linux-kernel - Re: LTTng and SystemTAP (Everyone who is scared to read this huge thread, skip to here)

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Date:	Sat, 23 Sep 2006 11:50:06 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Theodore Tso <tytso@....edu>, Nicholas Miell <nmiell@...cast.net>,
	Paul Mundt <lethal@...ux-sh.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>, Jes Sorensen <jes@....com>,
	Andrew Morton <akpm@...l.org>,
	Tom Zanussi <zanussi@...ibm.com>,
	Richard J Moore <richardj_moore@...ibm.com>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Michel Dagenais <michel.dagenais@...ymtl.ca>,
	Christoph Hellwig <hch@...radead.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	William Cohen <wcohen@...hat.com>,
	"Martin J. Bligh" <mbligh@...igh.org>
Subject: Re: LTTng and SystemTAP (Everyone who is scared to read this huge thread, skip to here)

* Ingo Molnar (mingo@...e.hu) wrote:
> 
> * Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:
> 
> > > Some of the extensive hooking you do in LTT could be aleviated to a 
> > > great degree if you used dynamic probes. For example the syscall 
> > > entry hackery in LTT looks truly scary.
> > 
> > Yes, agreed. The last time I checked, I thought about moving this 
> > tracing code to the syscall_trace_entry/exit (used for security hooks 
> > and ptrace if I remember well). I just didn't have the time to do it 
> > yet.
> 
> correct, that's where all such things (auditing, seccomp, ptrace, 
> sigstop, freezing, etc.) hook into. Much (all?) of the current entry.S 
> hacks can go away in favor of a much easier .c patch to 
> do_syscall_trace() and this would reduce a significantion portion of the 
> present intrusiveness of LTTng.
> 

Hi Ingo,

The only problem with do_syscall_trace is that it is only called at the
beginning of the system call. LTT also needs a marker at the end of the system
call to know when the control went back to user space.

Any idea of a nice location (in C code preferably) for such a marker ?

Mathieu


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