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SubjectRe: [RFC PATCH 2/5] tracing/events: nicer print format for parsing
From
2009/6/10 Ingo Molnar <mingo@elte.hu>:
>
> * Christoph Hellwig <hch@infradead.org> wrote:
>
>> On Tue, Jun 09, 2009 at 09:22:01PM +0200, Frederic Weisbecker wrote:
>> > But I wonder if the above new language is not breaking the charm
>> > of the TRACE_EVENT(), which charm is that it's easy to implement (hopefully).
>> >
>> > Everyone knows the printk formats. And I guess this new thing is
>> > easy and quick to learn. But because it's a new unknown
>> > language, the TRACE_EVENT will become less readable, less
>> > reachable for newcomers in TRACE_EVENT.
>>
>> I must also say I don't particularly like it.  printk is nice and
>> easy an everybody knows it, but it's not quite flexible enough as
>> we might have to do all kinds of conversions on the reader side.
>> What might be a better idea is to just have C function pointer for
>> output conversions that could be put into the a file in debugfs
>> and used by the binary trace buffer reader.  Or maybe not as we
>> would pull in too many depenencies.
>
> Another bigger problem with the new tag format, beyond introducing
> an arbitrary descriptor language (which is easy to mess up) is the
> loss of type checking.
>
> With the tags the field printouts can go stray easily - while with
> TP_printk() we had printf type checking. (which, as imperfect as it
> may be to specify a format, does create a real connection between
> the record and the output format specification.)
>
>> I think we should go with the printk solution for 2.6.31 and use
>> the full development cycle for 2.6.32 to come up with something
>> better.
>>
>> As soon as a couple of large subsystems use the even tracer we
>> also have a broader base examples to see how new syntax works on
>> them.
>
> I think much of the tooling problem could be solved with a little
> trick: the format string can be injected into an artificial .c file
> (runtime), and the tool could compile that .c file (in user-space)
> and get access to the result.
>
> For example, one of the more complex block tracepoints,
> /debug/tracing/events/block/block_bio_backmerge:
>
> print fmt: "%d,%d %s %llu + %u [%s]", ((unsigned int) ((REC->dev) >>
> 20)), ((unsigned int) ((REC->dev) & ((1U << 20) - 1))), REC->rwbs,
> (unsigned long long)REC->sector, REC->nr_sector, REC->comm
>
> when pasted verbatim into the stub below, produces:
>
>   0,6 a 7 + 8 [abc]
>
> Note that i pasted the format string into the code below unchanged,
> and i used the format descriptor to create the record type. (this
> too is easy to automate).
>
> If this is generated into the following function:
>
>  format_block_bio_backmerge(struct record *rec);
>
> and a small dynamic library is built out of it, tooling can use
> dlopen() to load those format printing stubs.
>
> It's all pretty straightforward and can be used for arbitrarily
> complex formats.



Hmm



> And i kind of like the whole notion on a design level as weell: the
> kernel exporting C source code for tools :-)
>
>        Ingo
>
> ------------------>
>
> struct record {
>        unsigned short common_type;
>        unsigned char common_flags;
>        unsigned char common_preempt;
>        int common_pid;
>        int common_tgid;
>        int dev;
>        unsigned long long sector;
>        unsigned int nr_sector;
>        char rwbs[6];
>        char comm[16];
> } this_record = { 1, 2, 3, 4, 5, 6, 7, 8, { 'a', }, "abc" };
>
> void main(void)
> {
>        struct record *REC = &this_record;
>
>        printf("%d,%d %s %llu + %u [%s]", ((unsigned int) ((REC->dev) >> 20)), ((unsigned int) ((REC->dev) & ((1U << 20) - 1))), REC->rwbs, (unsigned long long)REC->sector, REC->nr_sector, REC->comm);
> }


Yeah it's a quite nice idea.
But it's assuming everyone parses binary files using C programs.
Usually, such parsing
more likely involves the use of scripting languages.
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