linux-kernel - Re: [RFC][Patch 2/2] markers: example of irq regular kernel markers

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Message-ID: <20080621180203.GA11804@redhat.com>
Date:	Sat, 21 Jun 2008 14:02:03 -0400
From:	"Frank Ch. Eigler" <fche@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	systemtap-ml <systemtap@...rces.redhat.com>,
	Hideo AOKI <haoki@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC][Patch 2/2] markers: example of irq regular kernel markers

Hi -

On Sat, Jun 21, 2008 at 06:13:54PM +0200, Peter Zijlstra wrote:

> [...]  I think what Frank refers to here is why not scatter the
> kernel code with trace_mark()s on every conceivable location like
> you do with printk-style debugging.

It's not fair to caricaturize my suggestions this way ("every
conceivable location").

> Those trace marks might help out when $customer's kernel goes splat
> and you don't want to provide him with a custom kernel.

Right.

> I do think we must make a clear distinction between these cases because:
>
> 1) tracers provide a kernel<->user interface - and whilst we don't
> have a stable in-kernel API/ABI we are anal about the kernel/user
> boundary.  Andrew also greatly worries about this aspect.

Well, how to set Andrew's mind at ease then, beyond what we've already
said?  Back a few months ago, both systemtap and lttng guys - the
primary user-space clients - have said that we are fine with this
interface changing.  We each have mechanisms to adapt.

> 2) it highly uglyfies the code, Frank doesn't need to maintain it,
> so its easy for him to say. But IMHO its much harder to read code
> that is littered with debug statements that it is to read regular
> code.

Then don't put too many in, or hide them with inline functions.

>  3) it bloats the kernel,.. while it may not be fast path bloat, all
> that marker stuff does go somewhere.

That bloat has been quantified and appears negligible in space and time.

> So, while I see the value of 'stable' mark sites for 'stable'
> events, I'm dead-set against littering the kernel with markers just
> because we can, and hoping they might some day be useful for
> someone.

We're in violent agreement.  No one suggested "littering just because
we can".  The initial lttng suite of markers consisted of about one
hundred *in total*.  If some other subsystem maintainer runs amok and
adds thousands, please take it up with them, not with us.


- FChE
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