linux-kernel - Re: [PATCH] ring_buffer: allocate buffer page pointer

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Date:	Thu, 2 Oct 2008 09:06:11 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Jonathan Corbet <corbet@....net>,
	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	prasad@...ux.vnet.ibm.com, "Frank Ch. Eigler" <fche@...hat.com>,
	David Wilder <dwilder@...ibm.com>, hch@....de,
	Martin Bligh <mbligh@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Steven Rostedt <srostedt@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
Subject: Re: [PATCH] ring_buffer: allocate buffer page pointer


On Thu, 2 Oct 2008, Andrew Morton wrote:
> > 
> > This patch adds a macro to assign all entries of ftrace using the type
> > of the variable and checking the entry id. The typecasts are now done
> > in the macro for only those types that it knows about, which should
> > be all the types that are allowed to be read from the tracer.
> > 
> 
> I'm somewhat at a loss here because I'm unable to find any version of
> kernel/trace/trace.c which looks anything like the one which is being
> patched, but...

As Ingo mentioned, you don't have this yet. And be happy that you don't 
;-)

This patch is to fix the patch that did this.

> 
> > --- a/kernel/trace/trace.c
> > +++ b/kernel/trace/trace.c
> > @@ -1350,7 +1350,9 @@ print_lat_fmt(struct trace_iterator *iter, unsigned int trace_idx, int cpu)
> >  	}
> >  	switch (entry->type) {
> >  	case TRACE_FN: {
> > -		struct ftrace_entry *field = (struct ftrace_entry *)entry;
> 
> Why was this code using a cast in the first place?  It should be using
> entry->some_field_i_dont_have_here?  That was the whole point in using 
> the anonymous union in struct trace_entry?

Because the ring_buffer now allows for variable length entries, having a 
one size fits all entry is not optimal.

But because C is not the best for typecasting, we have this macro to help 
solve the issue. Instead of registering everything into a single union and 
causing small fields to be large, we have a macro you can register your
type with instead.

-- Steve

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