It should be able to access user-space
> registers including fs/gs.
> 

We need to get fs/gs during a sample interrupt from perf. If the sample
interrupt lands during kernel code (IE: syscall) we would also like to
get these TLS values when in process context.

I have some patches into the kernel to make this possible via
perf_events that works well, however, I don't want to reinvent the wheel
if there is some way to get these via perf samples already.

In OTel, we are trying to attribute samples to transactions that are
occurring. So the TLS fetch has to be aligned exactly with the sample.
You can do this via eBPF when it's available, however, we have
environments where eBPF is not available.

It's sounding like to do this properly without eBPF a new feature would
be required. If so, I do have some patches I can share in a bit as an
RFC.

Thanks,
-Beau

> Thank you,
> 
> -- 
> Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>