Re: [ltt-dev] trace a futex

From: Peter Zijlstra
Date: Fri Dec 05 2008 - 10:58:53 EST

On Fri, 2008-12-05 at 07:28 -0500, Mathieu Desnoyers wrote:
> * Peter Zijlstra (peterz@xxxxxxxxxxxxx) 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?
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at