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SubjectRe: [PATCH 05/10][RFC] tracing: Move fields from event to class structure
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On Wed, 2010-04-28 at 16:58 -0400, Mathieu Desnoyers wrote:
> * Steven Rostedt (rostedt@goodmis.org) wrote:
> > From: Steven Rostedt <srostedt@redhat.com>
> >
> > 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




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