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Re: [PATCH] Linux Kernel Markers 0.5 for Linux 2.6.17 (with probe management)
- From: Mathieu Desnoyers <compudj at krystal dot dyndns dot org>
- To: Ingo Molnar <mingo at elte dot hu>
- Cc: Martin Bligh <mbligh at google dot com>, "Frank Ch. Eigler" <fche at redhat dot com>, Masami Hiramatsu <masami dot hiramatsu dot pt at hitachi dot com>, prasanna at in dot ibm dot com, Andrew Morton <akpm at osdl dot org>, Paul Mundt <lethal at linux-sh dot org>, linux-kernel <linux-kernel at vger dot kernel dot org>, Jes Sorensen <jes at sgi dot com>, Tom Zanussi <zanussi at us dot ibm dot com>, Richard J Moore <richardj_moore at uk dot ibm dot com>, Michel Dagenais <michel dot dagenais at polymtl dot ca>, Christoph Hellwig <hch at infradead dot org>, Greg Kroah-Hartman <gregkh at suse dot de>, Thomas Gleixner <tglx at linutronix dot de>, William Cohen <wcohen at redhat dot com>, ltt-dev at shafik dot org, systemtap at sources dot redhat dot com, Alan Cox <alan at lxorguk dot ukuu dot org dot uk>
- Date: Thu, 21 Sep 2006 17:42:48 -0400
- Subject: Re: [PATCH] Linux Kernel Markers 0.5 for Linux 2.6.17 (with probe management)
- References: <20060921160009.GA30115@Krystal> <20060921160656.GA24774@elte.hu>
* Ingo Molnar (mingo@elte.hu) wrote:
> "As an example, LTTng traces the page fault handler, when kprobes just
> can't instrument it."
>
> but tracing a raw pagefault at the arch level is a bad idea anyway, we
> want to trace __handle_mm_fault(). That way you can avoid having to
> modify every architecture's pagefault handler ...
>
Then you lose the ability to trace in-kernel minor page faults.
> but the other points remained unanswered as far as i can see.
>
Hi Ingo,
I clearly expressed my position in the previous emails, so did you. You argued
about a use of tracing that is not relevant to my vision of reality, which is :
- Embedded systems developers won't want a breakpoint-based probe
- High performance computing users won't want a breakpoint-based probe
- djprobe is far away from being in an acceptable state on architectures with
very inconvenient erratas (x86).
- kprobe and djprobe cannot access local variables in every cases
For those reasons, I prefer a jump-over-call approach which lets gcc give us the
local variables. No need of DWARF or SystemTAP macro Kung Fu. Just C and a
loadable module.
By no means is it a replacement for a completely dynamic breakpoint-based
instrumentation mechanism. I really think that both mechanism should coexist.
This is my position : I let the distribution/user decide what is appropriate for
their use. My goal is to provide them a flexible mechanism that takes the
multiple variety of uses in account without performance impact if they are not
willing to pay it to benefit from tracing.
With all due respect, yes, there are Linux users different from the typical
Redhat client. If your vision is still limited to this scope after a 500
emails debate, I am afraid that there is very little I can do about it in
one more.
Mathieu
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