Re: oprofile's mechanism to get file path information

["David J. Wilder" <dwilder at us dot ibm dot com>]

William Cohen wrote:
> 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
Following with the lookup later idea,  could we just save the pointer in 
the probe handler and do the lookup when the END probe runs?   This  has 
to assume  the file is not  deleted between  the collection of the 
pointer  and the end probe running.   Another idea is creating some sort 
of kernel process running in a known context that we could pass  a 
lookup request and a place to store the result.