We have a sample value and its satellite data stored in a
struct transient, and we must locate an
actual sample file to store the sample in, using the context
information in the transient structure as a key. The transient data to
sample file lookup is handled in
daemon/opd_sfile.c. A hash is taken of the
transient values that are relevant (depending upon the setting of
--separate, some values might be irrelevant), and the
hash value is used to lookup the list of currently open sample files.
Of course, the sample file might not be found, in which case we need
to create and open it.
OProfile uses a rather complex scheme for naming sample files, in order
to make selecting relevant sample files easier for the post-profiling
utilities. The exact details of the scheme are given in
oprofile-tests/pp_interface, but for now it will
suffice to remember that the filename will include only relevant
information for the current settings, taken from the transient data. A
fully-specified filename looks something like :
/var/lib/oprofile/samples/current/{root}/usr/bin/xmms/{dep}/{root}/lib/tls/libc-2.3.2.so/CPU_CLK_UNHALTED.100000.0.28082.28089.0
It should be clear that this identifies such information as the
application binary, the dependent (library) binary, the hardware event,
and the process and thread ID. Typically, not all this information is
needed, in which cases some values may be replaced with the token
all.
The code that generates this filename and opens the file is found in
daemon/opd_mangling.c. You may have realised that
at this point, we do not have the binary image file names, only the
dcookie values. In order to determine a file name, a dcookie value is
looked up in the dcookie cache. This is to be found in
daemon/opd_cookie.c. Since dcookies are both
persistent and unique during a sampling session, we can cache the
values. If the value is not found in the cache, then we ask the kernel
to do the lookup from value to file name for us by calling
lookup_dcookie(). This looks up the value in a
kernel-side cache (see fs/dcookies.c) and returns
the fully-qualified file name to userspace.