linux-kernel - Re: [PATCH V2] irqchip/loongson-eiointc: Use early_cpu_to_node() instead of cpu_to

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Date: Sat, 22 Jun 2024 12:17:51 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Huacai Chen <chenhuacai@...il.com>
Cc: Huacai Chen <chenhuacai@...ngson.cn>, loongarch@...ts.linux.dev,
 linux-kernel@...r.kernel.org, Xuefeng Li <lixuefeng@...ngson.cn>, Jiaxun
 Yang <jiaxun.yang@...goat.com>
Subject: Re: [PATCH V2] irqchip/loongson-eiointc: Use early_cpu_to_node()
 instead of cpu_to_node()

Huacai!

On Sat, Jun 22 2024 at 10:49, Huacai Chen wrote:
> On Sat, Jun 22, 2024 at 4:42 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>>
>> On Thu, Jun 20 2024 at 10:06, Huacai Chen wrote:
>> > When we use "nr_cpus=n" to hard limit the CPU number, cpu_to_node() is
>> > not usable because it can only applied on "possible" CPUs. On the other
>> > hand, early_cpu_to_node() can be always used instead.
> cpu_to_node() depends on per-cpu area, and per-cpu area is only usable
> for "possible" CPUs.

When nr_cpus=n is on the command line then what needs to access
something for CPUs which are not possible to come ever online?

That does not make sense because it's exactly the same situation when
you compile a kernel with NR_CPU=8 and boot it on a system with 16
CPUs. Then early_cpu_to_node() does not give you anything either.

So what's the technical problem you are trying to solve?

Thanks,

        tglx