linux-kernel - Re: [patch-RFC 00/26] LTTng Kernel Trace Thread Flag

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Date:	Fri, 7 Dec 2007 08:44:26 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	"Frank Ch. Eigler" <fche@...hat.com>
Cc:	akpm@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org
Subject: Re: [patch-RFC 00/26] LTTng Kernel Trace Thread Flag

* Frank Ch. Eigler (fche@...hat.com) wrote:
> Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> writes:
> 
> > This is an RFC for addition of a new thread flag, TIF_KERNEL_TRACE, to each
> > architecture to activate system-wide system call tracing.
> > [...]
> 
> Instead of creating a new flag, could you overload TIF_SYSCALL_TRACE,
> putting the marker into syscall_trace(), and letting !PT_TRACED cause
> a skip over the ptrace notification logic?
> 
> - FChE

I don't see any PT_TRACED flag in current kernel HEAD ?

Hrm, let's see. If we share TIF_SYSCALL_TRACE with ptrace, we would then
have to figure out how to get this working :

- kernel tracing activated
- ptracing some random processes
- kernel tracing deactivated
- stop ptracing those processes

It means that we would have to keep some state information about the
ptrace status of each process. This is currently kept by
TIF_SYSCALL_TRACE, but since we would be overloading it, it would be
lost when we deactivate kernel tracing. Adding a supplementary field to
the thread_info structure is out of question here : we have to keep it
as small as possible. So where do you propose to keep this information
other than... another thread flag ?

Mathieu

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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