Hello,
Sorry about the delay.
On Tue, Feb 07, 2017 at 04:34:59PM +0100, Michal Hocko wrote:
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index c3358d4f7932..b6411816787a 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2343,7 +2343,16 @@ void drain_local_pages(struct zone *zone)
>
> static void drain_local_pages_wq(struct work_struct *work)
> {
> + /*
> + * drain_all_pages doesn't use proper cpu hotplug protection so
> + * we can race with cpu offline when the WQ can move this from
> + * a cpu pinned worker to an unbound one. We can operate on a different
> + * cpu which is allright but we also have to make sure to not move to
> + * a different one.
> + */
> + preempt_disable();
> drain_local_pages(NULL);
> + preempt_enable();
> }
>
> /*
> @@ -2379,12 +2388,6 @@ void drain_all_pages(struct zone *zone)
> }
>
> /*
> - * As this can be called from reclaim context, do not reenter reclaim.
> - * An allocation failure can be handled, it's simply slower
> - */
> - get_online_cpus();
> -
> - /*
> * We don't care about racing with CPU hotplug event
> * as offline notification will cause the notified
> * cpu to drain that CPU pcps and on_each_cpu_mask
> @@ -2423,7 +2426,6 @@ void drain_all_pages(struct zone *zone)
> for_each_cpu(cpu, &cpus_with_pcps)
> flush_work(per_cpu_ptr(&pcpu_drain, cpu));
>
> - put_online_cpus();
> mutex_unlock(&pcpu_drain_mutex);
I think this would work; however, a more canonical way would be
something along the line of...
drain_all_pages()
{
...
spin_lock();
for_each_possible_cpu() {
if (this cpu should get drained) {
queue_work_on(this cpu's work);
}
}
spin_unlock();
...
}
offline_hook()
{
spin_lock();
this cpu should get drained = false;
spin_unlock();
queue_work_on(this cpu's work);
flush_work(this cpu's work);
}
I think what workqueue should do is automatically flush in-flight CPU
work items on CPU offline and erroring out on queue_work_on() on
offline CPUs. And we now actually can do that because we have lifted
the guarantee that queue_work() is local CPU affine some releases ago.
I'll look into it soonish.
For the time being, either approach should be fine. The more
canonical one might be a bit less surprising but the
preempt_disable/enable() change is short and sweet and completely fine
for the case at hand.
Please feel free to add
Acked-by: Tejun Heo <tj@xxxxxxxxxx>
Thanks.
--
tejun
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