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    SubjectRe: [RFC PATCH v4] Unified trace buffer
    Steven Rostedt wrote:
    > On Thu, 25 Sep 2008, Masami Hiramatsu wrote:
    >
    >> Hi Steven,
    >>
    >> Steven Rostedt wrote:
    >>> This version has been cleaned up a bit. I've been running it as
    >>> a back end to ftrace, and it has been handling pretty well.
    >> Thank you for your great work.
    >> It seems good to me(especially, encapsulating events :)).
    >
    > Thanks!
    >
    >> I have one request of enhancement.
    >>
    >>> +static struct ring_buffer_per_cpu *
    >>> +ring_buffer_allocate_cpu_buffer(struct ring_buffer *buffer, int cpu)
    >>> +{
    >> [...]
    >>> + cpu_buffer->pages = kzalloc_node(ALIGN(sizeof(void *) * pages,
    >>> + cache_line_size()), GFP_KERNEL,
    >>> + cpu_to_node(cpu));
    >> Here, you are using a slab object for page managing array,
    >> the largest object size is 128KB(x86-64), so it can contain
    >> 16K pages = 64MB.
    >>
    >> As I had improved relayfs, in some rare case(on 64bit arch),
    >> we'd like to use larger buffer than 64MB.
    >>
    >> http://sourceware.org/ml/systemtap/2008-q2/msg00103.html
    >>
    >> So, I think similar hack can be applicable.
    >>
    >> Would it be acceptable for the next version?
    >
    > I would like to avoid using vmalloc as much as possible, but I do see the
    > limitation here. Here's my compromise.
    >
    > Instead of using vmalloc if the page array is greater than one page,
    > how about using vmalloc if the page array is greater than
    > KMALLOC_MAX_SIZE?
    >
    > This would let us keep the vmap area free unless we have no choice.

    Hmm, that's a good idea.
    In most cases, per-cpu buffer may be less than 64MB,
    so I think it is reasonable.

    Thank you,

    >
    > -- Steve
    >

    --
    Masami Hiramatsu

    Software Engineer
    Hitachi Computer Products (America) Inc.
    Software Solutions Division

    e-mail: mhiramat@redhat.com



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