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

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Date:	Tue, 28 Apr 2009 14:42:59 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Pekka Enberg <penberg@...helsinki.fi>,
	Andi Kleen <andi@...stfloor.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	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


* KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:

> 2009/4/28 Ingo Molnar <mingo@...e.hu>:
> >
> > * Pekka Enberg <penberg@...helsinki.fi> wrote:
> >
> >> Hi!
> >>
> >> 2009/4/28 KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>:
> >> >> I guess the main question here is whether this approach will scale to
> >> >> something like kmalloc() or the page allocator in production
> >> >> environments. For any serious workload, the frequency of events is
> >> >> going to be pretty high.
> >> >
> >> > Immediate Values patch series makes zero-overhead to tracepoint
> >> > while it's not used.
> >> >
> >> > So, We have to implement to stop collect stastics way. it restore
> >> > zero overhead world.
> >> > We don't lose any performance by trace.
> >>
> >> Sure but I meant the _enabled_ case here. kmalloc() (and the page
> >> allocator to some extent) is very performance sensitive in many
> >> workloads so you probably don't want to use tracepoints if you're
> >> collecting some overall statistics (i.e. tracing all events) like
> >> we do here.
> >
> > That's where 'collect current state' kind of tracepoints would help
> > - they could be used even without enabling any of the other
> > tracepoints. And they'd still be in a coherent whole with the
> > dynamic-events tracepoints.
> >
> > So i'm not arguing against these techniques at all - and we can move
> > on a wide scale from zero-overhead to lots-of-tracing-enabled models
> > - what i'm arguing against is the splintering.
> 
> umm.
> I guess Pekka and you talk about different thing.
> 
> if tracepoint is ON, tracepoint makes one function call. but few 
> hot spot don't have patience to one function call overhead.
> 
> scheduler stat and slab stat are one of good example, I think.
> 
> I really don't want convert slab_stat and sched_stat to ftrace 
> base stastics. currently it don't need extra function call and it 
> only touch per-cpu variable. So, a overhead is extream small.
> 
> Unfortunately, tracepoint still don't reach this extream 
> performance.

I understand that - please see my "[rfc] object collection tracing" 
reply in this thread, for a more detailed description about what i 
meant under 'object state tracing'.

	Ingo
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