linux-kernel - Re: tracepoint maintainance models

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Message-ID: <20060918161511.GA21204@elte.hu>
Date:	Mon, 18 Sep 2006 18:15:11 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	"Frank Ch. Eigler" <fche@...hat.com>,
	Paul Mundt <lethal@...ux-sh.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Jes Sorensen <jes@....com>, Andrew Morton <akpm@...l.org>,
	Tom Zanussi <zanussi@...ibm.com>,
	Richard J Moore <richardj_moore@...ibm.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: tracepoint maintainance models


* Alan Cox <alan@...rguk.ukuu.org.uk> wrote:

> Ar Llu, 2006-09-18 am 17:22 +0200, ysgrifennodd Ingo Molnar:
> > yeah - but i think to make it easier for SystemTap to insert a 
> > low-overhead probe there needs to be a 5-byte NOP inserted. There wont 
> > be any function call or condition at that place. At most there will be 
> > some minimal impact on the way gcc compiles the code in that function,
> 
> And more L1 misses. It seems that this problem should be solved by 
> jprobes and your int3 optimisation work.

Do you consider a single 5-byte NOP for a judiciously chosen 50 places 
in the kernel unacceptable? Note that the argument has shifted from 
static tracers to dynamic tracers: this _is_ about SystemTap: it adds 
points to the kernel where we can _guarantee_ that a dynamic probe can 
be inserted. In general there is no guarantee from gcc that any probe 
can be inserted into a function (djprobes and int3 optimization 
nonwithstanding) and this is a real practical problem for SystemTap. 
Frank can attest to that.

	Ingo
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