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

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Message-ID: <20100714233843.GD14533@nowhere>
Date:	Thu, 15 Jul 2010 01:38:45 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.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, Jul 14, 2010 at 07:11:17PM -0400, Mathieu Desnoyers wrote:
> * Frederic Weisbecker (fweisbec@...il.com) wrote:
> > On Wed, Jul 14, 2010 at 06:31:07PM -0400, Mathieu Desnoyers wrote:
> > > * Frederic Weisbecker (fweisbec@...il.com) wrote:
> > > > On Wed, Jul 14, 2010 at 12:54:19PM -0700, Linus Torvalds wrote:
> > > > > On Wed, Jul 14, 2010 at 12:36 PM, Frederic Weisbecker
> > > > > <fweisbec@...il.com> wrote:
> > > > > >
> > > > > > There is also the fact we need to handle the lost NMI, by defering its
> > > > > > treatment or so. That adds even more complexity.
> > > > > 
> > > > > I don't think your read my proposal very deeply. It already handles
> > > > > them by taking a fault on the iret of the first one (that's why we
> > > > > point to the stack frame - so that we can corrupt it and force a
> > > > > fault).
> > > > 
> > > > 
> > > > Ah right, I missed this part.
> > > 
> > > Hrm, Frederic, I hate to ask that but.. what are you doing with those percpu 8k
> > > data structures exactly ? :)
> > > 
> > > Mathieu
> > 
> > 
> > 
> > So, when an event triggers in perf, we sometimes want to capture the stacktrace
> > that led to the event.
> > 
> > We want this stacktrace (here we call that a callchain) to be recorded
> > locklessly. So we want this callchain buffer per cpu, with the following
> > type:
> 
> Ah OK, so you mean that perf now has 2 different ring buffer implementations ?
> How about using a single one that is generic enough to handle perf and ftrace
> needs instead ?
> 
> (/me runs away quickly before the lightning strikes) ;)
> 
> Mathieu


:-)

That's no ring buffer. It's a temporary linear buffer to fill a stacktrace,
and get its effective size before committing it to the real ring buffer.

Sure that involves two copies.

But I don't have a better solution in mind than using a pre-buffer for that,
since we can't know the size of the stacktrace in advance. We could
always reserve the max stacktrace size, but that would be wasteful.

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