netdev - Re: [PATCH v4] ixgbe: let the xdpdrv work with more than 64 cpus

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Message-ID: <da5da485-9dc7-e731-a8d9-f5ad7c7dffde@gmail.com>
Date:   Thu, 26 Aug 2021 11:19:35 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Jason Xing <kerneljasonxing@...il.com>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        "Nguyen, Anthony L" <anthony.l.nguyen@...el.com>,
        David Miller <davem@...emloft.net>, kuba@...nel.org,
        ast@...nel.org, daniel@...earbox.net, hawk@...nel.org,
        john.fastabend@...il.com, andrii@...nel.org, kafai@...com,
        songliubraving@...com, yhs@...com, kpsingh@...nel.org,
        intel-wired-lan@...ts.osuosl.org, netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, bpf@...r.kernel.org,
        Jason Xing <xingwanli@...ishou.com>,
        Shujin Li <lishujin@...ishou.com>
Subject: Re: [PATCH v4] ixgbe: let the xdpdrv work with more than 64 cpus



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.