Hello, On Thu, Apr 4, 2024 at 12:26 PM Beau Belgrave <be...@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. Thanks, Namhyung > > 1. https://opentelemetry.io/blog/2024/profiling/ > 2. > https://www.elastic.co/blog/continuous-profiling-distributed-tracing-correlation
