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

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Date:	Tue, 28 Apr 2009 17:55:51 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Andi Kleen <andi@...stfloor.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Fr馘駻ic 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>,
	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, Apr 28, 2009 at 05:38:33PM +0800, Ingo Molnar wrote:
> 
> * KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:
> 
> > > 
> > > * 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.
> > > 
> > > My point is that the allocation patterns can be derived from dynamic 
> > > events. We can build a map of everything if we know all the events 
> > > that led up to it. Doing:
> > > 
> > >   echo 3 > /proc/sys/vm/drop_caches
> > > 
> > > will clear 99% of the memory allocations, so we can build a new map 
> > > from scratch just about anytime. (and if boot allocations are 
> > > interesting they can be traced too)
> > > 
> > > _And_ via this angle we'll also have access to the dynamic events, 
> > > in a different 'view' of the same tracepoints - which is obviously 
> > > very useful for different purposes.
> > 
> > I am one of most strongly want guys to MM tracepoint. but No, many 
> > cunstomer never permit to use drop_caches.
> 
> See my other mail i just sent: it would be a natural extension of 
> tracing to also dump all current object state when tracing is turned 
> on. That way no drop_caches is needed at all.

I can understand the merits here - I also did readahead
tracing/accounting in _one_ piece of code. Very handy.

The readahead traces are now raw printks - converting to the ftrace
framework would be a big win.

But. It's still not a fit-all solution. Imagine when full data _since_
booting is required, but the user cannot afford a reboot.

> But it has to be expressed in one framework that cares about the 
> totality of the kernel - not just these splintered bits of 
> instrumentation and pieces of statistics.

Though minded to push the kpageflags interface, I totally agree the
above fine principle and discipline :-)

Thanks,
Fengguang
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