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SubjectRe: [PATCH] perf_counter: allow and require one-page mmap on counting counters
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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.





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