Message-ID: <20100803202127.GA18773@elte.hu>
Date:	Tue, 3 Aug 2010 22:21:27 +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


* Ingo Molnar <mingo@...e.hu> wrote:

> 
> * 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

/me once again experiences the subtle difference between 'Y' and 'N' when postponing a mail

So adding fragments would be possible as well. We've got the space for such 
extensions in the ABI and the basic model of streaming information is not 
affected.

[ The control structure of the mmap area is there for performance/wakeup 
  optimizations (and to allow the kernel to lose information on producer 
  overload, while still giving user-space an idea that we lost data and how 
  much) - it does not affect semantics and does not limit us. ]

So there's no design limitation - Peter simply prefers one possible solution 
over another and outlined his reasons - we should hash that out based on the 
technical arguments.

Thanks,

	Ingo
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