Message-Id: <14561845-1C68-42FE-8E05-309FDC3E9D28@linux.dev>
Date:   Thu, 1 Sep 2022 15:57:22 +0800
From:   Muchun Song <muchun.song@...ux.dev>
To:     David Hildenbrand <david@...hat.com>
Cc:     Muchun Song <songmuchun@...edance.com>,
        Greg KH <gregkh@...uxfoundation.org>, rafael@...nel.org,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>, andi@...stfloor.org,
        linux-kernel@...r.kernel.org, Linux MM <linux-mm@...ck.org>,
        David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH RESEND] mm: hugetlb: simplify per-node sysfs creation and
 removal



> On Sep 1, 2022, at 14:57, David Hildenbrand <david@...hat.com> wrote:
> 
> On 01.09.22 08:35, Muchun Song wrote:
>> 
>> 
>>> On Aug 24, 2022, at 11:23, Muchun Song <muchun.song@...ux.dev> wrote:
>>> 
>>> 
>>> 
>>>> On Aug 23, 2022, at 18:21, David Hildenbrand <david@...hat.com> wrote:
>>>> 
>>>> On 19.08.22 10:00, Muchun Song wrote:
>>>>> The following commit offload per-node sysfs creation and removal to a kworker and
>>>>> did not say why it is needed.  And it also said "I don't know that this is
>>>>> absolutely required".  It seems like the author was not sure as well.  Since it
>>>>> only complicates the code, this patch will revert the changes to simplify the code.
>>>>> 
>>>>> 39da08cb074c ("hugetlb: offload per node attribute registrations")
>>>>> 
>>>>> We could use memory hotplug notifier to do per-node sysfs creation and removal
>>>>> instead of inserting those operations to node registration and unregistration.
>>>>> Then, it can reduce the code coupling between node.c and hugetlb.c.  Also, it can
>>>>> simplify the code.
>>>>> 
>>>>> Signed-off-by: Muchun Song <songmuchun@...edance.com>
>>>> 
>>>> 
>>>> [...]
>>>> 
>>>>> @@ -683,7 +626,6 @@ static int register_node(struct node *node, int num)
>>>>> void unregister_node(struct node *node)
>>>>> {
>>>>> 	compaction_unregister_node(node);
>>>>> -	hugetlb_unregister_node(node);		/* no-op, if memoryless node */
>>>>> 	node_remove_accesses(node);
>>>>> 	node_remove_caches(node);
>>>>> 	device_unregister(&node->dev);
>>>>> @@ -905,74 +847,8 @@ void register_memory_blocks_under_node(int nid, unsigned long start_pfn,
>>>>> 			   (void *)&nid, func);
>>>>> 	return;
>>>>> }
>>>> 
>>>> [...]
>>>> 
>>>>> 	/*
>>>>> 	 * Create all node devices, which will properly link the node
>>>>> 	 * to applicable memory block devices and already created cpu devices.
>>>>> diff --git a/include/linux/node.h b/include/linux/node.h
>>>>> index 40d641a8bfb0..ea817b507f54 100644
>>>>> --- a/include/linux/node.h
>>>>> +++ b/include/linux/node.h
>>>>> @@ -2,15 +2,15 @@
>>>>> /*
>>>>> * include/linux/node.h - generic node definition
>>>>> *
>>>>> - * This is mainly for topological representation. We define the 
>>>>> - * basic 'struct node' here, which can be embedded in per-arch 
>>>>> + * This is mainly for topological representation. We define the
>>>>> + * basic 'struct node' here, which can be embedded in per-arch
>>>>> * definitions of processors.
>>>>> *
>>>>> * Basic handling of the devices is done in drivers/base/node.c
>>>>> - * and system devices are handled in drivers/base/sys.c. 
>>>>> + * and system devices are handled in drivers/base/sys.c.
>>>>> *
>>>>> * Nodes are exported via driverfs in the class/node/devices/
>>>>> - * directory. 
>>>>> + * directory.
>>>> 
>>>> Unrelated changes.
>>> 
>>> Yep, a minor cleanup BTW.
>>> 
>>>> 
>>>>> */
