Re: [PATCH v9 1/8] mm/demotion: Add support for explicit memory tiers

["Huang, Ying" <ying.huang@intel.com>]

Aneesh Kumar K V <aneesh.kumar@linux.ibm.com> writes:

> On 7/15/22 1:23 PM, Huang, Ying wrote:

[snip]

>> 
>> You dropped the original sysfs interface patches from the series, but
>> the kernel internal implementation is still for the original sysfs
>> interface.  For example, memory tier ID is for the original sysfs
>> interface, not for the new proposed sysfs interface.  So I suggest you
>> to implement with the new interface in mind.  What do you think about
>> the following design?
>> 
>
> Sorry I am not able to follow you here. This patchset completely drops
> exposing memory tiers to userspace via sysfs. Instead it allow
> creation of memory tiers with specific tierID from within the kernel/device driver.
> Default tierID is 200 and dax kmem creates memory tier with tierID 100. 
>
>
>> - Each NUMA node belongs to a memory type, and each memory type
>>   corresponds to a "abstract distance", so each NUMA node corresonds to
>>   a "distance".  For simplicity, we can start with static distances, for
>>   example, DRAM (default): 150, PMEM: 250.  The distance of each NUMA
>>   node can be recorded in a global array,
>> 
>>     int node_distances[MAX_NUMNODES];
>> 
>>   or, just
>> 
>>     pgdat->distance
>> 
>
> I don't follow this. I guess you are trying to have a different design.
> Would it be much easier if you can write this in the form of a patch? 

Written some pseudo code as follow to show my basic idea.

#define MEMORY_TIER_ADISTANCE_DRAM	150
#define MEMORY_TIER_ADISTANCE_PMEM	250

struct memory_tier {
	/* abstract distance range covered by the memory tier */
	int adistance_start;
	int adistance_len;
	struct list_head list;
	nodemask_t nodemask;
};

/* RCU list of memory tiers */
static LIST_HEAD(memory_tiers);

/* abstract distance of each NUMA node */
int node_adistances[MAX_NUMNODES];

struct memory_tier *find_create_memory_tier(int adistance)
{
	struct memory_tier *tier;

	list_for_each_entry(tier, &memory_tiers, list) {
		if (adistance >= tier->adistance_start &&
		    adistance < tier->adistance_start + tier->adistance_len)
			return tier;
	}
	/* allocate a new memory tier and return */
}

void memory_tier_add_node(int nid)
{
	int adistance;
	struct memory_tier *tier;

	adistance = node_adistances[nid] || MEMORY_TIER_ADISTANCE_DRAM;
	tier = find_create_memory_tier(adistance);
	node_set(nid, &tier->nodemask);
	/* setup demotion data structure, etc */
}

static int __meminit migrate_on_reclaim_callback(struct notifier_block *self,
						 unsigned long action, void *_arg)
{
	struct memory_notify *arg = _arg;
	int nid;

	nid = arg->status_change_nid;
	if (nid < 0)
		return notifier_from_errno(0);

	switch (action) {
	case MEM_ONLINE:
		memory_tier_add_node(nid);
		break;
	}

	return notifier_from_errno(0);
}

/* kmem.c */
static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
{
	node_adistances[dev_dax->target_node] = MEMORY_TIER_ADISTANCE_PMEM;
	/* add_memory_driver_managed() */
}

[snip]

Best Regards,
Huang, Ying