linux-kernel - Re: Unified tracing buffer

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Date:	Tue, 23 Sep 2008 14:13:13 -0400
From:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Tom Zanussi <zanussi@...cast.net>, prasad@...ux.vnet.ibm.com,
	Martin Bligh <mbligh@...gle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>, od@...ell.com,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>, hch@....de,
	David Wilder <dwilder@...ibm.com>,
	Christoph Lameter <cl@...ux-foundation.org>, linux-mm@...ck.org
Subject: Re: Unified tracing buffer

* Peter Zijlstra (a.p.zijlstra@...llo.nl) wrote:
> On Tue, 2008-09-23 at 00:25 -0500, Tom Zanussi wrote:
> 
> > - get rid of anything having to do with padding, nobody needs it and its
> > only affect has been to horribly distort and complicate a lot of the
> > code
> > - get rid of sub-buffers, they just cause confusion
> > - get rid of mmap, nobody uses it
> > - no sub-buffers and no mmap support means we can get rid of most of the
> > callbacks, and a lot of API confusion along with them
> > - add relay flags - they probably should have been used from the
> > beginning and options made explicit instead of being shoehorned into the
> > callback functions.
> 
>  - get rid of the vmap buffers as they cause tlb pressure and eat up
> precious vspace on 32 bit platforms.
> 

Although I agree on the basic idea, namely to use a sane amount of TLB
entries for tracing, I disagree on the way proposed to reach this goal.
Such memory management concerns belong to the mm field and should not be
done "oh so cleverly" by a buffer management infrastructure in the back
of the kernel memory management infrastructure.

I think we should instead try to figure out what is currently missing in
the kernel vmap mechanism (probably the ability to vmap from large 4MB
pages after boot), and fix _that_ instead (if possible), which would not
only benefit to tracing, but also to module support.

Also, I would like to keep a contiguous address mapping within buffers
so we could keep the buffer read/write code as simple as possible,
leveraging the existing CPU MM unit.

I added Christoph Lameter to the CC list, he always comes with clever
ideas. :)

Mathieu

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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