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

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Date:	Wed, 04 Aug 2010 16:56:28 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	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>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.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>
Subject: Re: [patch 1/2] x86_64 page fault NMI-safe

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.

>  I think that the stack dump
> should simply be saved directly to the ring buffer, without copy. The
> dump_stack() functions might have to be extended so they don't just save text
> dumbly, but can also be used to save events into the trace in binary format,
> perhaps with the continuation cookie Linus was proposing.

Because I don't want to support truncating reservations (because that
leads to large nops for nested events) and when the event needs to go to
multiple buffers you can re-use the stack-dump without having to do the
unwind again.

The problem with the continuation thing Linus suggested is that it would
bloat the output 3 fold. A stack entry is a single u64. If you want to
wrap that in a continuation event you need: a header (u64), a cookie
(u64) and the entry (u64).

Continuation events might make heaps of sense for larger data pieces,
but I don't see them being practical for such small pieces.
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