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

[Ingo Molnar <mingo@elte.hu>]

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

> 
> * 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

/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