linux-kernel - Re: [PATCH] cpu_hotplug: don't affect current task's affinity

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Date:	Wed, 22 Jul 2009 15:02:46 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	rusty@...tcorp.com.au, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] cpu_hotplug: don't affect current task's affinity

On Tue, 14 Jul 2009 16:47:34 +0800
Lai Jiangshan <laijs@...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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