[lttng-dev] [lttng-ust] [babeltrace] use case question -field names in event classes

[Thibault, Daniel]

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On Tue, Oct 11, 2016 at 4:46 PM, Staffan Tjernstrom <staffan at eternaltraveller.com> wrote:
> We have a situation where we have a significant number (1000's) of 
> log-style trace messages generated by a large group of (sometime
> offsite) developers.
>
> Unfortunately the tracepoint definitions are maintained by a small 
> group of central developers.
>
> So we have a need for generic tracepoint definitions, without losing 
> the ability to tailor the output from individual log messages.
>
> One approach we've come up with involves including the labels as part 
> of the parameter pack:
>
> TRACEPOINT_EVENT(
>     my_app,
>     two_ints_and_string,
>     TP_ARGS(
>         // some pre-amble fields removed for clarity
>         char const *, label1_arg,
>         int, value1_arg,
>         char const *, label2_arg,
>         int, value2_arg,
>         char const *, label3_arg,
>         char const *, value3_arg
>     ),
>     TP_FIELDS(
>         // pre-amble removed for clarity
>         ctf_string( label1, label1_arg )
>         ctf_integer(int, value1, value1_arg)
>         ctf_string( label2, label2_arg )
>         ctf_integer(int, value2, value2_arg)
>     )
> )
>
> and we can then call this from multiple locations in our general 
> code-base using:
>
> tracepoint(my_app, two_ints_and_string, "widget_height", 19, 
> "widget_width", 23, "widget_name", widget_name_as_cstring);
>
> tracepoint(my_app, two_ints_and_string, "x_coordinate", x, 
> "y_coordinate", y, "point_type", "star");
>
> The issue with this is that the output obviously treats the field 
> names and their values as individual items, so we get output like
>
> label1 = "widget_height", value1 = 19, label2 = "widget_width", value2
> =- 23, label3 = "widget_name", value3 = "topper"
> label1 = "x_coordinate", value1 = -23, label2 = "y_coordinate", value2 
> = 25532, label3 = "point_type", value3 = "star"
>
> But of course we need to mechanically scrape / inspect the output, so 
> what we'd really like would be output something more like:
>
> widget_height = 19, widget_width = 23, widget_name = "topper"
> x_coordinate = -23, y_coordinate = 25532, point_type = "star"
>
> Post-processing of the babeltrace output is of course one option 
> (using -n none), but it feels like the wrong choice (adding more steps 
> to the tracing chain).
>
> Does anyone out there have any better ideas how to achieve having our 
> cake ( a small number of generic tracepoints ), and eating it ( 
> user-defined field names )?

Have you thought about using TRACEPOINT_EVENT_CLASS and TRACEPOINT_EVENT_INSTANCE?

Untested:

TRACEPOINT_EVENT_CLASS(
     my_app,
     two_ints_and_string,
     TP_ARGS(
         // some pre-amble fields removed for clarity
         int, value1_arg,
         int, value2_arg,
         char const *, value3_arg
     ),

TRACEPOINT_EVENT_INSTANCE(
     my_app,
     two_ints_and_string,
     two_ints_and_string_widget,
     TP_ARGS(
         // repeats the TRACEPOINT_EVENT_CLASS TP_ARGS
     ),

TRACEPOINT_EVENT_INSTANCE(
     my_app,
     two_ints_and_string,
     two_ints_and_string_point,
     TP_ARGS(
         // repeats the TRACEPOINT_EVENT_CLASS TP_ARGS
     ),

tracepoint(my_app, two_ints_and_string_widget, 19, 23, widget_name_as_cstring);
tracepoint(my_app, two_ints_and_string_point, x, y, "star");

You would only need one TRACEPOINT_EVENT_CLASS per signature, and could tailor the event names to make the arguments as obvious as they need to be.

Daniel U. Thibault, M.Sc.²

Informaticien scientifique, CME-PSC, Centre de recherches de Valcartier
Recherche et développement pour la défense Canada / Gouvernement du Canada
daniel.thibault at drdc-rddc.gc.ca / Tél: 418-844-4000x4245 Téléc: 418-844-4538
daniel.thibault at forces.gc.ca / Tél: 418-844-4000x4245 Téléc: 418-844-4538

Computer Scientist, MCCS-SPC, Valcartier Research Centre 
Defence Research and Development Canada / Government of Canada
daniel.thibault at drdc-rddc.gc.ca / Tel: 418-844-4000x4245 Fax: 418-844-4538
daniel.thibault at forces.gc.ca / Tel: 418-844-4000x4245 Fax: 418-844-4538

DRDC is an agency of the Department of National Defence / RDDC est une agence du ministère de la Défense nationale