Re: Perf and ftrace [was Re: PyTimechart]

Peter Zijlstra
2010-05-12T18:49:11Z

On Wed, 2010-05-12 at 14:37 -0400, Mathieu Desnoyers wrote:
> * Peter Zijlstra (peterz@infradead.org) wrote:
> > On Wed, 2010-05-12 at 14:04 -0400, Mathieu Desnoyers wrote:
> > > Can't we keep multiple references to each page ? (shared page) so it's still in
> > > the buffer, also accessed by mmap(), and in addition accessed by splice.
> >
> > I'm not sure, the problem seems to be that a splice-consumer might want
> > to inject the page into a whole different address-space, over-writing
> > page->mapping/->index etc.
>
> OK, I see. In LTTng, I dropped the mmap() support when I integrated splice(). In
> both case, I can share the pages between the "output" (mmap or splice) and the
> ring buffer because my ring buffer does not care about
> page->mapping/->index/etc, so I never have to swap them.
>
> However, doing mmap() and splice() at the same time on the same pages seems
> problematic for the reason you point out here (and not very useful anyway).
> But I think restrictions could be done more transparently than what you propose,
> e.g.:
>
> 1) create buffer -> return fd
> (perform pfn alignment for the architecture worse-case, e.g. support mmap()
> on sparc)
>
> 2a) mmap(fd)
> return -EBUSY if any of the pages has non-NULL mapping.
> 3a) munmap(fd)
>
> 2b) splice(fd)
> return -EBUSY if any of the pages has non-NULL mapping.
>
> 2c) read(fd)
> Could probably be done concurrently with splice() or mmap().
>
> This way we would ensure that only mmap or splice is used on the buffer at a
> given time without crippling the API.
>
> Thoughts ?

Right, so the problem is that we now use mmap() to size the buffer. I
guess we could go adding a size attribute to perf_event_attr, but I
think its makes more sense to separate the actual event and the output
buffer objects.