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

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Message-ID: <20091016074222.GD30019@elte.hu>
Date:	Fri, 16 Oct 2009 09:42:22 +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:

> 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.

Yeah, but please reuse linux/bitmap.h instead of some open-coded array 
of 4x u64's.

	Ingo
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