linux-kernel - Re: [PATCH 5/5] proc: export more page flags in /proc/kpageflags

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Date:	Tue, 28 Apr 2009 11:15:08 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Larry Woodman <lwoodman@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Matt Mackall <mpm@...enic.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH 5/5] proc: export more page flags in /proc/kpageflags


* Pekka Enberg <penberg@...helsinki.fi> wrote:

> Hi Andi,
> 
> On Tue, 2009-04-28 at 09:40 +0200, Andi Kleen wrote:
> > > I think i have to NAK this kind of ad-hoc instrumentation of kernel 
> > > internals and statistics until we clear up why such instrumentation 
> > 
> > I think because it has zero fast path overhead and can be used 
> > any time without enabling anything special.

( That's a dubious claim in any case - tracepoints are very cheap.
  And they could be made even cheaper and such efforts would benefit
  all the tracepoint users so it's a prime focus of interest.
  Andi is a SystemTap proponent, right? I saw him oppose pretty much 
  everything built-in kernel tracing related. I consider that a 
  pretty extreme position. )

> Yes, zero overhead is important for certain things (like 
> CONFIG_SLUB_STATS, for example). However, putting slab allocator 
> specific checks in fs/proc looks pretty fragile to me. It would be 
> nice to have this under the "kmemtrace umbrella" so that there's 
> just one place that needs to be fixed up when allocators change.
> 
> Also, while you probably don't want to use tracepoints for this 
> kind of instrumentation, you might want to look into reusing the 
> ftrace reporting bits.

Exactly - we have a tracing and statistics framework for a reason.

	Ingo
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