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oprofile's mechanism to get file path information
- From: William Cohen <wcohen at redhat dot com>
- To: SystemTAP <systemtap at sources dot redhat dot com>
- Date: Thu, 02 Aug 2007 15:10:34 -0400
- Subject: oprofile's mechanism to get file path information
There was some discussion at today's SystemTap meeting on getting the file path
information for VFS tapset. The problem is that the functions in getting the
path require some locks, and in general want to avoid getting locks when in a
probe. It was mentioned that OProfile has a mechanism to get file path
information (dcookies). OProfile's mechanism may not be entirely appropriate,
but it have some similar issues.
OProfile has a interrupt mechanisum that does the actual sampling. On x86_64 and
i386 machines this is done as a non-maskable interrupt (NMI). As a result what
can be done in the interrupt context is very limited. The interrupt mechanism
just records the context that the interrupt occurred in, the linear address of
the program counter, and the performance counter that caused the sample.
OProfile records this information in per processor circular queues. This is done
to eliminate the need for any locks. The linear address is of limited use
because linear address is very ephemeral, different programs may map the same
shared library to different locations. OProfile converts the linear address into
a file and an offset into the file. This conversion happens when the data from
the per processor buffers is collected into a system-wide buffer.
Having arbitrary length strings in the buffer sent into user space is awkward.
OProfile uses the dcookie mechanism to use fixed size integer numbers for the
file path. The daemon in userspace can make a systemcall to convert the number
back into a string. This makes the data format much more compact. It doesn't
need to pass all large strings around; the user-space daemon only needs to do
the dcookie lookup if it hasn't seen the dcookie value before. This user-space
code in oprofile/daemon/opd_cookie.c does the operation. The kernel side of the
code is in sys_lookup_dcookie, in linux/fs/dcookies.c. There is some code in
linux/driver/oprofile/buffer_sync.c that is converting that linear address into
a filename and offset.
There is a dcookie_mutex for the dcookie stuff, so there is still some locking.
However, for oprofile this locking happens when the the buffers are being read
out (less time critical) or in the user-space is trying to get a string name
rather than when the sample is actually being collected.
Maybe some of the approach used in dcookies would be useful for VFS path names.
-Will