linux-kernel - Re: [patch 0/2] Immediate Values

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Date:	Wed, 14 May 2008 16:53:57 +0200
From:	Pavel Machek <pavel@....cz>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 0/2] Immediate Values - jump patching update

Hi!

> >> At least one complication though is that in the case of markers, 
> >> tracing parameter evaluation is itself conditional (and placed out of 
> >> the hot path due to -freorder-blocks).  With your suggested kind of 
> >> assembly ("g" constraints for all the expressions), those expressions 
> >> would be evaluated unconditionally, just to make them live somewhere. 
> >> That unconditional evaluation can easily entail memory reads and 
> >> dependent arithmetic, which could swamp the savings of eliminating 
> >> the marker-style conditional branch.
> >
> > Well, it depends a bit on what kind of expressions you put in there. 
> > You don't really want to put *expressions* in there as much as you 
> > want to put *data* references in there, although, of course, if your 
> > have something like "foo->bar[baz]->quux" then it's easy to trip upon.
> 
> and that's exactly what was tripped upon in sched.o and analyzed.
> 
> Furthermore, the suggestion of doing this exclusively within the DWARF2 
> space - besides the not particularly minor complication of it not being 
> implemented yet - is:
> 
>  - quite substantially complex on its own
> 
>  - would make Linux instrumentation dependent on all sorts of DWARF2
>    details which we had our 'fun' with before. (I proffer that that's
>    more fragile than any code patching can ever be.)

>  - if done self-sufficiently (i.e. if a kernel image can be used to
>    trace things, which i believe any usable kernel tracer must offer),
>    it would, with the current debug info format, enlargen the kernel RAM
>    image with quite a substantial amount of unswappable kernel memory. 

I am not sure self-sufficiency is  good goal here.

If tracing becomes part of kernel-user ABI, we are in big trouble...

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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