linux-kernel - Re: [PATCH v10 3/8] mm/demotion: Add hotplug callbacks to handle new numa node onlined

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Date:   Wed, 27 Jul 2022 10:08:16 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
To:     "Huang, Ying" <ying.huang@...el.com>,
        Alistair Popple <apopple@...dia.com>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        Wei Xu <weixugc@...gle.com>, Yang Shi <shy828301@...il.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Tim C Chen <tim.c.chen@...el.com>,
        Michal Hocko <mhocko@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Hesham Almatary <hesham.almatary@...wei.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Johannes Weiner <hannes@...xchg.org>, jvgediya.oss@...il.com
Subject: Re: [PATCH v10 3/8] mm/demotion: Add hotplug callbacks to handle
 new numa node onlined

"Huang, Ying" <ying.huang@...el.com> writes:

> Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com> writes:
>
>> On 7/26/22 9:33 AM, Huang, Ying wrote:
>>> "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com> writes:
>>> 
>>>> If the new NUMA node onlined doesn't have a performance level assigned,
>>>> the kernel adds the NUMA node to default memory tier.
>>>>
>>>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@...ux.ibm.com>
>>>> ---
>>>>  include/linux/memory-tiers.h |  1 +
>>>>  mm/memory-tiers.c            | 75 ++++++++++++++++++++++++++++++++++++
>>>>  2 files changed, 76 insertions(+)
>>>>
>>>> diff --git a/include/linux/memory-tiers.h b/include/linux/memory-tiers.h
>>>> index ef380a39db3a..3d5f14d57ae6 100644
>>>> --- a/include/linux/memory-tiers.h
>>>> +++ b/include/linux/memory-tiers.h
>>>> @@ -14,6 +14,7 @@
>>>>  #define MEMTIER_PERF_LEVEL_DRAM	(1 << (MEMTIER_CHUNK_BITS + 2))
>>>>  /* leave one tier below this slow pmem */
>>>>  #define MEMTIER_PERF_LEVEL_PMEM	(1 << MEMTIER_CHUNK_BITS)
>>>> +#define MEMTIER_HOTPLUG_PRIO	100
>>>>  
>>>>  extern bool numa_demotion_enabled;
>>>>  
>>>> diff --git a/mm/memory-tiers.c b/mm/memory-tiers.c
>>>> index 41a21cc5ae55..cc3a47ec18e4 100644
>>>> --- a/mm/memory-tiers.c
>>>> +++ b/mm/memory-tiers.c
>>>> @@ -5,6 +5,7 @@
>>>>  #include <linux/lockdep.h>
>>>>  #include <linux/moduleparam.h>
>>>>  #include <linux/node.h>
>>>> +#include <linux/memory.h>
>>>>  #include <linux/memory-tiers.h>
>>>>  
>>>>  struct memory_tier {
>>>> @@ -64,6 +65,78 @@ static struct memory_tier *find_create_memory_tier(unsigned int perf_level)
>>>>  	return new_memtier;
>>>>  }
>>>>  
>>>> +static struct memory_tier *__node_get_memory_tier(int node)
>>>> +{
>>>> +	struct memory_tier *memtier;
>>>> +
>>>> +	list_for_each_entry(memtier, &memory_tiers, list) {
>>>> +		if (node_isset(node, memtier->nodelist))
>>>> +			return memtier;
>>>> +	}
>>>> +	return NULL;
>>>> +}
>>>> +
>>>> +static void init_node_memory_tier(int node)
>>> 
>>> set_node_memory_tier()?
>>
>> That was done based on feedback from Alistair 
>>
>> https://lore.kernel.org/linux-mm/87h73iapg1.fsf@nvdebian.thelocal
>>
>>> 
>>>> +{
>>>> +	int perf_level;
>>>> +	struct memory_tier *memtier;
>>>> +
>>>> +	mutex_lock(&memory_tier_lock);
>>>> +
>>>> +	memtier = __node_get_memory_tier(node);
>>>> +	if (!memtier) {
>>>> +		perf_level = node_devices[node]->perf_level;
>>>> +		memtier = find_create_memory_tier(perf_level);
>>>> +		node_set(node, memtier->nodelist);
>>>> +	}
>
> It's related to Alistair's comments too.  When will memtier != NULL
> here?  We may need just VM_WARN_ON() here?

When the platform driver sets memory tier directly. With the old code
it can happen when dax/kmem register a node to a memory tier. With
memory_type proposal this can happen if the node is part of memory
type that is already added to a memory tier. 

>
>>>> +	mutex_unlock(&memory_tier_lock);
>>>> +}
>>>> +
>>>> +static void clear_node_memory_tier(int node)
>>>> +{
>>>> +	struct memory_tier *memtier;
>>>> +
>>>> +	mutex_lock(&memory_tier_lock);
>>>> +	memtier = __node_get_memory_tier(node);
>>>> +	if (memtier)
>>>> +		node_clear(node, memtier->nodelist);
>>> 
>>> When memtier->nodelist becomes empty, we need to free memtier?
>>> 
>>>> +	mutex_unlock(&memory_tier_lock);
>>>> +}
>>>> +
>>>> +/*
>>>> + * This runs whether reclaim-based migration is enabled or not,
>>>> + * which ensures that the user can turn reclaim-based migration
>>>> + * at any time without needing to recalculate migration targets.
>>>> + */
>>> 
>>> The comments doesn't apply here.
>>> 
>>>> +static int __meminit migrate_on_reclaim_callback(struct notifier_block *self,
>>>> +						 unsigned long action, void *_arg)
>>> 
>>> Now we are building memory tiers instead of working on demotion.  So I
>>> think we should rename the function to memtier_hotplug_callback().
>>> 
>>>> +{
>>>> +	struct memory_notify *arg = _arg;
>>>> +
>>>> +	/*
>>>> +	 * Only update the node migration order when a node is
>>>> +	 * changing status, like online->offline.
>>>> +	 */
>>>> +	if (arg->status_change_nid < 0)
>>>> +		return notifier_from_errno(0);
>>>> +
>>>> +	switch (action) {
>>>> +	case MEM_OFFLINE:
>>>> +		clear_node_memory_tier(arg->status_change_nid);
>>>> +		break;
>>>> +	case MEM_ONLINE:
>>>> +		init_node_memory_tier(arg->status_change_nid);
>>>> +		break;
>>>> +	}
>>>> +
>>>> +	return notifier_from_errno(0);
>>>> +}
>>>> +
>>>> +static void __init migrate_on_reclaim_init(void)
>>>> +{
>>>> +	hotplug_memory_notifier(migrate_on_reclaim_callback, MEMTIER_HOTPLUG_PRIO);
>>>> +}
>>> 
>>> I suggest to call hotplug_memory_notifier() in memory_tier_init()
>>> directly.  We are not working on demotion here.
>>> 
>>>> +
>>>>  static int __init memory_tier_init(void)
>>>>  {
>>>>  	int node;
>>>> @@ -96,6 +169,8 @@ static int __init memory_tier_init(void)
>>>>  			node_property->perf_level = default_memtier_perf_level;
>>>>  	}
>>>>  	mutex_unlock(&memory_tier_lock);
>>>> +
>>>> +	migrate_on_reclaim_init();
>>>>  	return 0;
>>>>  }
>>>>  subsys_initcall(memory_tier_init);
>>> 
>>> Best Regards,
>>> Huang, Ying
>>
>>
>> Will update the patch in next iteration to take care of other feedback.
>
> Thanks!
>
> Best Regards,
> Huang, Ying