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Subject: Re: [PATCH v4] ixgbe: let the xdpdrv work with more than 64 cpus
To:     Jason Xing <kerneljasonxing@gmail.com>,
        Jesse Brandeburg <jesse.brandeburg@intel.com>
Cc:     Eric Dumazet <eric.dumazet@gmail.com>,
        "Nguyen, Anthony L" <anthony.l.nguyen@intel.com>,
        David Miller <davem@davemloft.net>, kuba@kernel.org,
        ast@kernel.org, daniel@iogearbox.net, hawk@kernel.org,
        john.fastabend@gmail.com, andrii@kernel.org, kafai@fb.com,
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        intel-wired-lan@lists.osuosl.org, netdev <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, bpf@vger.kernel.org,
        Jason Xing <xingwanli@kuaishou.com>,
        Shujin Li <lishujin@kuaishou.com>
References: <20210826141623.8151-1-kerneljasonxing@gmail.com>
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From:   Eric Dumazet <eric.dumazet@gmail.com>
Message-ID: <da5da485-9dc7-e731-a8d9-f5ad7c7dffde@gmail.com>
Date:   Thu, 26 Aug 2021 11:19:35 -0700
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On 8/26/21 10:03 AM, Jason Xing wrote:

> 
> Honestly, I'm a little confused right now. @nr_cpu_ids is the fixed
> number which means the total number of cpus the machine has.
> I think, using @nr_cpu_ids is safe one way or the other regardless of
> whether the cpu goes offline or not. What do you think?
> 

More exactly, nr_cpu_ids is the max number cpu id can reach,
even in presence of holes.

I think that most/many num_online_cpus() in drivers/net are simply broken
and should be replaced by nr_cpu_ids.

The assumptions of cpus being nicely numbered from [0 to X-1],
with X==num_online_cpus() is wrong.

Same remark for num_possible_cpus(), see commit
88d4f0db7fa8 ("perf: Fix alloc_callchain_buffers()") for reference.