Date: Tue, 15 Dec 2009 20:08:58 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Paul Mackerras <paulus@samba.org>, Peter Zijlstra <peterz@infradead.org>,
       linux-kernel@vger.kernel.org, Michael Neuling <mikey@neuling.org>
Subject: Re: [PATCH 1/2] perf_event: Fix incorrect range check on cpu number
Message-ID: <20091215190858.GA15443@elte.hu>
References: <20091215084032.GA18661@brick.ozlabs.ibm.com>
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* Corey Ashford <cjashfor@linux.vnet.ibm.com> wrote:

> 
> 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.

Yes, see patch 2/2 of this series.

	Ingo
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