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SubjectRe: [PATCH] cpu_hotplug: don't affect current task's affinity
On Tue, 14 Jul 2009 16:47:34 +0800
Lai Jiangshan <laijs@cn.fujitsu.com> wrote:

>
> _cpu_down() changes current task's affinity and then
> recovers it at the end.
>
> It brings two defects:
>
> 1) The recovering will failed in some condition.
>
> # grep Cpus_allowed_list /proc/$$/status
> Cpus_allowed_list: 0-3
>
> # taskset -pc 2 $$
> pid 29075's current affinity list: 0-3
> pid 29075's new affinity list: 2
>
> # grep Cpus_allowed_list /proc/$$/status
> Cpus_allowed_list: 2
>
> # echo 0 > /sys/devices/system/cpu/cpu2/online
>
> # grep Cpus_allowed_list /proc/$$/status
> Cpus_allowed_list: 0
>
> In linux, tasks' "Cpus_allowed_list" which are "2" originally
> will become "0-1,3" after the cpu#2 is offlined.
>
> This "Cpus_allowed_list: 0" is suspicionful.
>
> 2) current task is a userspace task, the user may change
> its cpu-affinity at the same time. The user may get unexpected
> result if _cpu_down() changes current task's affinity.
>
> Actually, we don't have to change the affinity.
> We create a kernel thread to do the works.
>

I've rewritten the description as below. Can you check it please?

: _cpu_down() changes the current task's affinity and then recovers it at the
: end.
:
: It has two problems:
:
: 1) The recovery of the current tasks's cpus_allowed will fail under
: some conditions.
:
: # grep Cpus_allowed_list /proc/$$/status
: Cpus_allowed_list: 0-3
:
: # taskset -pc 2 $$
: pid 29075's current affinity list: 0-3
: pid 29075's new affinity list: 2
:
: # grep Cpus_allowed_list /proc/$$/status
: Cpus_allowed_list: 2
:
: # echo 0 > /sys/devices/system/cpu/cpu2/online
:
: # grep Cpus_allowed_list /proc/$$/status
: Cpus_allowed_list: 0
:
: Here, the Cpus_allowed_list was originally "2" and has become
: "0-1,3" after cpu #2 is offlined.
:
: This "Cpus_allowed_list: 0" is incorrect.
:
: 2) If the current task is a userspace task, the user may change its
: cpu-affinity during the CPU hot-unplugging. This change can be
: overwritten when _cpu_down() changes the current task's affinity.
:
:
: Fix all this by not changing the current tasks's affinity. Instead we
: create a kernel thread to do the work.



I don't fully understand 1). You say that the tasks's cpus_allowed has
become "0-1,3". But it hasn't - it has become "0".

Are you saying that the mask is internally 0-1,3 and that the
/proc/$$/status file is incorrectly displaying it?

Or are you saying that the mask _should_ have been 0-1,3 but the kernel
incorrectly set it to "0"?


Also, it says "The recovery of the current tasks's cpus_allowed will
fail under some conditions". What are those conditions?

Thanks.


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