Subject: [PATCH v3] ARM: smp: Only expose /sys/.../cpuX/online if hotpluggable

From: [email-masked] (Stephen Boyd)
Date: Mon, 06 Apr 2015 10:47:50 -0700

On 04/06/15 10:19, Tyler Baker wrote:
> On 19 February 2015 at 14:14, Simon Horman <[email-masked]> wrote:
>> On Wed, Feb 18, 2015 at 03:27:57PM -0800, Stephen Boyd wrote:
>>> On 02/18/15 14:27, Simon Horman wrote:
>>>> On Fri, Feb 13, 2015 at 04:42:54PM -0800, Stephen Boyd wrote:
>>>>> Writes to /sys/.../cpuX/online fail if we determine the platform
>>>>> doesn't support hotplug for that CPU. Furthermore, if the cpu_die
>>>>> op isn't specified the system hangs when we try to offline a CPU
>>>>> and it comes right back online unexpectedly. Let's figure this
>>>>> stuff out before we make the sysfs nodes so that the online file
>>>>> doesn't even exist if it isn't (at least sometimes) possible to
>>>>> hotplug the CPU.
>>>>>
>>>>> Add a new cpu_can_disable op and repoint all cpu_disable
>>>>> implementations at it because all current users use the op to
>>>>> indicate if a CPU can be hotplugged or not in a static fashion.
>>>>> With PSCI we may need to introduce a cpu_disable op so that the
>>>>> secure OS can be migrated off the CPU we're trying to hotplug.
>>>>> In this case, the cpu_can_disable op will indicate that all CPUs
>>>>> are hotpluggable by returning 1, but the cpu_disable op will make
>>>>> a PSCI migration call and occasionally fail, denying the hotplug
>>>>> of a CPU. This shouldn't be any worse than x86 where we may
>>>>> indicate that all CPUs are hotpluggable but occasionally we can't
>>>>> offline a CPU due to check_irq_vectors_for_cpu_disable() failing
>>>>> to find a CPU to move vectors to.
>>>>>
>>>>> Cc: Mark Rutland <[email-masked]>
>>>>> Cc: Nicolas Pitre <[email-masked]>
>>>>> Cc: Dave Martin <[email-masked]>
>>>>> Cc: Simon Horman <[email-masked]>
>>>>> Cc: Magnus Damm <[email-masked]>
>>>>> Cc: <[email-masked]>
>>>>> Signed-off-by: Stephen Boyd <[email-masked]>
>>>>> ---
>>>>>
>>>>> Changes since v2:
>>>>>  * Left cpu_disable op in place
>>>>>  * Split out shmobile function deletion
>>>>>
>>>>>  arch/arm/common/mcpm_platsmp.c       | 12 ++++--------
>>>>>  arch/arm/include/asm/smp.h           | 10 ++++++++++
>>>>>  arch/arm/kernel/setup.c              |  2 +-
>>>>>  arch/arm/kernel/smp.c                | 15 ++++++++++++++-
>>>>>  arch/arm/mach-shmobile/common.h      |  2 +-
>>>>>  arch/arm/mach-shmobile/platsmp.c     |  4 ++--
>>>>>  arch/arm/mach-shmobile/smp-r8a7790.c |  2 +-
>>>>>  arch/arm/mach-shmobile/smp-r8a7791.c |  2 +-
>>>>>  arch/arm/mach-shmobile/smp-sh73a0.c  |  2 +-
>>>>>  9 files changed, 35 insertions(+), 16 deletions(-)
>>>> I think it would make sense to separate the ARM-core changes
>>>> from the mach-shmobile integration changes.
>>> Are you saying two (three?) patches to add the op, and then move over
>>> each struct smp_operations? It's all going through rmk's tree so I'll
>>> leave that up to him.
>> I'm also happy to let RMK to decide what he thinks is best.
> Apologies for bringing up an older thread, but was this change ever
> picked up? I recently hit this issue when I was running the kselftest
> suite (specifically the cpu-hotplug test case) on the ARM boards from
> kernelci.org. I don't see it in -next so I've tested the core changes
> and confirmed it the fixes the cpu-hotplug behavior on platforms that
> do not support it.

No it hasn't gone anywhere. It would be good if Mark Rutland could ack
the patch, which I hope would give enough confidence to Russell that the
patch is good. Your tested-by would also be welcome. I'll go make the
bool change that Geert suggested and resend.

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