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From: Li Zefan on 18 May 2010 00:50
Steven Rostedt wrote:
> From: Steven Rostedt <srostedt(a)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 3K:
>
> text data bss dec hex filename
> 4913961 1088356 861512 6863829 68bbd5 vmlinux.orig
> 4900252 1057412 861512 6819176 680d68 vmlinux.regs
> 4900375 1053380 861512 6815267 67fe23 vmlinux.fields
>

It also saves memory allocated by trace_define_field(). :)

> 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.
>

All syscall exit events have the same fields. I'll make a patch for this.

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