We're trying to use oprofile to track down performance problems on a server cluster. However, the servers in question have a read-only file system, where /var/tmp is the only writeable directory.
OProfile wants to create two directories whenever it runs: /root/.oprofile and /var/lib/oprofile, but it can't, because the filesystem is read-only. I can use the --session-dir command line option to make it write its logs to elsewhere than /var/lib, but I can't find any such option to make it use some other directory than /root/.oprofile.
The filesystem is read-only because it is on nonwriteable media, not because of permissions -- ie, not even superuser can write to those directories. We can cook a new ROM image of the filesystem (which is how we installed oprofile, obviously), but there is no way for a runtime program to write to /root, whether it is superuser or not.
I tried creating a symlink in the ROM that points /root/.oprofile -> /var/tmp/oprofile, but apparently oprofile doesn't see this symlink as a directory, and fails when run:
redacted@redacted:~$ sudo opcontrol --no-vmlinux --start --session-dir=/var/tmp/oprofile/foo
mkdir: cannot create directory `/root/.oprofile': File exists
Couldn't mkdir -p /root/.oprofile
We must run our profilers on this particular system, because the performance issues we're trying to investigate don't manifest if we build and run the app on a development server. We can't just run our tests on a programmer's workstation and profile the app there, because the problem doesn't happen there.
Is there some way to configure oprofile so that it doesn't use /root ?
It turns out that this directory is hardcoded into the opcontrol bash script:
# location for daemon setup information
SETUP_DIR="/root/.oprofile"
SETUP_FILE="$SETUP_DIR/daemonrc"
Editing those lines seemed to get it working, more or less.