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

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Message-ID: <20100803201022.GA18583@elte.hu>
Date:	Tue, 3 Aug 2010 22:10:22 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	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


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Tue, Aug 3, 2010 at 12:45 PM, Mathieu Desnoyers
> <mathieu.desnoyers@...icios.com> wrote:
> >
> > The real issue here, IMHO, is that Perf has tied gory ring buffer 
> > implementation details to the userspace perf ABI, and there is now strong 
> > unwillingness from Perf developers to break this ABI.

(Wrong.)

> The thing is - I think my outlined buffer fragmentation model would work 
> fine with the perf ABI too.  Exactly because there is no deep structure, 
> just the same "stream of small events" both from a kernel and a user model 
> standpoint. Sure, the stream would now contain a new event type, but that's 
> trivial. It would still be _entirely_ reasonable to have the actual data in 
> the exact same ring buffer, including the whole mmap'ed area.

Yeah.

> Of course, when user space actually parses it, user space would have to 
> eventually defragment the event by allocating a new area and copying the 
> fragments together in the right order, but that's pretty trivial to do. It 
> certainly doesn't affect the current mmap'ed interface in the least.
> 
> Now, whether the perf people feel they want that kind of functionality, I 
> don't know. It's possible that they simply do not want to handle events that 
> are complex enough that they would have arbitrary size.

Looks useful. There's a steady trickle of new events and we already use type 
encapsulation for things like trace events - which are only made sense of 
later on in user-space.

We may want to add things like a NOP event to pad out the end of page

Thanks,

	Ingo
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