# Using perf probe for fun and profit

| tagged with
• gtk
• perf

Gnome Terminal has for a long time now exhibited a peculiar and annoying tendency to leak X11 resources. Over the course of weeks it will accrue hundreds of thousands of resources which xrestop could only describe as “Miscellaneous”. Eventually I broke down and found that these are SyncCounters. The challenge was then to determine why this was happening.

A bit of poking around in gdb indicated that gnome-terminal-server calls XSyncCreateCounter and XSyncDestroyCounter are when one would roughly expect (namely window creation and destruction). So where is this leak coming from?

The obvious way to approach this is be to capture callgraphs to calls to these functions and see if we can find missing frees. It turns out that perf has just the tool for this task: perf probe.

$sudo chown ben /sys/kernel/debug -R$ ~/bin/perf probe  -x /usr/lib/x86_64-linux-gnu/libXext.so --add XSyncCreateCounter
\$ ~/bin/perf probe  -x /usr/lib/x86_64-linux-gnu/libXext.so --add XSyncDestroyCounter
~/bin/perf record -e probe_libXext:XSyncCreateCounter,probe_libXext:XSyncDestroyCounter /usr/lib/gnome-terminal/gnome-terminal-server

This lead to the opening (and, happily, resolution) of Gnome ticket #760188.