[ltt-dev] trace a futex

[Peter Zijlstra]

[ resend with a real email address for Thomas ]

On Fri, 2008-12-05 at 07:28 -0500, Mathieu Desnoyers wrote:
> * Peter Zijlstra (peterz at infradead.org) wrote:
> > 
> > FWIW, the ftrace infrastructure has on many an occasion (even before it
> > was called ftrace and specific to -rt) helped in debugging and fixing
> > futex races.
> > 
> 
> Hrm, I'm not sure futex races is the key aspect of interest here.
> Knowing which amount of pthread mutex lock calls ends up calling the
> scheduler looks a bit more like the topic brought by this particular
> use-case. Therefore, correlating the information from the nptl with the
> kernel information would be useful.

We already have the scheduler instrucmentation, so correlating that to
known futex calls (from userspace) shouldn't be too hard. 

> Is lockdep called when a futex is taken ? 

lockdep only does kernel-internal locks - so no.

> Should we add instrumentation
> (tracepoints) to futex.c ? If yes, was there specific instrumentation
> you used with ftrace that should be added ?

Not sure we should, Thomas did the bulk of that ftrace debugging, Thomas
do you think it would be worthwhile to add some tracepoints in there?