linux-kernel - Re: [PATCH 0/11] LTTng-core (basic tracing infrastructure) 0.5.108

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Message-ID: <450AC2FA.70203@opersys.com>
Date:	Fri, 15 Sep 2006 11:12:58 -0400
From:	Karim Yaghmour <karim@...rsys.com>
To:	Jes Sorensen <jes@....com>
CC:	Ingo Molnar <mingo@...e.hu>, Roman Zippel <zippel@...ux-m68k.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	linux-kernel@...r.kernel.org,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...l.org>, Ingo Molnar <mingo@...hat.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tom Zanussi <zanussi@...ibm.com>, ltt-dev@...fik.org,
	Michel Dagenais <michel.dagenais@...ymtl.ca>
Subject: Re: [PATCH 0/11] LTTng-core (basic tracing infrastructure) 0.5.108

Jes Sorensen wrote:
> There very few tracepoints in this category,

Wow, that's progress.

> the only things you can
> claim are more or less generic are syscalls, and tracing syscall
> handling is tricky.

If there are implementation issue, I trust an adequate solution can be
found by using the tested-and-proven method of posting stuff on the
lkml for review.

> This is grossly over simplifying things and why the whole things doesn't
> hold water. There is no such thing as 'the place' to put a specific
> tracepoint.
> 
> Especially when we start talking about things like tracepoints in the
> scheduler.

I do not underestimate the difficulty of selecting such tracepoints.
This is why I chose not to maintain other people's specific tracepoints.
I realize this is a tough problem, but I also trust subsystem maintainers
are smart enough to make the appropriate decision. Obviously for such
things like the scheduler, any fine-grained instrumentation will draw
a barrage of criticism from anyone since a lot of stuff depends on it.
Either the lkml process works or it doesn't, but it isn't for me to
decide.

> Note that I haven't been referring to debug tracepoints at any point in
> this debate.

You're right, but others have happily intermingled the whole lot, and
I just wanted to document my personal categorization on lkml for all
to see.

> You seem to think that it's fine to add instrumentation in the syscall
> path as an example as long as it's compiled out. Well on some
> architectures, the syscall path is very sensitive to alignment and there
> may be restrictions on how large the stub of code is allowed to be, like
> a few hundred bytes. Just because things work one way on x86, doesn't
> mean they work like that everywhere.

If ltt failed to implement such things appropriately, then we apologize.
That fact doesn't preclude proper implementation in the future, however.

Karim

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