linux-kernel - Re: [PATCH 0/2 v2] Syscalls tracing

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Date:	Mon, 23 Mar 2009 20:40:20 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Steven Rostedt <rostedt@...dmis.org>, tglx@...utronix.de,
	Jason Baron <jbaron@...hat.com>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Jiaying Zhang <jiayingz@...gle.com>,
	Michael Rubin <mrubin@...gle.com>,
	Martin Bligh <mbligh@...gle.com>,
	Michael Davidson <md@...gle.com>
Subject: Re: [PATCH 0/2 v2] Syscalls tracing

* Frederic Weisbecker <fweisbec@...il.com> wrote:

> And actually I don't think two copy_from_user will really change a 
> lot the tracing throughput.

Correct. It's already in the CPU cache so it is a performance 
non-issue and essentially for free. Copy avoidance is only an issue 
when touchig cache-cold data.

( Yes, a few cycles could be shaven off but the beauty of 
  all-encompassing non-source-code-invasive syscall tracing covering 
  hundreds of syscalls straight away trumps those concerns. )

> The idea would be now to join the syscalls metadata with such 
> quick handlers. We will have to think about how to join these in a 
> proper way.

We could allow per syscall tracepoints via the attribute table. The 
call signature could be a standard:

   long sys_call(unsinged long arg1, unsigned long arg2,
		 unsigned long arg3, unsigned long arg4,
		 unsigned long arg5, unsigned long arg6);

This would allow interested plugins/tools to install a system call 
specific callback. (we might allow two tracepoints - one before and 
one after the syscall)

The registration API could be driven by the name or by the syscall 
index - NR_sys_open or so.

	Ingo
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