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

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Date:	Thu, 25 Sep 2008 12:38:21 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	prasad@...ux.vnet.ibm.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	David Wilder <dwilder@...ibm.com>, hch@....de,
	Martin Bligh <mbligh@...gle.com>,
	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:

> > > Or if we already have the data together:
> > > 
> > > ring_buffer_write(buffer, event_id, length, data);
> > 
> > Don't like the event_id, just stick to plain binary data, and leave 
> > interpretation open to whoemever uses it.
> 
> This is up to debate. I know you don't like this extra event layer, 
> but seriously, all my uses with ring buffers has had some kind of 
> event. But then I'm sure you can argue that if you are using a single 
> type you can can the event.
> 
> I'm open to doing this, but I would like a consensus on this.
> 
> Martin, Thomas, Ingo, Linus?

i'd prefer Peter's simplification and not pass event_id along. Since 
static types are lost anyway (which is the biggest cost and risk of any 
such abstraction), we have to convert between types early on. Whether 
event_id is visible in the API is no big difference.

(It might be cheaper to not pass it along even if everyone ends up using 
it - as it has no semantic meaning anyway.)

pretty much the only generic tracing information is time and payload 
size. ( but even a time key is debatable - there are various resolutions 
needed by different usecases. Some usecases are even fine without any 
timestamps at all - they just want to know the ordering of events and 
that's it.)

i'd like to see some automatic type protection though, as an 
off-by-default debug option: encode the record type on storing and 
double-check it on extraction. So it should be possible to reliably 
store/restore a typed trace buffer and notice corruption early in 
testing.

because there's one thing that is far more important tracer feature than 
sheer performance: robustness.

Automated type checking in debug mode would also mean we could go for 
RLE encoding much more agressively. Most of the risks of a more complex, 
more compressed and pointer-laden data format come from type mismatches 
and the loss of compiler protection against human errors/stupidity. 
(running off the end of the page, misinterpreting a pointer, a record, 
etc.)

	Ingo
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