Re: [PATCH 1/2] perf_event: Fix incorrect range check on cpu number

[Corey Ashford]

Paul Mackerras wrote:
> On Tue, Dec 15, 2009 at 11:31:32AM +0100, Peter Zijlstra wrote:
>
> > On Tue, 2009-12-15 at 19:40 +1100, Paul Mackerras wrote:
> > > It is quite legitimate for CPUs to be numbered sparsely, meaning that
> > > it possible for an online CPU to have a number which is greater than
> > > the total count of possible CPUs.
> > >
> > > Currently find_get_context() has a sanity check on the cpu number
> > > where it checks it against num_possible_cpus().  This test can fail
> > > for a legitimate cpu number if the cpu_possible_mask is sparsely
> > > populated.
> > >
> > > This fixes the problem by checking the CPU number against
> > > nr_cpumask_bits instead, since that is the appropriate check to ensure
> > > that the cpu number is same to pass to cpu_isset() subsequently.
> > Cute, do you actually have hardware that does this?
>
> Yeah, Mikey ran across this on a POWER7 box here.

Does the perf tool need to be fixed too?  The "perf stat" tool, at least, has a 
"-a" switch that tells the tool to count the event on all cpus, and it does this 
by iterating over the number of cpus, 0..n, assuming they are all contiguous.

- Corey

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