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

[Tyler Baker]

On 19 February 2015 at 14:14, Simon Horman <horms@xxxxxxxxxxxx> 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 <mark.rutland@xxxxxxx>
>> >> Cc: Nicolas Pitre <nico@xxxxxxxxxx>
>> >> Cc: Dave Martin <Dave.Martin@xxxxxxx>
>> >> Cc: Simon Horman <horms@xxxxxxxxxxxx>
>> >> Cc: Magnus Damm <magnus.damm@xxxxxxxxx>
>> >> Cc: <linux-sh@xxxxxxxxxxxxxxx>
>> >> Signed-off-by: Stephen Boyd <sboyd@xxxxxxxxxxxxxx>
>> >> ---
>> >>
>> >> 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.

>
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- Tyler
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