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

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Message-ID: <84144f020904280225h490ef682p8973cb1241a1f3ea@mail.gmail.com>
Date:	Tue, 28 Apr 2009 12:25:06 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Ingo Molnar <mingo@...e.hu>
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

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

* Pekka Enberg <penberg@...helsinki.fi> wrote:
>>> > I think because it has zero fast path overhead and can be used
>>> > any time without enabling anything special.
>
> On Tue, Apr 28, 2009 at 12:15 PM, Ingo Molnar <mingo@...e.hu> wrote:
>> ( 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. )

On Tue, Apr 28, 2009 at 12:19 PM, Pekka Enberg <penberg@...helsinki.fi> wrote:
> I have no idea how expensive tracepoints are but I suspect they don't
> make too much sense for this particular scenario. After all, kmemtrace
> is mainly interested in _allocation patterns_ whereas this patch seems
> to be more interested in "memory layout" type of things.

That said, I do foresee a need to be able to turn on more detailed
tracing after you've identified problematic areas from kpageflags type
of overview report. And for that, you almost certainly want
kmemtrace/tracepoints style solution with pid/function/whatever regexp
matching ftrace already provides.

                        Pekka
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