On Tue, Feb 07, 2017 at 01:37:08PM +0100, Michal Hocko wrote:

quoted

You cannot put sleepable lock inside the preempt disbaled section...
We can make it a spinlock right?

Scratch that! For some reason I thought that cpu notifiers are run in an
atomic context. Now that I am checking the code again it turns out I was
wrong. __cpu_notify uses __raw_notifier_call_chain so this is not an
atomic context.

Indeed.

Anyway, shouldn't be it sufficient to disable preemption
on drain_local_pages_wq?

That would be sufficient for a hot-removed CPU moving the drain request
to another CPU and avoiding any scheduling events.

The CPU hotplug callback will not preempt us
and so we cannot work on the same cpus, right?

I don't see a specific guarantee that it cannot be preempted and it
would depend on an the exact cpu hotplug implementation which is subject
to quite a lot of change. Hence, the mutex provides a guantee that the
hot-removed CPU teardown cannot run on the same CPU as a workqueue drain
running on a CPU it was not originally scheduled for.

-- 
Mel Gorman
SUSE Labs

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