Re: [PATCH 7/9] perf tools: Add size to struct perf_record_header_build_id

From: Arnaldo Carvalho de Melo <acme@kernel.org>
To: Jiri Olsa <jolsa@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>, Ian Rogers <irogers@google.com>,
	lkml <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Ingo Molnar <mingo@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Namhyung Kim <namhyung@kernel.org>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Michael Petlan <mpetlan@redhat.com>,
	Stephane Eranian <eranian@google.com>
Subject: Re: [PATCH 7/9] perf tools: Add size to struct perf_record_header_build_id
Date: Wed, 14 Oct 2020 12:32:50 -0300	[thread overview]
Message-ID: <20201014153250.GK3100363@kernel.org> (raw)
In-Reply-To: <20201014132146.GB1382146@krava>

Em Wed, Oct 14, 2020 at 03:21:46PM +0200, Jiri Olsa escreveu:
> On Wed, Oct 14, 2020 at 08:59:08AM -0300, Arnaldo Carvalho de Melo wrote:
> > Em Tue, Oct 13, 2020 at 09:24:39PM +0200, Jiri Olsa escreveu:
> > > We do not store size with build ids in perf data,
> > > but there's enough space to do it. Adding misc bit
> > > PERF_RECORD_MISC_BUILD_ID_SIZE to mark build id event
> > > with size.
> > > 
> > > With this fix the dso with md5 build id will have correct
> > > build id data and will be usable for debuginfod processing
> > > if needed (coming in following patches).
> > > 
> > > Acked-by: Ian Rogers <irogers@google.com>
> > > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > > ---
> > >  tools/lib/perf/include/perf/event.h | 12 +++++++++++-
> > >  tools/perf/util/build-id.c          |  8 +++++---
> > >  tools/perf/util/header.c            | 10 +++++++---
> > >  3 files changed, 23 insertions(+), 7 deletions(-)
> > > 
> > > diff --git a/tools/lib/perf/include/perf/event.h b/tools/lib/perf/include/perf/event.h
> > > index a6dbba6b9073..988c539bedb6 100644
> > > --- a/tools/lib/perf/include/perf/event.h
> > > +++ b/tools/lib/perf/include/perf/event.h
> > > @@ -201,10 +201,20 @@ struct perf_record_header_tracing_data {
> > >  	__u32			 size;
> > >  };
> > >  
> > > +#define PERF_RECORD_MISC_BUILD_ID_SIZE (1 << 15)
> > > +
> > >  struct perf_record_header_build_id {
> > >  	struct perf_event_header header;
> > >  	pid_t			 pid;
> > > -	__u8			 build_id[24];
> > > +	union {
> > > +		__u8		 build_id[24];
> > > +		struct {
> > > +			__u8	 data[20];
> > > +			__u8	 size;
> > > +			__u8	 reserved1__;
> > > +			__u16	 reserved2__;
> > > +		};
> > > +	};
> > >  	char			 filename[];
> > >  };

> > Hey, shouldn't we just append the extra info at the end, i.e. keep it
> > like:

> >  struct perf_record_header_build_id {
> >  	struct perf_event_header header;
> >  	pid_t			 pid;
> > 	__u8			 build_id[24];
> >  	char			 filename[];
> > 	__u8			 size;
> >  };

> > No need for PERF_RECORD_MISC_BUILD_ID_SIZE, older tools will continue
> > working with new perf data files.

> hum, then how would we tell if the last byte (size) is present or not?

IT would be different than '\0' ;-)

- Arnaldo

> > 
> > OTOH BUILD_ID_SIZE is 20 and the space on this header is 24, so the last
> > 4 bytes were not being used, so older tools don't look into it, they
> > should continue working, have you tested this case? I.e. getting the
> > perf binary in, say, fedora and check that it works with this new
> > perf_record_header_build_id layout?
> 
> yes, that still works (tested), because we copied only 20 bytes
> of the build_id[24] and did not care about the rest

Great that you actually tested it.

- Arnaldo