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

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Message-ID: <20091016073337.GA5069@nowhere>
Date:	Fri, 16 Oct 2009 09:33:41 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
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

On Fri, Oct 16, 2009 at 09:10:46AM +0200, Ingo Molnar wrote:
> 
> * 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


Yeah, I've thought about that too but feared about the limitation
of bitweight(u64), although it's probably enough, we never know.
That said I can take a bunch of four u64 to draw this bitmap and 256
features is enough.

Indeed that's way much better as a bitmap. Will do that instead.

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