On Thu, 9 Feb 2017, Michal Hocko wrote:

Christoph, you are completely ignoring the reality and the code. There
is no need for stop_machine nor it is helping anything. As the matter
of fact there is a synchronization with the cpu hotplug needed if you
want to make a per-cpu specific operations. get_online_cpus is the
most straightforward and heavy weight way to do this synchronization
but not the only one. As the patch [1] describes we do not really need
get_online_cpus in drain_all_pages because we can do _better_. But
this is not in any way a generic thing applicable to other code paths.

If you disagree then you are free to post patches but hand waving you
are doing here is just wasting everybody's time. So please cut it here
unless you have specific proposals to improve the current situation.

I am fine with the particular solution here for this particular problem.

My problem is the general way of having to synchronize via
get_online_cpus() because of cpu hotplug operations.

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