linux-kernel - Re: Fix powerTOP regression with 2.6.39-rc5

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Date:	Tue, 10 May 2011 09:54:57 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	David Sharp <dhsharp@...gle.com>,
	Vaibhav Nagarnaik <vnagarnaik@...gle.com>,
	Michael Rubin <mrubin@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Christoph Hellwig <hch@...radead.org>,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: Fix powerTOP regression with 2.6.39-rc5

* Steven Rostedt <rostedt@...dmis.org> wrote:

> > I think its rather obvious how the unification should be done: check 
> > tip:tmp.perf/trace for the 'trace' command that does tracing.
> 
> I'll tell you what. I've been talking with other developers and one thing we 
> came up with that we all seem to agree with is that ftrace is designed to 
> trace the entire system, and it does it very well. Perf is designed to trace 
> individual tasks, and it does it very well (trace is an example of this. It's 
> focus is on tasks not the system). Ftrace can also trace individual tasks and 
> perf can also trace the entire system, but they both do those poorly.

Not sure where you picked that up but it's 100% nonsense and you could not be 
more wrong.

The reason why you see most instrumentation users use per task tracing and 
profiling is very simple: they *can* do it and local views are what most 
developer are interested in!

Otherwise perf has been designed to do system-wide (global) tracing pretty much 
from day one on. In fact one of the first applications of perf: kerneltop, the 
tool that evolved into 'perf top' has a system-wide view and never had any 
other default but system-wide tracing+profiling ...

'perf top' is what many kernel developers use and it's very popular because the 
kernel itself is 'system-wide' so obviously kernel developers want to have (and 
need to have) a system-wide view.

ftrace uses system-wide tracing because that's pretty much the only model it 
has. That is one of its many design mistakes, not a feature.

But the world is a lot more than just kernel focused workflows and perf 
supports various other popular views:

  - per task
  - per task hierarchy (tree spanning fork()/exec()/clone() trees of tasks)
  - per cgroup
  - system-wide

And you want to keep ftrace a forked identity on the weird notion that somehow 
perf can not do system-wide event collection and that somehow fundamentally 
instrumentation can not serve these goals of event grouping?

Steve, your opinion is, sadly, very narrow.

Thanks,

	Ingo
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