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Subject: Re: [PATCH RESEND] mm: hugetlb: simplify per-node sysfs creation and
 removal
From:   Muchun Song <muchun.song@linux.dev>
In-Reply-To: <685F3238-D727-4C1F-9BA4-36651ABCD91A@linux.dev>
Date:   Thu, 1 Sep 2022 14:35:42 +0800
Cc:     Muchun Song <songmuchun@bytedance.com>,
        Greg KH <gregkh@linuxfoundation.org>, rafael@kernel.org,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oscar Salvador <osalvador@suse.de>, andi@firstfloor.org,
        linux-kernel@vger.kernel.org, Linux MM <linux-mm@kvack.org>,
        David Rientjes <rientjes@google.com>
Content-Transfer-Encoding: quoted-printable
Message-Id: <E4D5CD53-99A9-479E-AF4C-6653852D8A98@linux.dev>
References: <20220819080029.12241-1-songmuchun@bytedance.com>
 <60933ffc-b850-976c-78a0-0ee6e0ea9ef0@redhat.com>
 <685F3238-D727-4C1F-9BA4-36651ABCD91A@linux.dev>
To:     David Hildenbrand <david@redhat.com>
Precedence: bulk


> On Aug 24, 2022, at 11:23, Muchun Song <muchun.song@linux.dev> wrote:
>=20
>=20
>=20
>> On Aug 23, 2022, at 18:21, David Hildenbrand <david@redhat.com> =
wrote:
>>=20
>> 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.
>>>=20
>>> 39da08cb074c ("hugetlb: offload per node attribute registrations")
>>>=20
>>> 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.
>>>=20
>>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>>=20
>>=20
>> [...]
>>=20
>>> @@ -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;
>>> }
>>=20
>> [...]
>>=20
>>> 	/*
>>> 	 * 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=20
>>> - * basic 'struct node' here, which can be embedded in per-arch=20
>>> + * 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.=20
>>> + * and system devices are handled in drivers/base/sys.c.
>>> *
>>> * Nodes are exported via driverfs in the class/node/devices/
>>> - * directory.=20
>>> + * directory.
>>=20
>> Unrelated changes.
>=20
> Yep, a minor cleanup BTW.
>=20
>>=20
>>> */
>>> #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>
>>>=20
>>> /**
>>> * 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 {
>>>=20
>>> struct memory_block;
>>> extern struct node *node_devices[];
>>> -typedef  void (*node_registration_func_t)(struct node *);
>>>=20
>>> #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
>>>=20
>>> #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>
>>>=20
>>> #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 =3D &node_hstates[node->dev.id];
>>> 	int err;
>>>=20
>>> 	if (nhs->hugepages_kobj)
>>> -		return;		/* already allocated */
>>> +		return 0;		/* already allocated */
>>>=20
>>> 	nhs->hugepages_kobj =3D kobject_create_and_add("hugepages",
>>> 							=
&node->dev.kobj);
>>> 	if (!nhs->hugepages_kobj)
>>> -		return;
>>> +		return -ENOMEM;
>>>=20
>>> 	for_each_hstate(h) {
>>> 		err =3D 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 =3D 0;
>>> +	struct memory_notify *mnb =3D arg;
>>> +	int nid =3D mnb->status_change_nid;
>>> +
>>> +	if (nid =3D=3D NUMA_NO_NODE)
>>> +		return NOTIFY_DONE;
>>> +
>>> +	if (action =3D=3D MEM_GOING_ONLINE)
>>> +		ret =3D hugetlb_register_node(node_devices[nid]);
>>> +	else if (action =3D=3D MEM_CANCEL_ONLINE || action =3D=3D =
MEM_OFFLINE)
>>> +		hugetlb_unregister_node(node_devices[nid]);
>>> +
>>> +	return notifier_from_errno(ret);
>>> }
>>>=20
>>> /*
>>> @@ -4003,18 +4023,11 @@ static void __init =
hugetlb_register_all_nodes(void)
>>> {
>>> 	int nid;
>>>=20
>>> -	for_each_node_state(nid, N_MEMORY) {
>>> -		struct node *node =3D node_devices[nid];
>>> -		if (node->dev.id =3D=3D 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 */
>>=20
>> Do we really *need* the memory hotplug notifier and the added =
complexity
>> due for handling memory-less nodes?

Hi David,

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=E2=80=9D). =
This approach
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=E2=80=99s your opinion?

Thanks,
Muchun

>=20
> I have found the commit introduced this mechanism, see commit:
>=20
> 	4faf8d950ec4 ("hugetlb: handle memory hot-plug events")
>=20
> =46rom the commit message, I think it is a suggestion from David =
Rientjes.
> I didn=E2=80=99t see any reasons why we need it. So Cc David Rientjes =
(Maybe
> he knew more context). The committer Lee and the reviewer Andi=E2=80=99s=
 email
> is invalid (don=E2=80=99t Cc them)
>=20
>>=20
>> Why can't we simply register/unregister sysfs entries in
>> register_node/unregister_node and call it a day?
>>=20
>=20
> At least, I agree with you. Before I change to this way, let=E2=80=99s =
wait for
> some potential comments from David Rientjes.
>=20
>=20
> Thanks.
>=20
>> TBH, we should just have sysfs entries for memory-less nodes and not
>> care about such (corner) cases.
>>=20
>>=20
>> --=20
>> Thanks,
>>=20
>> David / dhildenb