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

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