On Tue 07-02-17 12:03:19, Tejun Heo wrote:
> 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 see
> 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.
Thanks for double checking!
> Please feel free to add
>
> Acked-by: Tejun Heo <tj@xxxxxxxxxx>
Thanks!
--
Michal Hocko
SUSE Labs
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