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

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Date:	Thu, 25 Sep 2008 07:33:24 -0700
From:	"Martin Bligh" <mbligh@...igh.org>
To:	"Peter Zijlstra" <peterz@...radead.org>
Cc:	"Martin Bligh" <mbligh@...gle.com>,
	"Steven Rostedt" <rostedt@...dmis.org>,
	"Linus Torvalds" <torvalds@...ux-foundation.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

> I rather like this idea, as it gives small entries (the common case) the
> least overhead but does allow for larger ones.
>
> By also putting the time in there you can do the merge sort iterator,
> Linus was right that everybody wants this anyway.
>
> As for delta encoding the time, we could make the tick log the absolute
> time packet, that's at least 100Hz and it already has to compute the
> full gtod thing anyway.
>
> I don't much like Linus' idea of bringing type information back into the
> primitive header (sorry Linus ;-)). I'd much rather keep that
> abstraction in the next layer.

There is part of the type stuff that belongs in the lower layer, it seems -
the padding events for the up-to-end-of-page buffering, and the timestamp
extensions. It seems wrong to split those across two layers.

But perhaps we can keep a couple of bits for this, and three of the bits
to represent the length of the data payload (maybe in 4 byte multiples
rather than bytes?) That'd let up to 28 bytes as a payload in a short event.
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