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

["Huang, Ying" <ying.huang@intel.com>]

"Huang, Ying" <ying.huang@intel.com> writes:

> Dave Hansen <dave.hansen@intel.com> writes:
>
>> 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.
>
> Yes.  This sounds good.  I will try to test this patch on my side.

I have tested this patch, it works as expected for memory hotplug.  I
have asked 0-Day guys to test the original test case, but 0-Day doesn't
work very well for now, we need to wait for a while for 0-Day test
result.

Best Regards,
Huang, Ying