linux-kernel - Re: [PATCH] perf tools: Add a new generic section in perf.data

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Date:	Fri, 16 Oct 2009 09:10:46 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Mike Galbraith <efault@....de>,
	Paul Mackerras <paulus@...ba.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] perf tools: Add a new generic section in perf.data

* Frederic Weisbecker <fweisbec@...il.com> wrote:

> +/*
> + * Version 0: contains trace_info section only
> + */
> +struct perf_file_additionals {
> +	u64				version;
> +	struct perf_file_section	trace_info;
> +};

i dont disagree with the change - but it would be even nicer to simply 
define a features bitmask, instead of a flat version - and add the 
trace_info section as a feature.

That way it's all a lot more manageable: we dont know about versions per 
se, we know about features. Individual features could be developed (and 
backported) in a distributed way - without having to worry about a flat 
version model.

So i'd suggest something like a bitmask in the perf.data file header:

	DECLARE_BITMAP(features, 256);

Plus every perf version knows about the features it supports:

	DECLARE_BITMAP(features_supported, 256);

The compatibility rule is: perf only touches attributes that belong to 
features it knows about.

	Ingo
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