linux-kernel - Re: [PATCH 08/12] add trace events for each syscall entry/exit

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Date:	Wed, 26 Aug 2009 13:08:12 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Hendrik Brueckner <brueckner@...ux.vnet.ibm.com>,
	Jason Baron <jbaron@...hat.com>, linux-kernel@...r.kernel.org,
	mingo@...e.hu, laijs@...fujitsu.com, rostedt@...dmis.org,
	jiayingz@...gle.com, mbligh@...gle.com, lizf@...fujitsu.com,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH 08/12] add trace events for each syscall entry/exit

* Peter Zijlstra (peterz@...radead.org) wrote:
> On Tue, 2009-08-25 at 14:31 -0400, Mathieu Desnoyers wrote:
> 
> > (Well, I do not have time currently to look into the gory details
> > (sorry), but let's try to take a step back from the problem.)
> > 
> > The design proposal for this kthread behavior wrt syscalls is based on a
> > very specific and current kernel behavior, that may happen to change and
> > that I have actually seen proven incorrect. For instance, some
> > proprietary Linux driver does very odd things with system calls within
> > kernel threads, like invoking them with int 0x80.
> > 
> > Yes, this is odd, but do we really want to tie the tracer that much to
> > the actual OS implementation specificities ?
> > 
> > That sounds like a recipe for endless breakages and missing bits of
> > instrumentation.
> > 
> > So my advice would be: if we want to trace the syscall entry/exit paths,
> > let's trace them for the _whole_ system, and find ways to make it work
> > for corner-cases rather than finding clever ways to diminish
> > instrumentation coverage.
> > 
> > Given the ret from fork example happens to be the first event fired
> > after the thread is created, we should be able to deal with this problem
> > by initializing the thread structure used by syscall exit tracing to an
> > initial "ret from fork" value.
> 
> So you're saying we should let proprietary crap influence the design of
> the kernel in any way?

Nah. And I start to feel comfortable with syscall entry/exit being only
be traced for userspace threads. But as I pointed out in a follow-up
email, the lack of sys_*() tracing for invocation from within the kernel
might be problematic. This is actually my main point.

Mathieu

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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