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

    * Dave Chinner <> wrote:

    > On Tue, Aug 03, 2010 at 11:56:11AM -0700, Linus Torvalds wrote:
    > > On Tue, Aug 3, 2010 at 10:18 AM, Peter Zijlstra <> wrote:
    > > >
    > > > FWIW I really utterly detest the whole concept of sub-buffers.
    > >
    > > I'm not quite sure why. Is it something fundamental, or just an
    > > implementation issue?
    > >
    > > One thing that I think could easily make sense in a _lot_ of buffering
    > > areas is the notion of a "continuation" buffer. We know we have cases
    > > where we want to attach a lot of data to one particular event, but the
    > > buffering itself is inevitably always going to have some limits on
    > > atomicity etc. And quite often, the event that _generates_ the data is
    > > not necessarily going to have all that data in one contiguous region,
    > > and doing a scatter-gather memcpy to get it that way is not good
    > > either.
    > >
    > > At the same time, I do _not_ believe that the kernel ring-buffer code
    > > should handle pointers to sub-buffers etc, or worry about iovec-like
    > > arrays of smaller ranges. So if _that_ is what you mean by "concept of
    > > sub-buffers", then I agree with you.
    > >
    > > But what I do think might make a lot of sense is to allow buffer
    > > fragments, and just teach user space to do de-fragmentation. Where it
    > > would be important that the de-fragmentation really is all in user
    > > space, and not really ever visible to the ring-buffer implementation
    > > itself (and there would not, for example, be any guarantees that the
    > > fragments would be contiguous - there could be other events in the
    > > buffer in between fragments). Maybe we could even say that fragments
    > > might be across different CPU ring-buffers, and user-space needs to
    > > sort it out if it wants to (where "sort it out" literally would mean
    > > having to sort and re-attach them in the right order, since there
    > > wouldn't be any ordering between them).
    > >
    > > From a kernel perspective, the only thing you need for fragment
    > > handling would be to have a buffer entry that just says "I'm fragment
    > > number X of event ID Y". Nothing more. Everything else would be up to
    > > the parser in user space to work out.
    > Heh. For a moment there I thought you were describing the the way XFS writes
    > transactions into it's log. Replace "CPU ring-buffers" with "in-core log
    > buffers", "userspace parsing" with "log recovery" and "event ID" with
    > "transaction ID", and the concept you describe is eerily similar. That
    > includes the fact that transactions are not contiguous in the log, can
    > interleave fragments between concurrent transaction commits and they can
    > span multiple log buffers, too. It works pretty well for scaling concurrent
    > writers....

    That's certainly a good model when you have to stream into a
    persistent-storage transaction log space with multiple writers.

    The difference is that with instrumentation we are generally able to make
    things per task or per cpu so there's no real multi-CPU 'concurrent writers'

    You dont have that luxory/simplicity when logging to storage, of course!



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