Hi all, I have recently been exploring the possibility of using a BPF_PROG_TYPE_PERF_EVENT program to implement stack sampling for languages which do not use the platform's %sp for their stack pointer (in my case, GHC/Haskell [1], which on x86-64 uses %rbp for its stack pointer). Specifically, the idea is to use a sampling perf_events session with an eBPF overflow handler which locates the currently-running thread's stack and records it in the sample ringbuffer (see [2] for my current attempt). At this point I only care about user-space samples. However, I quickly ran up against the fact that perf_event's stack sampling logic (namely perf_output_sample_ustack) is called from an IRQ context. This appears to preclude use of a sleepable BPF program, which would be necessary to use bpf_copy_from_user. Indeed, the fact that the usual stack sampling logic uses copy_from_user_inatomic rather than copy_from_user suggests that this isn't a safe context for sleeping. So, I'm at this point a bit unclear on how to proceed. I can see a few possible directions forward, although none are particularly enticing: * Add a bpf_copy_from_user_atomic helper, which can be called from a non-sleepable context like a perf_events overflow handler. This would take the same set_fs() and pagefault_disable() precautions as perf_output_sample_ustack to ensure that the access is safe and aborts on fault. * Introduce a new BPF program type, BPF_PROG_TYPE_PERF_EVENT_STACK_LOCATOR, which can be invoked by perf_output_sample_ustack to locate the stack to be sampled. Do either of these ideas sound upstreamable? Perhaps there are other ideas on how to attack this general problem? I do not believe Haskell is alone in its struggle with the current inflexibility of stack sampling; the JVM introduced framepointer support specifically to allow callgraph sampling; however, dedicating a register and code to this seems like an unfortunate compromise, especially on x86-64 where registers are already fairly precious. Any thoughts or suggestions would be greatly appreciated. Cheers, - Ben [1] https://www.haskell.org/ghc/ [2] https://gitlab.haskell.org/bgamari/hs-bpf-prof/
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