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 7/18/22 12:27 PM, Huang, Ying wrote:
>> 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
>
>
> Implementing that I ended up with the below. The difference is adistance_len is not a memory tier property
> instead it is a kernel parameter like memory_tier_chunk_size which can
> be tuned to create more memory tiers.

It's not determined how to represent the range of abstract distance of
memory tier.  perf_level_chunk_size or perf_level_granularity is another
possible solution.  But I don't think it should be a kernel parameter
for the fist step.

> How about this? Not yet tested.
>
> struct memory_tier {
> 	struct list_head list;
> 	int id;

We don't need "id" for now in fact.  So I suggest to remove it.  We can
add it when we really need it.

> 	int perf_level;
> 	nodemask_t nodelist;
> };
>
> static LIST_HEAD(memory_tiers);
> static DEFINE_MUTEX(memory_tier_lock);
> static unsigned int default_memtier_perf_level = DEFAULT_MEMORY_TYPE_PERF;
> core_param(default_memory_tier_perf_level, default_memtier_perf_level, uint, 0644);
> static unsigned int memtier_perf_chunk_size = 150;
> core_param(memory_tier_perf_chunk, memtier_perf_chunk_size, uint, 0644);
>
> /*
>  * performance levels are grouped into memtiers each of chunk size
>  * memtier_perf_chunk
>  */
> static struct memory_tier *find_create_memory_tier(unsigned int perf_level)
> {
> 	bool found_slot = false;
> 	struct list_head *ent;
> 	struct memory_tier *memtier, *new_memtier;
> 	static int next_memtier_id = 0;
> 	/*
> 	 * zero is special in that it indicates uninitialized
> 	 * perf level by respective driver. Pick default memory
> 	 * tier perf level for that.
> 	 */
> 	if (!perf_level)
> 		perf_level = default_memtier_perf_level;
>
> 	lockdep_assert_held_once(&memory_tier_lock);
>
> 	list_for_each(ent, &memory_tiers) {
> 		memtier = list_entry(ent, struct memory_tier, list);
> 		if (perf_level >= memtier->perf_level &&
> 		    perf_level < memtier->perf_level + memtier_perf_chunk_size)
> 			return memtier;
> 		else if (perf_level < memtier->perf_level) {
> 			found_slot = true;
> 			break;
> 		}
> 	}
>
> 	new_memtier = kzalloc(sizeof(struct memory_tier), GFP_KERNEL);
> 	if (!new_memtier)
> 		return ERR_PTR(-ENOMEM);
>
> 	new_memtier->id = next_memtier_id++;
> 	new_memtier->perf_level = ALIGN_DOWN(perf_level, memtier_perf_chunk_size);
> 	if (found_slot)
> 		list_add_tail(&new_memtier->list, ent);
> 	else
> 		list_add_tail(&new_memtier->list, &memory_tiers);
> 	return new_memtier;
> }
>
> static int __init memory_tier_init(void)
> {
> 	int node;
> 	struct memory_tier *memtier;
>
> 	/*
> 	 * Since this is early during  boot, we could avoid
> 	 * holding memtory_tier_lock. But keep it simple by
> 	 * holding locks. So we can add lock held debug checks
> 	 * in other functions.
> 	 */
> 	mutex_lock(&memory_tier_lock);
> 	memtier = find_create_memory_tier(default_memtier_perf_level);
> 	if (IS_ERR(memtier))
> 		panic("%s() failed to register memory tier: %ld\n",
> 		      __func__, PTR_ERR(memtier));
>
> 	/* CPU only nodes are not part of memory tiers. */
> 	memtier->nodelist = node_states[N_MEMORY];
>
> 	/*
> 	 * nodes that are already online and that doesn't
> 	 * have perf level assigned is assigned a default perf
> 	 * level.
> 	 */
> 	for_each_node_state(node, N_MEMORY) {
> 		struct node *node_property = node_devices[node];
>
> 		if (!node_property->perf_level)
> 			node_property->perf_level = default_memtier_perf_level;
> 	}
> 	mutex_unlock(&memory_tier_lock);
> 	return 0;
> }
> subsys_initcall(memory_tier_init);

I think that this can be a starting point of our future discussion and
review.  Thanks!

Best Regards,
Huang, Ying