Message-ID: <47B97E63.3070205@siemens.com>
Date: Mon, 18 Feb 2008 13:47:31 +0100
From: Jan Kiszka <jan.kiszka@siemens.com>
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To: prasad@linux.vnet.ibm.com
CC: linux-kernel@vger.kernel.org, mingo@elte.hu,
       Gautham R Shenoy <ego@in.ibm.com>, mathieu.desnoyers@polymtl.ca,
       linux-rt-users@vger.kernel.org, dipankar@in.ibm.com,
       paulmck@linux.vnet.ibm.com
Subject: Re: [PATCH 0/2] Markers Implementation for RCU Tracing - Ver II
References: <20071231060911.GB6461@in.ibm.com> <20080103163055.GB27651@Krystal> <20080104105858.GA13865@in.ibm.com> <20080105124632.GA16576@Krystal> <20080107195038.GA5119@in.ibm.com>
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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.

-- 
Siemens AG, Corporate Technology, CT SE 2
Corporate Competence Center Embedded Linux
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