linux-kernel - possible_cpus broken in linux-next

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Date:	Wed, 29 May 2013 10:10:22 -0700
From:	Dave Hansen <dave.hansen@...el.com>
To:	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Toshi Kani <toshi.kani@...com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: possible_cpus broken in linux-next

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.wysocki@...el.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.wysocki@...el.com>
>     Acked-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>     Reviewed-by: Toshi Kani <toshi.kani@...com>
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