Re: [RFC PATCH] LTTng instrumentation mm (updated)

Mathieu Desnoyers
2007-12-04T20:11:21Z

* Dave Hansen (haveblue@us.ibm.com) wrote:
> On Tue, 2007-12-04 at 14:25 -0500, Mathieu Desnoyers wrote:
> >
> > - I also dump the equivalent of /proc/swaps (with kernel internal
> > information) at trace start to know what swap files are currently
> > used.
>
> What about just enhancing /proc/swaps so that this information can be
> useful to people other than those doing traces?
>

It includes an in-kernel struct file pointer, exporting it to userspace
would be somewhat ugly.

> Now that we have /proc/$pid/pagemap, we expose some of the same
> information about which userspace virtual addresses are stored where and
> in which swapfile.
>

The problems with /proc :

- It exports all the data in formatted text. What I need for my traces
is pure binary, compact representation.
- It's not very neat to export in-kernel pointer information like a
kernel tracer would need.
- The locking is very often wrong. I started correcting /proc/modules a
while ago, but I fear there are quite a few cases where a procfile
reader could release the locks between two consecutive reads of the
same list and therefore cause missing information or corruption. While
being manageable for a proc text file, this is _highly_ unwanted in a
trace. See my previous "seq file sorted" and "module.c sort module
list" patches about this. My tracer deals with addition/removal of
elements to a list between dumps done by "chunks" by tracing the
modifications done to the list at the same time. However, /proc seq
files will just get corrupted or forget about an element not touched
by the modification, which my tracer cannot cope with.

Mathieu

> -- Dave