linux-kernel - Re: [PATCH v5 3/3] mm/mempolicy: Support memory hotplug in weighted interleave

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Message-ID: <Z-0IlZgYLjogZSZO@localhost.localdomain>
Date: Wed, 2 Apr 2025 11:51:17 +0200
From: Oscar Salvador <osalvador@...e.de>
To: Rakie Kim <rakie.kim@...com>
Cc: gourry@...rry.net, akpm@...ux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, linux-cxl@...r.kernel.org,
	joshua.hahnjy@...il.com, dan.j.williams@...el.com,
	ying.huang@...ux.alibaba.com, david@...hat.com,
	Jonathan.Cameron@...wei.com, kernel_team@...ynix.com,
	honggyu.kim@...com, yunjeong.mun@...com
Subject: Re: [PATCH v5 3/3] mm/mempolicy: Support memory hotplug in weighted
 interleave

On Wed, Apr 02, 2025 at 10:49:04AM +0900, Rakie Kim wrote:
> The weighted interleave policy distributes page allocations across multiple
> NUMA nodes based on their performance weight, thereby improving memory
> bandwidth utilization. The weight values for each node are configured
> through sysfs.
> 
> Previously, sysfs entries for configuring weighted interleave were created
> for all possible nodes (N_POSSIBLE) at initialization, including nodes that
> might not have memory. However, not all nodes in N_POSSIBLE are usable at
> runtime, as some may remain memoryless or offline.
> This led to sysfs entries being created for unusable nodes, causing
> potential misconfiguration issues.
> 
> To address this issue, this patch modifies the sysfs creation logic to:
> 1) Limit sysfs entries to nodes that are online and have memory, avoiding
>    the creation of sysfs entries for nodes that cannot be used.
> 2) Support memory hotplug by dynamically adding and removing sysfs entries
>    based on whether a node transitions into or out of the N_MEMORY state.
> 
> Additionally, the patch ensures that sysfs attributes are properly managed
> when nodes go offline, preventing stale or redundant entries from persisting
> in the system.
> 
> By making these changes, the weighted interleave policy now manages its
> sysfs entries more efficiently, ensuring that only relevant nodes are
> considered for interleaving, and dynamically adapting to memory hotplug
> events.
> 
> Signed-off-by: Rakie Kim <rakie.kim@...com>
> Signed-off-by: Honggyu Kim <honggyu.kim@...com>
> Signed-off-by: Yunjeong Mun <yunjeong.mun@...com>

Sorry to jump at v5, and maybe this was already discussed but:

> +static int wi_node_notifier(struct notifier_block *nb,
> +			       unsigned long action, void *data)
> +{
> +	int err;
> +	struct memory_notify *arg = data;
> +	int nid = arg->status_change_nid;
> +
> +	if (nid < 0)
> +		goto notifier_end;
> +
> +	switch(action) {
> +	case MEM_ONLINE:
> +		err = sysfs_wi_node_add(nid);
> +		if (err) {
> +			pr_err("failed to add sysfs [node%d]\n", nid);
> +			return NOTIFY_BAD;
> +		}
> +		break;
> +	case MEM_OFFLINE:
> +		if (!node_state(nid, N_MEMORY))
> +			sysfs_wi_node_release(nid);

This check is not needed.

If status_change_nid is different than NUMA_NO_NODE, it means that the memory
state of the numa node was effectively changed.
So doing:
 
  case MEM_OFFLINE:
          sysfs_wi_node_release(nid)
 
is enough.

-- 
Oscar Salvador
SUSE Labs