Messages in this thread | | | Date | Wed, 10 Apr 2024 08:37:29 -0700 | From | Beau Belgrave <> | Subject | Re: Copying TLS/user register data per perf-sample? |
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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@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. > > 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
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