linux-kernel - Re: LTTng finds abnormally long APIC interrupt handler : 58.2 ms

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Message-ID: <y0mwsisitcy.fsf@ton.toronto.redhat.com>
Date:	Thu, 07 Aug 2008 14:15:57 -0400
From:	fche@...hat.com (Frank Ch. Eigler)
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	akpm@...ux-foundation.org, Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, ltt-dev@...ts.casi.polymtl.ca
Subject: Re: LTTng finds abnormally long APIC interrupt handler : 58.2 ms


Steven Rostedt <rostedt@...dmis.org> writes:

> [...]
>> It all holds in a small module. Having the ability to connect any marker
>> on any generic probe from the lttng marker interface has been very
>> useful to do all this. Actually, this module has to be built into the
>> kernel because ftrace does not permit breeing the trace_ops data
>> structure. :/
>
> I did this purposely. The reason is that there is no safe way to release
> the trace_ops, because you never know if something is in the function
> you pointed to. If you remove the module after that, it crashes.
>
> I've been thinking of exporting it, and make it so that once you 
> register something to the trace_ops, I'll up the module count, and never 
> let that module unload.

This issue will need some sort of resolution if systemtap is to be
able to hook up to ftrace.  Perhaps we could have a second/nested API
that suffers the costs of explicit reference counting, but does permit
dynamic attach and robust detach.


- FChE
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