Re: [PATCH 0/2] Markers Implementation for RCU Tracing - Ver II

[Jan Kiszka <jan.kiszka@siemens.com>]

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!

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.

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