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

[Aneesh Kumar K V]

On 7/15/22 1:23 PM, Huang, Ying wrote:
> "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxx> writes:
> 
>> In the current kernel, memory tiers are defined implicitly via a
>> demotion path relationship between NUMA nodes, which is created
>> during the kernel initialization and updated when a NUMA node is
>> hot-added or hot-removed.  The current implementation puts all
>> nodes with CPU into the top tier, and builds the tier hierarchy
>> tier-by-tier by establishing the per-node demotion targets based
>> on the distances between nodes.
>>
>> This current memory tier kernel interface needs to be improved for
>> several important use cases,
>>
>> The current tier initialization code always initializes
>> each memory-only NUMA node into a lower tier.  But a memory-only
>> NUMA node may have a high performance memory device (e.g. a DRAM
>> device attached via CXL.mem or a DRAM-backed memory-only node on
>> a virtual machine) and should be put into a higher tier.
>>
>> The current tier hierarchy always puts CPU nodes into the top
>> tier. But on a system with HBM or GPU devices, the
>> memory-only NUMA nodes mapping these devices should be in the
>> top tier, and DRAM nodes with CPUs are better to be placed into the
>> next lower tier.
>>
>> With current kernel higher tier node can only be demoted to selected nodes on the
>> next lower tier as defined by the demotion path, not any other
>> node from any lower tier.  This strict, hard-coded demotion order
>> does not work in all use cases (e.g. some use cases may want to
>> allow cross-socket demotion to another node in the same demotion
>> tier as a fallback when the preferred demotion node is out of
>> space), This demotion order is also inconsistent with the page
>> allocation fallback order when all the nodes in a higher tier are
>> out of space: The page allocation can fall back to any node from
>> any lower tier, whereas the demotion order doesn't allow that.
>>
>> The current kernel also don't provide any interfaces for the
>> userspace to learn about the memory tier hierarchy in order to
>> optimize its memory allocations.
>>
>> This patch series address the above by defining memory tiers explicitly.
>>
>> This patch introduce explicity memory tiers. The tier ID value
>> of a memory tier is used to derive the demotion order between
>> NUMA nodes.
>>
>> For example, if we have 3 memtiers: memtier100, memtier200, memiter300
>> then the memory tier order is: memtier300 -> memtier200 -> memtier100
>> where memtier300 is the highest tier and memtier100 is the lowest tier.
>>
>> While reclaim we migrate pages from fast(higher) tiers to slow(lower)
>> tiers when the fast(higher) tier is under memory pressure.
>>
>> This patchset introduce 3 memory tiers (memtier100, memtier200 and memtier300)
>> which are created by different kernel subsystems. The default memory
>> tier created by the kernel is memtier200. A kernel parameter is provided
>> to override the default memory tier.
>>
>> Link: https://lore.kernel.org/linux-mm/CAAPL-u9Wv+nH1VOZTj=9p9S70Y3Qz3+63EkqncRDdHfubsrjfw@xxxxxxxxxxxxxx
>> Link: https://lore.kernel.org/linux-mm/7b72ccf4-f4ae-cb4e-f411-74d055482026@xxxxxxxxxxxxx
>>
>> Signed-off-by: Jagdish Gediya <jvgediya@xxxxxxxxxxxxx>
>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>
>> ---
>>  include/linux/memory-tiers.h | 15 +++++++
>>  mm/Makefile                  |  1 +
>>  mm/memory-tiers.c            | 78 ++++++++++++++++++++++++++++++++++++
>>  3 files changed, 94 insertions(+)
>>  create mode 100644 include/linux/memory-tiers.h
>>  create mode 100644 mm/memory-tiers.c
>>
>> diff --git a/include/linux/memory-tiers.h b/include/linux/memory-tiers.h
>> new file mode 100644
>> index 000000000000..a81dbc20e0d1
>> --- /dev/null
>> +++ b/include/linux/memory-tiers.h
>> @@ -0,0 +1,15 @@
>> +/* SPDX-License-Identifier: GPL-2.0 */
>> +#ifndef _LINUX_MEMORY_TIERS_H
>> +#define _LINUX_MEMORY_TIERS_H
>> +
>> +#ifdef CONFIG_NUMA
>> +
>> +#define MEMORY_TIER_HBM_GPU	300
>> +#define MEMORY_TIER_DRAM	200
>> +#define MEMORY_TIER_PMEM	100
>> +
>> +#define DEFAULT_MEMORY_TIER	MEMORY_TIER_DRAM
>> +#define MAX_MEMORY_TIER_ID	400
>> +
>> +#endif	/* CONFIG_NUMA */
>> +#endif  /* _LINUX_MEMORY_TIERS_H */
>> diff --git a/mm/Makefile b/mm/Makefile
>> index 6f9ffa968a1a..d30acebc2164 100644
>> --- a/mm/Makefile
>> +++ b/mm/Makefile
>> @@ -92,6 +92,7 @@ obj-$(CONFIG_KFENCE) += kfence/
>>  obj-$(CONFIG_FAILSLAB) += failslab.o
>>  obj-$(CONFIG_MEMTEST)		+= memtest.o
>>  obj-$(CONFIG_MIGRATION) += migrate.o
>> +obj-$(CONFIG_NUMA) += memory-tiers.o
>>  obj-$(CONFIG_DEVICE_MIGRATION) += migrate_device.o
>>  obj-$(CONFIG_TRANSPARENT_HUGEPAGE) += huge_memory.o khugepaged.o
>>  obj-$(CONFIG_PAGE_COUNTER) += page_counter.o
>> diff --git a/mm/memory-tiers.c b/mm/memory-tiers.c
>> new file mode 100644
>> index 000000000000..011877b6dbb9
>> --- /dev/null
>> +++ b/mm/memory-tiers.c
>> @@ -0,0 +1,78 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +#include <linux/types.h>
>> +#include <linux/nodemask.h>
>> +#include <linux/slab.h>
>> +#include <linux/lockdep.h>
>> +#include <linux/moduleparam.h>
>> +#include <linux/memory-tiers.h>
>> +
>> +struct memory_tier {
>> +	struct list_head list;
>> +	int id;
>> +	nodemask_t nodelist;
>> +};
>> +
>> +static DEFINE_MUTEX(memory_tier_lock);
>> +static LIST_HEAD(memory_tiers);
>> +
>> +static void insert_memory_tier(struct memory_tier *memtier)
>> +{
>> +	struct list_head *ent;
>> +	struct memory_tier *tmp_memtier;
>> +
>> +	lockdep_assert_held_once(&memory_tier_lock);
>> +
>> +	list_for_each(ent, &memory_tiers) {
>> +		tmp_memtier = list_entry(ent, struct memory_tier, list);
>> +		if (tmp_memtier->id < memtier->id) {
>> +			list_add_tail(&memtier->list, ent);
>> +			return;
>> +		}
>> +	}
>> +	list_add_tail(&memtier->list, &memory_tiers);
>> +}
>> +
>> +static struct memory_tier *register_memory_tier(unsigned int tier)
>> +{
>> +	struct memory_tier *memtier;
>> +
>> +	if (tier > MAX_MEMORY_TIER_ID)
>> +		return ERR_PTR(-EINVAL);
>> +
>> +	memtier = kzalloc(sizeof(struct memory_tier), GFP_KERNEL);
>> +	if (!memtier)
>> +		return ERR_PTR(-ENOMEM);
>> +
>> +	memtier->id   = tier;
>> +
>> +	insert_memory_tier(memtier);
>> +
>> +	return memtier;
>> +}
>> +
>> +static unsigned int default_memtier = DEFAULT_MEMORY_TIER;
>> +core_param(default_memory_tier, default_memtier, uint, 0644);
>> +
>> +static int __init memory_tier_init(void)
>> +{
>> +	struct memory_tier *memtier;
>> +
>> +	/*
>> +	 * Register only default memory tier to hide all empty
>> +	 * memory tier from sysfs. 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 = register_memory_tier(default_memtier);
>> +	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];
>> +	mutex_unlock(&memory_tier_lock);
>> +	return 0;
>> +}
>> +subsys_initcall(memory_tier_init);
> 
> 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? 


