linux-kernel - Re: [RFC PATCH 1/3] Unified trace buffer

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Date:	Wed, 24 Sep 2008 09:24:46 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Martin Bligh <mbligh@...gle.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	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,
	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	David Wilder <dwilder@...ibm.com>, hch@....de,
	Tom Zanussi <zanussi@...cast.net>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [RFC PATCH 1/3] Unified trace buffer

On Wed, 24 Sep 2008, Peter Zijlstra wrote:
> 
> So when we reserve we get a pointer into page A, but our reserve length
> will run over into page B. A write() method will know how to check for
> this and break up the memcpy to copy up-to the end of A and continue
> into B.

I would suggest just not allowing page straddling.

Yeah, it would limit event size to less than a page, but seriously, do 
people really want more than that? If you have huge events, I suspect it 
would be a hell of a lot better to support some kind of indirection 
scheme than to force the ring buffer to handle insane cases.

Most people will want the events to be as _small_ as humanly possible. The 
normal event size should hopefully be in the 8-16 bytes, and I think the 
RFC patch is already broken because it allocates that insane 64-bit event 
counter for things. Who the hell wants a 64-bit event counter that much? 
That's broken.

		Linus
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