If I boot with: maxcpus=2 possible_cpus=4, I get
# grep . /sys/devices/system/cpu/cpu[0-9]*/online'
/sys/devices/system/cpu/cpu1/online:1
/sys/devices/system/cpu/cpu2/online:1
/sys/devices/system/cpu/cpu3/online:1
on bad kernels, and this on working ones:
/sys/devices/system/cpu/cpu1/online:1
/sys/devices/system/cpu/cpu2/online:0
/sys/devices/system/cpu/cpu3/online:0
I also get -EINVAL if I try to re-offline them in this state. 2 and 3
don't show up in /proc/cpuinfo, so sysfs just looks broken here. This
happens in a KVM guest, so it should be dirt-simple for anyone to reproduce.
I bisected it down to:
> commit 0902a9044fa5b7a0456ea4daacec2c2b3189ba8c
> Author: Rafael J. Wysocki <rafael.j.wyso...@intel.com>
> Date: Fri May 3 00:25:49 2013 +0200
>
> Driver core: Use generic offline/online for CPU offline/online
>
> Rework the CPU hotplug code in drivers/base/cpu.c to use the
> generic offline/online support introduced previously instead of
> its own CPU-specific code.
>
> For this purpose, modify cpu_subsys to provide offline and online
> callbacks for CONFIG_HOTPLUG_CPU set and remove the code handling
> the CPU-specific 'online' sysfs attribute.
>
> This modification is not supposed to change the user-observable
> behavior of the kernel (i.e. the 'online' attribute will be present
> in exactly the same place in sysfs and should trigger exactly the
> same actions as before).
>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wyso...@intel.com>
> Acked-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>
> Reviewed-by: Toshi Kani <toshi.k...@hp.com>
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