linux-kernel - Re: [PATCH 2/4] ftrace - add function

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Message-ID: <20091210175737.GB5256@elte.hu>
Date:	Thu, 10 Dec 2009 18:57:37 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	"Frank Ch. Eigler" <fche@...hat.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Tim Bird <tim.bird@...sony.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Li Zefan <lizf@...fujitsu.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] ftrace - add function_duration tracer


* Frank Ch. Eigler <fche@...hat.com> wrote:

> Ingo Molnar <mingo@...e.hu> writes:
> 
> > [...]
> > Just off the top of my head we want to be able to trace:
> >
> >  - max irq service latencies for a given IRQ
> >  - max block IO completion latencies for a app
> >  - max TLB flush latencies in the system
> >  - max sys_open() latencies in a task
> >  - max fork()/exit() latencies in a workload
> >  - max scheduling latencies on a given CPU
> >  - max page fault latencies
> >  - max wakeup latencies for a given task
> >  - max memory allocation latencies
> >
> >  - ... and dozens and dozens of other things where there's a "start"
> >    and a "stop" event and where we want to measure the time between
> >    them.
> > [...]
> 
> FWIW, those who want to collect such measurements today can do so with 
> a few lines of systemtap script for each of the above.

Well, i dont think stap can do workload instrumentation. It can do 
system-wide (and user local / task local) - but can it do per task 
hierarchies?

Also, i dont think stap supports proper separation of per workload 
measurements either. I.e. can you write a script that will work properly 
even if multiple monitoring tools are running, each trying to measure 
latencies?

Also, i personally find built-in kernel functionality more trustable 
than dynamically built stap kernel modules that get inserted.

	Ingo
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