Re: Markers: multi-probe locking fun (was: Re: [PATCH 0/2] Markers Implementation for RCU Tracing - Ver II)

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> On Tue, Feb 19, 2008 at 03:33:26PM -0500, Mathieu Desnoyers wrote:
> > * Jan Kiszka (jan.kiszka@siemens.com) wrote:
> > > Paul E. McKenney wrote:
> > > > 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.
> > > 
> > > You mean something like "#define call_sched call_rcu_classic"?
> > > 
> > > I just learned that there is another reason for killing
> > > rcu_read_lock&friends from the marker probes: It can deadlock on -rt
> > > with PREEMPT_RCU_BOOST (hit probe inside rq-lock protected region =>
> > > rcu_read_unlock triggers unboost => stuck on rq_lock :( ).
> > > 
> > 
> > Yep, ok, let's do this :
> > 
> > in include/linux/rcupdate.h
> > 
> > #ifndef PREEMPT_RT
> > #define call_sched call_rcu
> > #else
> > #define call_sched call_rcu_classic
> > #endif
> > 
> > And I'll adapt the markers accordingly.
> 
> Good point, this will indeed work for 2.6.24-rt1!
> 
> Will need to do a bit more for 2.6.25-rc1.  My current thought is to
> provide a kernel thread that loops over the CPUs, advancing/invoking
> per-CPU lists of callbacks as it does so.  Then call_sched() would simply
> enqueue its callback on the current CPU's next list.
> 

I just noticed that my multiple probes support patch, that just got into
mainline, does not include the fixes I did you asked for (which includes
protecting with rcu_read_lock). I guess Andrew did not notice the patch.
Therefore, I think we'll need to address this pretty soon to make the
markers play nicely with CONFIG_PREEMPT_RCU.

Mathieu

> 							Thanx, Paul

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68