linux-kernel - Re: [patch 1/2] x86_64 page fault NMI-safe

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Date:	Fri, 06 Aug 2010 11:50:40 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Steven Rostedt <rostedt@...tedt.homelinux.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Christoph Hellwig <hch@....de>, Li Zefan <lizf@...fujitsu.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Johannes Berg <johannes.berg@...el.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Tom Zanussi <tzanussi@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andi Kleen <andi@...stfloor.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	"Frank Ch. Eigler" <fche@...hat.com>, Tejun Heo <htejun@...il.com>,
	2nddept-manager@....hitachi.co.jp
Subject: Re: [patch 1/2] x86_64 page fault NMI-safe

On Fri, 2010-08-06 at 15:18 +0900, Masami Hiramatsu wrote:
> Peter Zijlstra wrote:
> > On Wed, 2010-08-04 at 10:45 -0400, Mathieu Desnoyers wrote:
> > 
> >> How do you plan to read the data concurrently with the writer overwriting the
> >> data while you are reading it without corruption ?
> > 
> > I don't consider reading while writing (in overwrite mode) a valid case.
> > 
> > If you want to use overwrite, stop the writer before reading it.
> 
> For example, would you like to read system audit log always after
> stop the audit?
> 
> NO, that's a most important requirement for tracers, especially for
> system admins (they're the most important users of Linux) to check
> the system health and catch system troubles.
> 
> For performance measurement and checking hotspot, one-shot tracing
> is enough. But it's just for developers. But for the real world
> computing, Linux is just an OS, users want to run their system,
> middleware and applications, without troubles. But when they hit
> a trouble, they wanna shoot it ASAP.
> The flight recorder mode is mainly for those users.

You cannot over-write and consistently read the buffer, that's plain
impossible. With sub-buffers you can swivel a sub-buffer and
consistently read that, but there is no guarantee the next sub-buffer
you steal was indeed adjacent to the previous buffer you stole as that
might have gotten over-written by the active writer while you were
stealing the previous one.

If you want to snapshot buffers, do that, simply swivel the whole trace
buffer, and continue tracing in a new one, then consume the old trace in
a consistent manner.

I really see no value in being able to read unrelated bits and pieces of
a buffer.

So no, I will _not_ support reading an over-write buffer while there is
an active reader.
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