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

Ingo Molnar
2009-03-25T09:04:06Z

* 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?

Ingo