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

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

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.