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

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Wed, 10 Apr 2024 08:35:42 -0700
Beau Belgrave <beaub@linux.microsoft.com> wrote:

> 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@linux.microsoft.com> 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.

OK, those are not directly accessible from pt_regs.

> 
> 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.

I would like to see it. I think it is possible to introduce a helper
to get a base address of user TLS for probe events, and start supporting
from x86.

> 
> 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.

It is better to be shared in RFC stage, so that we can discuss it from
the direction level.

Thank you,

> 
> Thanks,
> -Beau
> 
> > Thank you,
> > 
> > -- 
> > Masami Hiramatsu (Google) <mhiramat@kernel.org>


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>