Re: tracepoints for kernel/mutex.c

[Jason Baron <jbaron@redhat.com>]

On Thu, Oct 16, 2008 at 11:34:38PM +0200, Peter Zijlstra wrote:
> On Thu, 2008-10-16 at 17:04 -0400, Jason Baron wrote:
> 
> > Below are 3 tracepoints I've been playing with in kernel/mutex.c using
> > a SystemTap script. The idea is to detect and determine the cause of
> > lock contention. Currently I get the following output:
> > 
> > <contended mutex nam> <process name and pid of the contention> <time of
> > contention> <pid that woke me up(caused the contention?)>
> 
> > I think this approach has a number of advantages. It has low
> > overhead in the off case, since its based on tracepoints. It is
> > minimally invasive in the code path (3 tracepoints). It also allows me
> > to explore data structures and parts of the kernel by simply modifying
> > the SystemTap script. I do not need to re-compile the kernel and reboot.
> 
> *sigh* this is why I hate markers and all related things...
> 
> _IFF_ you want to place tracepoints, get them in the same place as the
> lock-dep/stat hooks, that way you get all the locks, not only mutexes.

makes sense. So we could layer lock-dep/stat on top of tracepoints? That
would potentially also make lock-dep/stat more dynamic.

> 
> This is the same reason I absolutely _hate_ Edwin's rwsem tracer.
> 

i'm trying to get some consensus on these types of patches. Do we
want to create a new tracer for each thing we want to trace, or add
tracepoints?

> Folks, lets please start by getting the tracing infrastructure in and
> those few high-level trace-points google proposed.
> 
> Until we get the basics in, I think I'm going to NAK any and all
> tracepoint/marker patches.
> 

I think that core locking functions are pretty basic...