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From: David Miller <davem@davemloft.net>
Subject: Re: [RFC PATCH] openvswitch: use percpu flow stats
Date: Fri, 19 Aug 2016 22:25:01 -0700 (PDT)
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From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Fri, 19 Aug 2016 21:07:52 -0700

> Here is an example of a system I had in the past.
> 
> 8 cpus  (num_possible_cpus() returns 8)
> 
> Cpus were numbered : 0, 1, 2, 3, 8, 9, 10, 11  : (nr_cpu_ids = 12)
> 
> I am pretty sure they are still alive.
> 
> Since you want an array indexed by cpu numbers, you need to size it by
> nr_cpu_ids, otherwise array[11] will crash / access garbage.
> 
> This is why you find many nr_cpu_ids in the linux tree, not
> num_possible_cpus() to size arrays.

My bad, I misunderstood what nr_cpu_ids means.  You are absolutely right.