linux-kernel - Re: [PATCH] perf tools: Provide backward compatibility with previous perf.data version

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Date:	Thu, 8 Oct 2009 22:31:37 +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>
Subject: Re: [PATCH] perf tools: Provide backward compatibility with
	previous perf.data version


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

> On Thu, Oct 08, 2009 at 10:12:22PM +0200, Ingo Molnar wrote:
> > 
> > * Frederic Weisbecker <fweisbec@...il.com> wrote:
> > 
> > > We have merged the trace.info file into perf.data by adding one 
> > > section in the perf headers. This makes it incompatible with previous 
> > > version: the new perf tools can't read the older perf.data.
> > > 
> > > To support the previous format, we check the headers size. If they 
> > > have the same size than in the previous format, then ignore the trace 
> > > info section that doesn't exist.
> > > 
> > > Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> > > Cc: Peter Zijlstra <peterz@...radead.org>
> > > Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
> > > Cc: Mike Galbraith <efault@....de>
> > > Cc: Paul Mackerras <paulus@...ba.org>
> > > ---
> > >  tools/perf/util/header.c |    8 +++++++-
> > >  1 files changed, 7 insertions(+), 1 deletions(-)
> > > 
> > > diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
> > > index 212fade..9aae360 100644
> > > --- a/tools/perf/util/header.c
> > > +++ b/tools/perf/util/header.c
> > > @@ -287,10 +287,16 @@ struct perf_header *perf_header__read(int fd)
> > >  	do_read(fd, &f_header, sizeof(f_header));
> > >  
> > >  	if (f_header.magic	!= PERF_MAGIC		||
> > > -	    f_header.size	!= sizeof(f_header)	||
> > >  	    f_header.attr_size	!= sizeof(f_attr))
> > >  		die("incompatible file format");
> > >  
> > > +	if (f_header.size != sizeof(f_header)) {
> > > +		/* Support the previous format */
> > > +		if (f_header.size == offsetof(typeof(f_header), trace_info))
> > > +			f_header.trace_info.size = 0;
> > > +		else
> > > +			die("incompatible file format");
> > > +	}
> > >  	nr_attrs = f_header.attrs.size / sizeof(f_attr);
> > >  	lseek(fd, f_header.attrs.offset, SEEK_SET);
> > 
> > Perfect - thanks Frederic!
> > 
> > This should handle most of the (non-trace) perf.data's out there. 
> 
> 
> Yeah, hopefully. I've only tested in a simple old perf.data but that 
> should do the trick for all of the non-trace files.

yeah, i tested it on an old perf.data too, and it worked.

> > trace.info is not something we really want to be backwards compatible 
> > about, if we can avoid it.
> > 
> > 	Ingo
> 
> 
> Yeah, that said, it's still possible and wouldn't require too much 
> changes. May be let's wait for complaints and, if any, I can provide 
> this backward support too.

ok.

	Ingo
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