Re: [PATCH RESEND] mm: hugetlb: simplify per-node sysfs creation and removal

Muchun Song
2022-09-01T07:58:05Z

> On Sep 1, 2022, at 14:57, David Hildenbrand <david@redhat.com> wrote:
>
> On 01.09.22 08:35, Muchun Song wrote:
>>
>>
>>> On Aug 24, 2022, at 11:23, Muchun Song <muchun.song@linux.dev> wrote:
>>>
>>>
>>>
>>>> On Aug 23, 2022, at 18:21, David Hildenbrand <david@redhat.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@bytedance.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
>