Date: Tue, 3 Aug 2010 22:10:22 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Steven Rostedt <rostedt@rostedt.homelinux.com>,
        Thomas Gleixner <tglx@linutronix.de>, Christoph Hellwig <hch@lst.de>,
        Li Zefan <lizf@cn.fujitsu.com>, Lai Jiangshan <laijs@cn.fujitsu.com>,
        Johannes Berg <johannes.berg@intel.com>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        Arnaldo Carvalho de Melo <acme@infradead.org>,
        Tom Zanussi <tzanussi@gmail.com>,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        Andi Kleen <andi@firstfloor.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Jeremy Fitzhardinge <jeremy@goop.org>,
        "Frank Ch. Eigler" <fche@redhat.com>, Tejun Heo <htejun@gmail.com>
Subject: Re: [patch 1/2] x86_64 page fault NMI-safe
Message-ID: <20100803201022.GA18583@elte.hu>
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 <20100714233843.GD14533@nowhere>
 <20100715162631.GB30989@Krystal>
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* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Tue, Aug 3, 2010 at 12:45 PM, Mathieu Desnoyers
> <mathieu.desnoyers@efficios.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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