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

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Date:	Tue, 15 Dec 2009 11:15:27 -0800
From:	Corey Ashford <cjashfor@...ux.vnet.ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, Michael Neuling <mikey@...ling.org>
Subject: Re: [PATCH 1/2] perf_event: Fix incorrect range check on cpu number

Ingo Molnar wrote:
> * Corey Ashford <cjashfor@...ux.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

Oops! missed that.  Thanks!

- Corey


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