Re: perf and containers

[Milian Wolff <milian.wolff@kdab.com>]

On Donnerstag, 29. Juni 2017 22:33:37 CEST Brendan Gregg wrote:
> G'Day perf-users,
> 
> I've been using perf with containers a lot, running perf from the host
> system-wide or with --cgroup for one container, and I'm wondering
> about symbol translation and namespaces. When running "perf script" or
> "perf report" from the host:
> 
> - kernel symbols: works fine
> 
> - JIT symbols: doesn't work as /tmp/perf-PID.map is in the container,
> not the host, and has a different PID. I currently have shell scripts
> to copy and rename map files, but... Would it be possible for perf to
> try opening /tmp/perf-PID.map, and if that fails, check if the PID
> still exists in /proc, read its NSpid from /proc/PID/status, and if
> that's different (meaning it's in a container), then read
> /proc/PID/root/tmp/perf-NSpid.map instead?
> 
> - binary symbols: /usr/bin/node,
> /usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java, /lib/..., etc, may not
> be in the host, so symbol translation fails. Could perf have a similar
> approach where it tries from /proc/PID/root/... instead, if the PID is
> in a container?
> 
> Or, are there other workarounds I don't know about for these? :) perf
> could have a --setns flag to set the mount namespace before it
> attempts symbol translation, which may also work for when I'm
> analyzing one container only from the host, and could run "perf script
> --setns=..." for that one container.
> 
> Using perf from within the container is totally different, and not
> always possible (perf_event_open() can be disabled, and, some
> containers are completely locked down, such that it's hard to run any
> command that isn't the application)...

It's a bit hacky, but in the past I resorted to passing the mount point of the 
container root to perf via `--symfs`. That made it work quite nicely for my 
use case, but I also recorded from within the container - not sure if that 
plays a role here.

Cheers

-- 
Milian Wolff | milian.wolff@kdab.com | Senior Software Engineer
KDAB (Deutschland) GmbH&Co KG, a KDAB Group company
Tel: +49-30-521325470
KDAB - The Qt Experts