[Ksummit-2008-discuss] DTrace

Frank Ch. Eigler fche at redhat.com
Mon Jun 30 10:12:46 PDT 2008

Hi -

On Mon, Jun 30, 2008 at 01:29:13PM +0200, Christoph Hellwig wrote:

> [...]  This might be getting a little offtopic for the kernel summit
> discuss list, but let's start anyway, we can move this to a better
> suited list, although I can't think of one except for linux-kernel.

systemtap at sources.redhat.com
utrace-devel at redhat.com

> I'm not sure if that's the current design, but I can't find any
> evidence in the code that it allows running handlers in process
> context, all that's available is a kernel callback.  [...]

For systemtap's purposes, that is sufficient.  Our probes are meant to
run non-intrusively (they do not mess with user thread scheduling,
their VM state, strictly limited time & space consumption), so
actually injecting equivalent snippets of code into userspace for
execution there does not seem to buy anything.  Plus, like dtrace, we
want scripts to be able to intermix probes (=> share data) amongst
kernel and multiple user-space threads, and this seems most naturally
done by running the probes themselves in kernel space.

> [...] What we really need is a userspace interface so that it
> actually can be used by thing like frysk or an implementation of the
> userspace dtrace hooks.

That will get built as other tools require it.  Systemtap per se will
likely not.

> [...] For complex traces doing this in userspace is for sure a better idea.

Can you elaborate upon this more complex scenario?

- FChE

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