Re: [Qemu-devel] [PATCH v6 11/20] trace: emit name <-> ID mapping in simpletrace header

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Tue, Oct 04, 2016 at 02:35:50PM +0100, Daniel P. Berrange wrote:
> Currently simpletrace assumes that events are given IDs
> starting from 0, based on the order in which they appear
> in the trace-events file, with no gaps. When the
> trace-events file is split up, this assumption becomes
> problematic.
> 
> To deal with this, extend the simpletrace format so that
> it outputs a table of event name <-> ID mappings. That
> will allow QEMU to assign arbitrary IDs to events without
> breaking simpletrace parsing.
> 
> The v3 simple trace format was
> 
>   FILE HEADER
>   EVENT TRACE RECORD 0
>   EVENT TRACE RECORD 1
>   ...
>   EVENT TRACE RECORD N
> 
> The v4 simple trace format is now
> 
>   FILE HEADER
>   EVENT MAPPING RECORD 0
>   EVENT MAPPING RECORD 1
>   ...
>   EVENT MAPPING RECORD M
>   EVENT TRACE RECORD RECORD 0
>   EVENT TRACE RECORD RECORD 1
>   ...
>   EVENT TRACE RECORD N
> 
> Although this shows all the mapping records being emitted
> upfront, this is not required by the format. While the main
> simpletrace backend will emit all mappings at startup,
> the systemtap simpletrace.stp script will emit the mappings
> at first use. eg
> 
>   FILE HEADER
>   ...
>   EVENT MAPPING RECORD 0
>   EVENT TRACE RECORD RECORD 0
>   EVENT TRACE RECORD RECORD 1
>   EVENT MAPPING RECORD 1
>   EVENT TRACE RECORD RECORD 2
>   ...
>   EVENT TRACE RECORD N
> 
> This is more space efficient given that most trace records
> only include a subset of events.
> 
> In modifying the systemtap simpletrace code, a 'begin' probe
> was added to emit the trace event header, so you no longer
> need to add '--no-header' when running simpletrace.py for
> systemtap generated trace files.
> 
> Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
> ---
>  scripts/simpletrace.py                       | 50 ++++++++++++++++++++--------
>  scripts/tracetool/format/simpletrace_stap.py | 24 +++++++++++--
>  trace/simple.c                               | 33 ++++++++++++++++--
>  3 files changed, 89 insertions(+), 18 deletions(-)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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