Re: [PATCH 05/10][RFC] tracing: Move fields from event to classstructure

[Steven Rostedt]

On Wed, 2010-04-28 at 16:58 -0400, Mathieu Desnoyers wrote:
> * Steven Rostedt (rostedt@xxxxxxxxxxx) wrote:
> > From: Steven Rostedt <srostedt@xxxxxxxxxx>
> > 
> > Move the defined fields from the event to the class structure.
> > Since the fields of the event are defined by the class they belong
> > to, it makes sense to have the class hold the information instead
> > of the individual events. The events of the same class would just
> > hold duplicate information.
> > 
> > After this change the size of the kernel dropped another 8K:
> > 
> >    text	   data	    bss	    dec	    hex	filename
> > 5788186	1337252	9351592	16477030	 fb6b66	vmlinux.orig
> > 5774316	1306580	9351592	16432488	 fabd68	vmlinux.reg
> > 5774503	1297492	9351592	16423587	 fa9aa3	vmlinux.fields
> > 
> > Although the text increased, this was mainly due to the C files
> > having to adapt to the change. This is a constant increase, where
> > new tracepoints will not increase the Text. But the big drop is
> > in the data size (as well as needed allocations to hold the fields).
> > This will give even more savings as more tracepoints are created.
> > 
> > Note, if just TRACE_EVENT()s are used and not DECLARE_EVENT_CLASS()
> > with several DEFINE_EVENT()s, then the savings will be lost. But
> > we are pushing developers to consolidate events with DEFINE_EVENT()
> > so this should not be an issue.
> > 
> > The kprobes define a unique class to every new event, but are dynamic
> > so it should not be a issue.
> > 
> > The syscalls however have a single class but the fields for the individual
> > events are different. The syscalls use a metadata to define the
> > fields. I moved the fields list from the event to the metadata and
> > added a "get_fields()" function to the class. This function is used
> > to find the fields. For normal events and kprobes, get_fields() just
> > returns a pointer to the fields list_head in the class. For syscall
> > events, it returns the fields list_head in the metadata for the event.
> 
> So, playing catch-up here, why don't we simply put each syscall event in
> their own class ? We could possibly share the class where it makes
> sense (e.g. exact same fields).

Well, they have their own class. But I guess you are talking about a
"meta-data class".

> 
> With the per-class sub-metadata, what's the limitations we have to
> expect with these system call events ? Can we map to a field size
> directly from the event ID, or do we have to somehow have the event size
> encoded in the header to make sense of the payload ?

That will be a lot of work. This is all generated automatically from the
SYSCALL() macros. To group them, we need a way to know what syscalls
have the same parameters, and manually add that. It may end up being a
maintenance nightmare.

-- Steve


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/