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

[Ingo Molnar <mingo@elte.hu>]

* 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