Re: [PATCH 1/2] mm/migrate: optimize hotplug-time demotion order updates

[David Hildenbrand]

On 20.09.21 23:37, Dave Hansen wrote:
> On 9/17/21 5:55 PM, Huang, Ying wrote:
> > > @@ -3147,6 +3177,16 @@ static void __set_migration_target_nodes
> > >   	int node;
> > >   
> > >   	/*
> > > +	 * The "migration path" array is heavily optimized
> > > +	 * for reads.  This is the write side which incurs a
> > > +	 * very heavy synchronize_rcu().  Avoid this overhead
> > > +	 * when nothing of consequence has changed since the
> > > +	 * last write.
> > > +	 */
> > > +	if (!node_demotion_topo_changed())
> > > +		return;
> > > +
> > > +	/*
> > >   	 * Avoid any oddities like cycles that could occur
> > >   	 * from changes in the topology.  This will leave
> > >   	 * a momentary gap when migration is disabled.
> > Now synchronize_rcu() is called in disable_all_migrate_targets(), which
> > is called for MEM_GOING_OFFLINE.  Can we remove the synchronize_rcu()
> > from disable_all_migrate_targets() and call it in
> > __set_migration_target_nodes() before we update the node_demotion[]?
>
> I see what you are saying.  This patch just targeted
> __set_migration_target_nodes() which is called in for
> MEM_ONLINE/OFFLINE.  But, it missed MEM_GOING_OFFLINE's call to
> disable_all_migrate_targets().
>
> I think I found something better than what I had in this patch, or the
> tweak you suggested: The 'memory_notify->status_change_nid' field is
> passed to all memory hotplug notifiers and tells us whether the node is
> going online/offline.  Instead of trying to track the changes, I think
> we can simply rely on it to tell us when a node is going online/offline.
>
> This removes the need for the demotion code to track *any* state.  I've
> attached a totally untested patch to do this.

Sounds sane to me (although I really detest that status_change_nid... 
interface).

I was just about to ask "but how does this interact with !CONFIG_NUMA" 
... until I realized that having a single node go completely offline is 
rather unrealistic ;)

--
Thanks,

David / dhildenb