linux-kernel - Re: [PATCH 0/2] Markers Implementation for RCU Tracing

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Date:	Mon, 18 Feb 2008 15:41:13 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Jan Kiszka <jan.kiszka@...mens.com>, prasad@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org, mingo@...e.hu,
	Gautham R Shenoy <ego@...ibm.com>,
	linux-rt-users@...r.kernel.org, dipankar@...ibm.com
Subject: Re: [PATCH 0/2] Markers Implementation for RCU Tracing - Ver II

* Paul E. McKenney (paulmck@...ux.vnet.ibm.com) 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.
> 

Yes, I use call_rcu, so I guess a call_sched would be useful here.

Mathieu

> 						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

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