linux-kernel - Re: [PATCH] Linux Kernel Markers 0.5 for Linux 2.6.17 (with probe management)

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Date:	Thu, 21 Sep 2006 12:54:04 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ingo Molnar <mingo@...e.hu>
CC:	"Frank Ch. Eigler" <fche@...hat.com>,
	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	Martin Bligh <mbligh@...gle.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	prasanna@...ibm.com, Andrew Morton <akpm@...l.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Paul Mundt <lethal@...ux-sh.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Jes Sorensen <jes@....com>, 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>, ltt-dev@...fik.org,
	systemtap@...rces.redhat.com, Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH] Linux Kernel Markers 0.5 for Linux 2.6.17 (with probe
 management)

Ingo Molnar wrote:
> let me qualify that: parameters must be prepared there too - but no 
> actual function call inserted. (at most a NOP inserted). The register 
> filling doesnt even have to be function-calling-convention compliant - 
> that makes the symbolic probe almost zero-impact to register 
> allocation/scheduling, the only thing it should ensure is that the 
> parameters that are annotated to be available in register, stack or 
> memory _somewhere_. (i.e. not hidden or destroyed at that point by gcc) 
> Does a simple asm() that takes read-only parameters but only adds a NOP 
> achieve this result?

Do you mean using the asm to make sure gcc puts a reference to a 
variable into the DWARF info, or some other way of encoding the value 
locations?

Hm, another problem.  If the mark is in a loop, and gcc decides to 
unroll the loop, then you'll probably only get a mark in one iteration 
of the loop (or 1/Nth of the iterations).  Or worse, assembler errors.  
The only way I can see to deal with this is to not use symbols, but put 
records in a special section.  That way, if the asm() inserting them 
gets duplicated, you'll get duplicate records in the marker section.

I guess you'd get a similar problem with markers inserted in inlined 
functions.

(How does gdb deal with breakpoints in unrolled loops?)

    J
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