linux-kernel - Re: [PATCH v10 Golden] Unified trace buffer

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Date:	Tue, 30 Sep 2008 19:00:43 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jonathan Corbet <corbet@....net>,
	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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 v10 Golden] Unified trace buffer

On Tue, 2008-09-30 at 12:48 -0400, Steven Rostedt wrote:
> On Tue, 30 Sep 2008, Linus Torvalds wrote:
> > 
> > Trace buffers are different, though. Do people realize that doing the 
> > overloading means that you never EVER can use those buffers for anything 
> > else? Do people realize that it means that splice() and friends are out of 
> > the question?
> > 
> > > Trouble is, looking at it I see no easy way out,
> > 
> > Quite frankly, we could just put it at the head of the page itself. Having 
> > a "whole page" for the trace data is not possible anyway, since the trace 
> > header itself will always eat 8 bytes.
> > 
> > And I do think it would potentially be a better model. Or at least safer.
> 
> Actually, looking at the code, there is no reason I need to keep this in 
> the frame buffer itself. I've also encapsulated the accesses to the 
> incrementing of the pointers so it would be trivial to try other 
> approaches.
> 
> The problem we had with the big array struct is that we can want large 
> buffers and to do that with pointers means we would need to either come up 
> with a large allocator or use vmap.
> 
> But I just realized that I could also just make a link list of page 
> pointers and do the exact same thing without having to worry about page 
> frames.  Again, the way I coded this up, it is quite trivial to replace 
> the handling of the pages with other schemes.

The list_head in the page frame should be available regardless of
splice() stuffs.

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