Re: [RFC PATCH 2/5] tracing/events: nicer print format for parsing

["Frédéric Weisbecker" <fweisbec@gmail.com>]

Le 10 juin 2009 13:31, Frédéric Weisbecker<fweisbec@gmail.com> a écrit :
> 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.
>

While thinking more about it your idea can produce a whole parser
automatically and dynamically written.
So actually, looks like a right direction :-)