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

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Message-ID: <20080927184212.GB13685@elte.hu>
Date:	Sat, 27 Sep 2008 20:42:12 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Martin Bligh <mbligh@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Martin Bligh <mbligh@...igh.org>, linux-kernel@...r.kernel.org,
	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


* Steven Rostedt <rostedt@...dmis.org> wrote:

> 
> On Sat, 27 Sep 2008, Ingo Molnar wrote:
> > 
> > If all you do is to trace high-freq events on all CPUs and you are _not_ 
> > interested in the precise interactions, the overhead of global 
> > synchronization can hurt a lot.
> > 
> > In any case, SMP coherency of trace events is an independent property of 
> > the tracer, and preferably something that can be turned on/off.
> 
> Just a note.  The current ring buffering system that I'm proposing 
> keeps its own time stamp counter (currently sched_clock) that will 
> most likely be updated later. I'm trying to keep this ring buffer 
> system as dumb as possible. It does not even implement the merge sort. 
> That's up to the tracer to handle. There's nothing stopping the trace 
> from adding some atomic counter to each event to help it sort.

correct. The price is all the notifier/callback overhead and the loss of 
type checking of the record contents. But that's an unavoidable price of 
abstraction, at least in C.

> So yes, the tracer can implement anything it wants on top of the ring 
> buffer ;-)

yes, very nice! :)

	Ingo
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