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Date: Tue, 10 May 2011 10:09:18 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: David Sharp <dhsharp@google.com>,
        Vaibhav Nagarnaik <vnagarnaik@google.com>,
        Michael Rubin <mrubin@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
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        Frederic Weisbecker <fweisbec@gmail.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Thomas Gleixner <tglx@linutronix.de>,
        Christoph Hellwig <hch@infradead.org>, Arnd Bergmann <arnd@arndb.de>
Subject: Re: Fix powerTOP regression with 2.6.39-rc5
Message-ID: <20110510080918.GB27426@elte.hu>
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* Steven Rostedt <rostedt@goodmis.org> wrote:

> > > > PowerTop uses the perf ABI because it's a rather convenient and unified 
> > > > method to get a rich selection of events via the same facility, same 
> > > > ring-buffer, using a system call ABI, etc.
> > > 
> > > It seams that Arjan read the perf kernel code to see how the raw data was 
> > > laid out and read it directly.
> > 
> > Yes, of course - i also have code that uses the syscall directly, it's 
> > easier to code up ad-hoc than to parse some XML-lookalike descriptor from 
> > /debug/tracing/events/.
> 
> So you admit this is a ad-hoc way of doing things. [...]

There's nothing wrong with being able to code and make use of instrumentation 
ad-hoc, using a simple interface.

The *worst* possible thing is to force user-space to use something complex.
So we kept the perf ABI simple.

> [...] Thus a library is a perfect solution. [...]

That's a non sequitor.

> [...] And the event formats are far from XML-look-a-like. You really think 
> this:
> 
> name: sched_switch
> ID: 57
> format:
> 	field:unsigned short common_type;	offset:0;	size:2;
> 	field:unsigned char common_flags;	offset:2;	size:1;
> 	field:unsigned char common_preempt_count;	offset:3;	size:1;
> 	field:int common_pid;	offset:4;	size:4;
> 	field:int common_lock_depth;	offset:8;	size:4;
> 
> 	field:char prev_comm[TASK_COMM_LEN];	offset:12;	size:16;
> 	field:pid_t prev_pid;	offset:28;	size:4;
> 	field:int prev_prio;	offset:32;	size:4;
> 	field:long prev_state;	offset:40;	size:8;
> 	field:char next_comm[TASK_COMM_LEN];	offset:48;	size:16;
> 	field:pid_t next_pid;	offset:64;	size:4;
> 	field:int next_prio;	offset:68;	size:4;
> 
> Is equivalent to this:
> 
> <?xml version="1.0" encoding="UTF-8"?>
> <KernelShark><CaptureSettings><Events><CaptureType>Events</CaptureType><System>sched</System></Events><Plugin>function_graph</Plugin><File>/tmp/trace.dat</File></CaptureSettings></KernelShark>

I did not say that it's equivalent, i said it's XML look-alike.

Steve, we even joked about that, that if we continue like this we'll end up 
with an XML parser ... I requested several changes to the description format so 
that it becomes more human readable.

Thanks,

	Ingo
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