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

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Message-ID: <alpine.LFD.1.10.0809251025170.3265@nehalem.linux-foundation.org>
Date:	Thu, 25 Sep 2008 10:29:57 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
cc:	Martin Bligh <mbligh@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	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, "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 Thu, 25 Sep 2008, Mathieu Desnoyers wrote:
> 
> We could use a page header instead to contain the "unused_size"
> information.

Absolutely. There's no one way to do this.

> I would prefer to put the extended timestamp within the event header
> instead of creating a separate entry for this for atomicity concerns
> (what happens if a long interrupt executes between the TSCExtend marker
> event and the event expecting to be written right next to it ?).

The log entries should be reserved with interrupts disabled anyway, and 
they are per-CPU, so there are no atomicity issues.

For NMI's, things get more exciting. I'd really prefer NMI's to go to a 
separate ring buffer entirely, because otherwise consistency gets really 
hard. Using lockless algorithms for a variable-sized pool of pages is a 
disaster waiting to happen.

I don't think we can currently necessarily reasonably trace NMI's, but 
it's something to keep in mind as required support eventually.

		Linus
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