linux-kernel - Re: maxcpus confusion

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Message-Id: <20171204210228.GA3286@osiris>
Date:   Mon, 4 Dec 2017 22:02:28 +0100
From:   Heiko Carstens <heiko.carstens@...ibm.com>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>
Subject: Re: maxcpus confusion

On Fri, Dec 01, 2017 at 12:58:47PM -0800, Randy Dunlap wrote:
> Hi,
> 
> I used "maxcpus=1" on a recent x86 boot (4.15-rc1) and got 4 CPUs (all of
> them AFAICT).  When I use "nr_cpus=1", I do get a hard limit of one CPU.
> 
> 
> A few boot log excerpts:
> 
> [    0.000000] smpboot: Allowing 8 CPUs, 4 hotplug CPUs
> ...
> [    0.030007] smp: Bringing up secondary CPUs ...
> [    0.030109] smp: Brought up 1 node, 1 CPU
> [    0.030181] smpboot: Max logical packages: 4
> [    0.030255] smpboot: Total of 1 processors activated (4789.06 BogoMIPS)
> 
> 
> Is systemd (or something else) later onlining the other CPUs?
> 
> /sys/devices/system/cpu/online is 0-3
> /sys/devices/system/cpu/offline is 4-7

Check /lib/udev/rules.d/ for a rule that auto-onlines CPUs.
This behavior is distribution specific.