On Mon, Feb 18, 2008 at 01:47:31PM +0100, Jan Kiszka wrote:
> K. Prasad wrote:
> > Hi Ingo,
> > Please accept these patches into the rt tree which convert the
> > existing RCU tracing mechanism for Preempt RCU and RCU Boost into
> > markers.
> >
> > These patches are based upon the 2.6.24-rc5-rt1 kernel tree.
> >
> > Along with marker transition, the RCU Tracing infrastructure has also
> > been modularised to be built as a kernel module, thereby enabling
> > runtime changes to the RCU Tracing infrastructure.
> >
> > Patch [1/2] - Patch that converts the Preempt RCU tracing in
> > rcupreempt.c into markers.
> >
> > Patch [1/2] - Patch that converts the Preempt RCU Boost tracing in
> > rcupreempt-boost.c into markers.
> >
>
> I have a technical problem with marker-based RCU tracing: It causes
> nasty recursions with latest multi-probe marker patches (sorry, no link
> at hand, can be found in latest LTTng, maybe also already in -mm). Those
> patches introduce a marker probe trampoline like this:
>
> void marker_probe_cb(const struct marker *mdata, void *call_private,
> const char *fmt, ...)
> {
> va_list args;
> char ptype;
>
> /*
> * rcu_read_lock does two things : disabling preemption to make sure the
> * teardown of the callbacks can be done correctly when they are in
> * modules and they insure RCU read coherency.
> */
> rcu_read_lock();
> preempt_disable();
> ...
>
> Can we do multi-probe with pure preempt_disable/enable protection? I
> guess it's fine with classic RCU, but what about preemptible RCU? Any
> suggestion appreciated!
If you substitute synchronize_sched() for synchronize_rcu(), this should
work fine. Of course, this approach would cause RCU tracing to degrade
latencies somewhat in -rt.
If tracing is using call_rcu(), we will need to add a call_sched()
or some such.
Thanx, Paul
> Jan
>
> PS: You will run into this issue if you try to marry latest -rt with
> latest LTTng. Straightforward workaround is to comment-out any RCU
> trace_mark occurrences.
>
> --
> Siemens AG, Corporate Technology, CT SE 2
> Corporate Competence Center Embedded Linux
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