>>>>> #ifndef _LINUX_NODE_H_
>>>>> #define _LINUX_NODE_H_
>>>>> @@ -18,7 +18,6 @@
>>>>> #include <linux/device.h>
>>>>> #include <linux/cpumask.h>
>>>>> #include <linux/list.h>
>>>>> -#include <linux/workqueue.h>
>>>>> 
>>>>> /**
>>>>> * struct node_hmem_attrs - heterogeneous memory performance attributes
>>>>> @@ -84,10 +83,6 @@ static inline void node_set_perf_attrs(unsigned int nid,
>>>>> struct node {
>>>>> 	struct device	dev;
>>>>> 	struct list_head access_list;
>>>>> -
>>>>> -#if defined(CONFIG_MEMORY_HOTPLUG) && defined(CONFIG_HUGETLBFS)
>>>>> -	struct work_struct	node_work;
>>>>> -#endif
>>>>> #ifdef CONFIG_HMEM_REPORTING
>>>>> 	struct list_head cache_attrs;
>>>>> 	struct device *cache_dev;
>>>>> @@ -96,7 +91,6 @@ struct node {
>>>>> 
>>>>> struct memory_block;
>>>>> extern struct node *node_devices[];
>>>>> -typedef  void (*node_registration_func_t)(struct node *);
>>>>> 
>>>>> #if defined(CONFIG_MEMORY_HOTPLUG) && defined(CONFIG_NUMA)
>>>>> void register_memory_blocks_under_node(int nid, unsigned long start_pfn,
>>>>> @@ -144,11 +138,6 @@ extern void unregister_memory_block_under_nodes(struct memory_block *mem_blk);
>>>>> extern int register_memory_node_under_compute_node(unsigned int mem_nid,
>>>>> 						   unsigned int cpu_nid,
>>>>> 						   unsigned access);
>>>>> -
>>>>> -#ifdef CONFIG_HUGETLBFS
>>>>> -extern void register_hugetlbfs_with_node(node_registration_func_t doregister,
>>>>> -					 node_registration_func_t unregister);
>>>>> -#endif
>>>>> #else
>>>>> static inline void node_dev_init(void)
>>>>> {
>>>>> @@ -176,11 +165,6 @@ static inline int unregister_cpu_under_node(unsigned int cpu, unsigned int nid)
>>>>> static inline void unregister_memory_block_under_nodes(struct memory_block *mem_blk)
>>>>> {
>>>>> }
>>>>> -
>>>>> -static inline void register_hugetlbfs_with_node(node_registration_func_t reg,
>>>>> -						node_registration_func_t unreg)
>>>>> -{
>>>>> -}
>>>>> #endif
>>>>> 
>>>>> #define to_node(device) container_of(device, struct node, dev)
>>>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>>>>> index 536a52c29035..9a72499486c1 100644
>>>>> --- a/mm/hugetlb.c
>>>>> +++ b/mm/hugetlb.c
>>>>> @@ -33,6 +33,7 @@
>>>>> #include <linux/migrate.h>
>>>>> #include <linux/nospec.h>
>>>>> #include <linux/delayacct.h>
>>>>> +#include <linux/memory.h>
>>>>> 
>>>>> #include <asm/page.h>
>>>>> #include <asm/pgalloc.h>
>>>>> @@ -3967,19 +3968,19 @@ static void hugetlb_unregister_node(struct node *node)
>>>>> * Register hstate attributes for a single node device.
>>>>> * No-op if attributes already registered.
>>>>> */
>>>>> -static void hugetlb_register_node(struct node *node)
>>>>> +static int hugetlb_register_node(struct node *node)
>>>>> {
>>>>> 	struct hstate *h;
>>>>> 	struct node_hstate *nhs = &node_hstates[node->dev.id];
>>>>> 	int err;
>>>>> 
>>>>> 	if (nhs->hugepages_kobj)
>>>>> -		return;		/* already allocated */
>>>>> +		return 0;		/* already allocated */
>>>>> 
>>>>> 	nhs->hugepages_kobj = kobject_create_and_add("hugepages",
>>>>> 							&node->dev.kobj);
>>>>> 	if (!nhs->hugepages_kobj)
>>>>> -		return;
>>>>> +		return -ENOMEM;
>>>>> 
>>>>> 	for_each_hstate(h) {
>>>>> 		err = hugetlb_sysfs_add_hstate(h, nhs->hugepages_kobj,
>>>>> @@ -3989,9 +3990,28 @@ static void hugetlb_register_node(struct node *node)
>>>>> 			pr_err("HugeTLB: Unable to add hstate %s for node %d\n",
>>>>> 				h->name, node->dev.id);
>>>>> 			hugetlb_unregister_node(node);
