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

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Date:   Tue, 21 Sep 2021 09:23:18 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, oliver.sang@...el.com,
        mhocko@...e.com, weixugc@...gle.com, osalvador@...e.de,
        rientjes@...gle.com, dan.j.williams@...el.com, gthelen@...gle.com,
        yang.shi@...ux.alibaba.com, akpm@...ux-foundation.org
Subject: Re: [PATCH 1/2] mm/migrate: optimize hotplug-time demotion order
 updates

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