lkml.org 
[lkml]   [2009]   [Mar]   [25]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    SubjectRe: [PATCH] perf_counter: allow and require one-page mmap on counting counters
    From
    Date
    On Wed, 2009-03-25 at 10:03 +0100, Ingo Molnar wrote:
    > * Paul Mackerras <paulus@samba.org> wrote:
    >
    > > Ingo Molnar writes:
    > >
    > > > * Paul Mackerras <paulus@samba.org> wrote:
    > > >
    > > > > +++ b/kernel/perf_counter.c
    > > > > @@ -1362,8 +1362,13 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
    > > > > vma_size = vma->vm_end - vma->vm_start;
    > > > > nr_pages = (vma_size / PAGE_SIZE) - 1;
    > >
    > > To answer your question below... this: ^^^
    > >
    > > > >
    > > > > - if (nr_pages == 0 || !is_power_of_2(nr_pages))
    > > > > - return -EINVAL;
    > > > > + if (counter->hw_event.record_type == PERF_RECORD_SIMPLE) {
    > > > > + if (nr_pages)
    > > > > + return -EINVAL;
    > > > > + } else {
    > > > > + if (nr_pages == 0 || !is_power_of_2(nr_pages))
    > > > > + return -EINVAL;
    > > > > + }
    > > >
    > > > Hm, is_power_of_2() is buggy then as 1 page is a power of two as
    > > > well: 1 == 2^0.
    > > >
    > > > Hm, it seems fine:
    > > >
    > > > static inline __attribute__((const))
    > > > bool is_power_of_2(unsigned long n)
    > > > {
    > > > return (n != 0 && ((n & (n - 1)) == 0));
    > > > }
    > > >
    > > > that should return true for an input of 1.
    > > >
    > > > What am i missing?
    > > >
    > > > Ingo
    > >
    > > We have one page as a header that contains the info for reading
    > > the counter value in userspace plus the head pointer, followed by
    > > (for a sampling counter) 2^N pages of ring buffer.
    >
    > ah - ok. Morning confusion. (any email from me that comes at single
    > digit hour local time should be considered fundamentally suspect ;-)
    >
    > Wouldnt it still be better to keep the symmetry between counting and
    > sampling counters? In theory we could transit between these stags
    > and 'switch off' a sampling counter or 'switch on' a counting
    > counter - via an ioctl or so. Shouldnt counting counters be sampling
    > counters that were created while disabled temporarily?

    I think I initially intended 0 pages to be ok, even for sampling
    counters. I just messed up that if stmt.

    if (nr_pages != 0 && !is_power_of_2(nr_pages))
    return -EINVAL;

    would I think, do what I intended.





    \
     
     \ /
      Last update: 2009-03-25 10:21    [W:0.024 / U:149.840 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site