>>>>> -			break;
>>>>> +			return -ENOMEM;
>>>>> 		}
>>>>> 	}
>>>>> +	return 0;
>>>>> +}
>>>>> +
>>>>> +static int __meminit hugetlb_memory_callback(struct notifier_block *self,
>>>>> +					     unsigned long action, void *arg)
>>>>> +{
>>>>> +	int ret = 0;
>>>>> +	struct memory_notify *mnb = arg;
>>>>> +	int nid = mnb->status_change_nid;
>>>>> +
>>>>> +	if (nid == NUMA_NO_NODE)
>>>>> +		return NOTIFY_DONE;
>>>>> +
>>>>> +	if (action == MEM_GOING_ONLINE)
>>>>> +		ret = hugetlb_register_node(node_devices[nid]);
>>>>> +	else if (action == MEM_CANCEL_ONLINE || action == MEM_OFFLINE)
>>>>> +		hugetlb_unregister_node(node_devices[nid]);
>>>>> +
>>>>> +	return notifier_from_errno(ret);
>>>>> }
>>>>> 
>>>>> /*
>>>>> @@ -4003,18 +4023,11 @@ static void __init hugetlb_register_all_nodes(void)
>>>>> {
>>>>> 	int nid;
>>>>> 
>>>>> -	for_each_node_state(nid, N_MEMORY) {
>>>>> -		struct node *node = node_devices[nid];
>>>>> -		if (node->dev.id == nid)
>>>>> -			hugetlb_register_node(node);
>>>>> -	}
>>>>> -
>>>>> -	/*
>>>>> -	 * Let the node device driver know we're here so it can
>>>>> -	 * [un]register hstate attributes on node hotplug.
>>>>> -	 */
>>>>> -	register_hugetlbfs_with_node(hugetlb_register_node,
>>>>> -				     hugetlb_unregister_node);
>>>>> +	get_online_mems();
>>>>> +	hotplug_memory_notifier(hugetlb_memory_callback, 0);
>>>>> +	for_each_node_state(nid, N_MEMORY)
>>>>> +		hugetlb_register_node(node_devices[nid]);
>>>>> +	put_online_mems();
>>>>> }
>>>>> #else	/* !CONFIG_NUMA */
>>>> 
>>>> Do we really *need* the memory hotplug notifier and the added complexity
>>>> due for handling memory-less nodes?
>> 
>> Hi David,
>> 
> 
> Hi,
> 
> thanks for playing with the idea.
> 
>> After some tries, I think it may not reduce the complexity. node_dev_init()
>> is called at early stage before hugetlb_register_all_nodes(). So we need to
>> add a mechanism to detect if the hugetlb subsystem finishes initialization
>> in node_dev_init() so that it can determine to help hugetlb create /sysfs
>> files, the mechanism is similar with the changes in drivers/base/node.c of
>> commit 9a30523066cd ("hugetlb: add per node hstate attributes”). This approach
> 
> If I'm not wrong, all you need is a single call from node_dev_init()
> into hugetlb code.
> 
> There, you create the hugetlb sysfs if hugetlb was already initialized,
> otherwise it's a NOP as you initialize when hugetlb gets initialized.

Thanks for your reminder, I know how to handle it now. I’ll send a new
patch later.

Thanks,
Muchun

> 
> When initializing hugetlb, you go over all added nodes and create
> hugetlb sysfs.
> 
> Testing/remembering if hugetlb was initialized should be easy, no?
> 
> What's the complicated part I am missing?
> 
>> may add more code than the memory-notify-based approach like this patch
>> implemented. And it also add the code coupling between node.c and hugetlb.c.
>> So I tend to use memory hotplug notifier. What’s your opinion?
> 
> We have hugetlb special casing all over the place, it's an integral MM
> part -- not some random driver where we'd really want decoupling.
> 
> So I don't see why the decouling would be beneficial here and how using
> the memory notifier is any better then a simple callback.
> 
> 
> But again, I did not look into the details of the necessary implementation.
> 
> Thanks!
> 
> 
> -- 
> Thanks,
> 
> David / dhildenb
> 

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