> - Each memory tier corresponds to a range of distance, for example,
>   0-100, 100-200, 200-300, >300, we can start with static ranges too.
> 
> - The core API of memory tier could be
> 
>     struct memory_tier *find_create_memory_tier(int distance);
> 
>   it will find the memory tier which covers "distance" in the memory
>   tier list, or create a new memory tier if not found.
> 

I was expecting this to be internal to dax kmem. How dax kmem maps
"abstract distance" to a memory tier. At this point this patchset is
keeping all that for a future patchset. 

> - kmem_dax driver will setup distance for PMEM NUMA nodes before online
>   them.
> 

Sure we can do that as part of future patchset ?

> - When a NUMA node is onlined, we will use find_create_memory_tier() to
>   find or create its memory tier and add the NUMA node into the memory
>   tier.
> 

This is what this patchset does. When we online a numa node the kernel 
find the memory tier for the node (__node_get_memory_tier). If it doesn't
exist, we create one. (The new one created is not dynamic as you outlined
earlier. But then that can be done in a future patchset). For now I am
keeping this simpler.

static int node_set_memory_tier(int node, int tier)
{
	struct memory_tier *memtier;
	int ret = 0;

	mutex_lock(&memory_tier_lock);
	memtier = __node_get_memory_tier(node);
	/*
	 * if node is already part of the tier proceed with the
	 * current tier value, because we might want to establish
	 * new migration paths now. The node might be added to a tier
	 * before it was made part of N_MEMORY, hence estabilish_migration_targets
	 * will have skipped this node.
	 */
	if (!memtier)
		ret = __node_set_memory_tier(node, tier);
	establish_migration_targets();

	mutex_unlock(&memory_tier_lock);

	return ret;
}





> - Or we can add memory type data structure now.
> 
> Best Regards,
> Huang, Ying

-aneesh