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

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Message-ID: <alpine.DEB.1.10.0809301339140.6224@gandalf.stny.rr.com>
Date:	Tue, 30 Sep 2008 13:41:23 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.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, 30 Sep 2008, Peter Zijlstra wrote:
> > 
> > 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.

Regardless, there's more info we want to store for each page than the list 
head.  Especially when we start converting this to lockless. I rather get 
out of the overlaying of the page frames, its nice to save the space, but 
really scares the hell out of me. I can just imagine this blowing up if we 
redo the paging, and I dislike this transparent coupling between the 
tracer buffer and the pages.

-- Steve

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