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

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Message-ID: <20091210180412.GB30999@redhat.com>
Date:	Thu, 10 Dec 2009 13:04:12 -0500
From:	"Frank Ch. Eigler" <fche@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
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

Hi -


> > 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?

It can track the evolution of task hierarchies by listening to process
forking events, and filter other kernel/user events according to
then-current hierarchy data.  One primitive implementation of this is
in the target_set.stp tapset, but it's easy to script up other
policies.


> 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?

Sure, always has.  You can run many scripts concurrently, each with its
own internal state.  (Overheads accumulate, sadly & naturally.)


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

I understand.  In the absence of a suitable bytecode engine in the
kernel, this was the only practical way to do everything we needed.


- FChE
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