linux-kernel - Re: [RFC][PATCH 3/9] mm/migrate: update migration order during on hotplug events

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Date:   Wed, 07 Oct 2020 20:08:10 +0200
From:   osalvador@...e.de
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        yang.shi@...ux.alibaba.com, rientjes@...gle.com,
        ying.huang@...el.com, dan.j.williams@...el.com, david@...hat.com
Subject: Re: [RFC][PATCH 3/9] mm/migrate: update migration order during on
 hotplug events

On 2020-10-07 18:17, Dave Hansen wrote:
> From: Dave Hansen <dave.hansen@...ux.intel.com>
> 
> Reclaim-based migration is attempting to optimize data placement in
> memory based on the system topology.  If the system changes, so must
> the migration ordering.
> 
> The implementation here is pretty simple and entirely unoptimized.  On
> any memory or CPU hotplug events, assume that a node was added or
> removed and recalculate all migration targets.  This ensures that the
> node_demotion[] array is always ready to be used in case the new
> reclaim mode is enabled.
> 
> This recalculation is far from optimal, most glaringly that it does
> not even attempt to figure out if nodes are actually coming or going.
> But, given the expected paucity of hotplug events, this should be
> fine.

Hi Dave,

I am still going through all the details, but just wanted to comment 
early on this one.
Could not you hook into __try_online_node/try_offline_node?

In there we check whether a node should be brought up or removed due to 
lack of cpus and memory.
That is being checked during hot-remove operations.

We also have node_states_check_changes_{offline,online} and their pair 
node_states_{set,clear}_node, that checks during online/offline stages 
which states should be removed from the node, but that is only wrt. 
memory (I guess we would only be interested in N_MEMORY).

Thanks