Re: Perf and ftrace [was Re: PyTimechart]

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

* 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 ?

Mathieu


-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com