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

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Date:	Tue, 19 Feb 2008 13:54:28 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Jan Kiszka <jan.kiszka@...mens.com>
Cc:	prasad@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	mingo@...e.hu, Gautham R Shenoy <ego@...ibm.com>,
	mathieu.desnoyers@...ymtl.ca, linux-rt-users@...r.kernel.org,
	dipankar@...ibm.com
Subject: Re: Markers: multi-probe locking fun (was: Re: [PATCH 0/2] Markers Implementation for RCU Tracing - Ver II)

On Tue, Feb 19, 2008 at 05:27:40PM +0100, Jan Kiszka 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"?

This would work for Classic RCU.  For preemptible RCU, a bit more
work is needed.

> 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 :( ).

OK, good to know.  Guess we need a call_sched() for -rt and for
preemptible RCU sooner rather than later...

						Thanx, Paul
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