linux-kernel - Re: Unified tracing buffer

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Date:	Sat, 20 Sep 2008 00:41:10 +0200
From:	Olaf Dabrunz <od@...e.de>
To:	Martin Bligh <mbligh@...gle.com>
Cc:	Randy Dunlap <randy.dunlap@...cle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	"od Frank Ch. Eigler" <fche@...hat.com>
Subject: Re: Unified tracing buffer

On 19-Sep-08, Martin Bligh wrote:
> >> Event ids are 16 bit, dynamically allocated.
> >
> > What are these (like)?
> 
> u16
>
> Sorry, probably lots of implicit assumptions in there that I forgot to explain

Ids for event types. Either allocated dynamically, if the tracer needs
new ids on each use, or statically assigned for others (like my fctrace
or Steven's ftrace, I believe). Should we have a reserved range / registry
for static allocation, maybe something like a very simple version of
devices.txt?

> > Arch-specific "word"?
> > or some fixed-size-for-all-systems (so that trace buffers can be
> > shared/used on other systems?)  Preferably the latter.
> 
> Mmmm. I don't see anything wrong with making it just 8 byte aligned, personally.
> Steven - this was your thing?

Unaligned can be much slower. I guess some very quick tracers can
benefit from alignment.

> as long as we record it in the buffer header at trace start. I guess we should
> document the buffer header ;-)

Yes. :)

-- 
Olaf Dabrunz (od/odabrunz), SUSE Linux Products GmbH, Nürnberg

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