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Nix Packages collection & NixOS
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ease the use of `perf probe` #66924

Open teto opened 5 years ago

teto commented 5 years ago

I need to trace some variables in the kernel. I tried using perf on nixos but this was not straightforward strace -olog perf probe -L tcp_cong_avoid_ai fails with

Failed to find the path for the kernel: No such file or directory
  Error: Failed to show lines.

I started developing a patch to add vmlinux (present in ${kernel.dev}/vmlinux) to tools/perf/util/symbol.c but there is no stable path to that file.

I've then stumbled on https://www.mail-archive.com/nix-commits-bounces@lists.science.uu.nl/msg04507.html that puts vmlinux in a stable path but it doesn't explain how perf looks into that folder: @viric do you have a custom patch in your repo ?

teto commented 5 years ago

nevermind, perf help probe explains how to pass a custom path :/

teto commented 5 years ago

usage of perf is not pleasant, thus I reopen the issue to discuss how to make it more straightforward. since the CLI get long. I wonder if we should not patch the perf source to not have to run this kind of command `perf --debug verbose probe -k /nix/store/5am9gvlr7wkcwy48kibhvq16l624iw6h-linux-5.1.0-mptcp_v0.96.0-dev/vmlinux -m /run/booted-system/kernel-modules -s /home/teto/mptcp 'tcp_cong_avoid_ai tp->snd_cwnd_clamp'

Also using compressed modules seem to trouble perf. Maybe with a nixos module we could add some warnings checking for MODULE_COMPRESS settings etc

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