Messages in this thread | | | Date | Wed, 25 Mar 2009 10:03:36 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH] perf_counter: allow and require one-page mmap on counting counters |
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* 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
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