Date:   Tue, 30 Aug 2022 12:16:35 +0530
From:   Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>
To:     Wei Xu <weixugc@...gle.com>
Cc:     Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Huang Ying <ying.huang@...el.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>,
        Alistair Popple <apopple@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Johannes Weiner <hannes@...xchg.org>, jvgediya.oss@...il.com,
        Bharata B Rao <bharata@....com>
Subject: Re: [PATCH v2] mm/demotion: Expose memory tier details via sysfs

On 8/30/22 12:01 PM, Wei Xu wrote:
> On Sun, Aug 28, 2022 at 11:08 PM Aneesh Kumar K.V
> <aneesh.kumar@...ux.ibm.com> wrote:
>>
>> This patch adds /sys/devices/virtual/memory_tiering/ where all memory tier
>> related details can be found. All allocated memory tiers will be listed
>> there as /sys/devices/virtual/memory_tiering/memory_tierN/
>>
>> The nodes which are part of a specific memory tier can be listed via
>> /sys/devices/virtual/memory_tiering/memory_tierN/nodes
>>
>> The abstract distance range value of a specific memory tier can be listed via
>> /sys/devices/virtual/memory_tiering/memory_tierN/abstract_distance
>>
>> A directory hierarchy looks like
>> :/sys/devices/virtual/memory_tiering$ tree memory_tier4/
>> memory_tier4/
>> ├── abstract_distance
>> ├── nodes
>> ├── subsystem -> ../../../../bus/memory_tiering
>> └── uevent
>>
>> All toptier nodes are listed via
>> /sys/devices/virtual/memory_tiering/toptier_nodes
>>
>> :/sys/devices/virtual/memory_tiering$ cat toptier_nodes
>> 0,2
>> :/sys/devices/virtual/memory_tiering$ cat memory_tier4/nodes
>> 0,2
>> :/sys/devices/virtual/memory_tiering$ cat memory_tier4/abstract_distance
>> 512 - 639
>>
>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@...ux.ibm.com>
>> ---
>>  .../ABI/testing/sysfs-kernel-mm-memory-tiers  |  41 +++++
>>  mm/memory-tiers.c                             | 155 +++++++++++++++---
>>  2 files changed, 174 insertions(+), 22 deletions(-)
>>  create mode 100644 Documentation/ABI/testing/sysfs-kernel-mm-memory-tiers
>>
>> diff --git a/Documentation/ABI/testing/sysfs-kernel-mm-memory-tiers b/Documentation/ABI/testing/sysfs-kernel-mm-memory-tiers
>> new file mode 100644
>> index 000000000000..6955f69a4423
>> --- /dev/null
>> +++ b/Documentation/ABI/testing/sysfs-kernel-mm-memory-tiers
>> @@ -0,0 +1,41 @@
>> +What:          /sys/devices/virtual/memory_tiering/
>> +Date:          August 2022
>> +Contact:       Linux memory management mailing list <linux-mm@...ck.org>
>> +Description:   A collection of all the memory tiers allocated.
>> +
>> +               Individual memory tier details are contained in subdirectories
>> +               named by the abstract distance of the memory tier.
>> +
>> +               /sys/devices/virtual/memory_tiering/memory_tierN/
>> +
>> +
>> +What:          /sys/devices/virtual/memory_tiering/memory_tierN/
>> +               /sys/devices/virtual/memory_tiering/memory_tierN/abstract_distance
>> +               /sys/devices/virtual/memory_tiering/memory_tierN/nodes
>> +Date:          August 2022
>> +Contact:       Linux memory management mailing list <linux-mm@...ck.org>
>> +Description:   Directory with details of a specific memory tier
>> +
>> +               This is the directory containing information about a particular
>> +               memory tier, memtierN, where N is derived based on abstract distance.
>> +
>> +               A smaller value of N implies a higher (faster) memory tier in the
>> +               hierarchy.
> 
> Given that abstract_distance is provided, it would be more flexible if
> we don't commit to the interface where N in memtierN also indicates
> the memory tier ordering.


IIUC this is one of the request that Johannes had ie, to be able to understand the
memory tier hierarchy based on memtier name. 

>> +
>> +               abstract_distance: The abstract distance range this specific memory
>> +               tier maps to.
> 
> I still think the name of "abstract distance" is kind of confusing
> because it is not clear what is the other object that this distance
> value is relative to.  Do we have to expose this value at this point
> if N in memtierN can already indicate the memory tier ordering?
> 

I do agree that abstract distance is confusing. But IIUC we agreed that it is much better
than other names suggested and is closer to already understood "numa distance" term.

https://lore.kernel.org/linux-mm/YuLF%2FGG8x5lQvg%2Ff@cmpxchg.org/


>> +               nodes: NUMA nodes that are part of this memory tier.
>> +
>> +
>> +What:          /sys/devices/virtual/memory_tiering/toptier_nodes
>> +Date:          August 2022
>> +Contact:       Linux memory management mailing list <linux-mm@...ck.org>
>> +Description:   Toptier node mask
>> +
>> +               A toptier is defined as the memory tier from which memory promotion
>> +               is not done by the kernel.
>> +
>> +               toptier_nodes: NUMA nodes that are part of all the memory tiers
>> +               above a topier tier.
> 
> Nit: topier -> toptier
> 
> toptier_nodes should be the union of NUMA nodes that are part of each
> toptier, not above a toptier, right?
> 

I will update the wording. Yes. it is the union of NUMA nodes that are 
part of each toptier. 

>> +
>> diff --git a/mm/memory-tiers.c b/mm/memory-tiers.c
>> index c4bd6d052a33..d4648d4e4d54 100644
>> --- a/mm/memory-tiers.c
>> +++ b/mm/memory-tiers.c
>> @@ -19,6 +19,7 @@ struct memory_tier {
>>          * adistance_start .. adistance_start + MEMTIER_CHUNK_SIZE
>>          */
>>         int adistance_start;
>> +       struct device dev;
>>         /* All the nodes that are part of all the lower memory tiers. */
>>         nodemask_t lower_tier_mask;
>>  };
>> @@ -36,6 +37,13 @@ static DEFINE_MUTEX(memory_tier_lock);
>>  static LIST_HEAD(memory_tiers);
>>  static struct node_memory_type_map node_memory_types[MAX_NUMNODES];
>>  static struct memory_dev_type *default_dram_type;
>> +
>> +#define to_memory_tier(device) container_of(device, struct memory_tier, dev)
>> +static struct bus_type memory_tier_subsys = {
>> +       .name = "memory_tiering",
>> +       .dev_name = "memory_tier",
>> +};
>> +
>>  #ifdef CONFIG_MIGRATION
>>  static int top_tier_adistance;
>>  /*
>> @@ -98,8 +106,73 @@ static int top_tier_adistance;
>>  static struct demotion_nodes *node_demotion __read_mostly;
>>  #endif /* CONFIG_MIGRATION */
>>
>> +static __always_inline nodemask_t get_memtier_nodemask(struct memory_tier *memtier)
>> +{
>> +       nodemask_t nodes = NODE_MASK_NONE;
>> +       struct memory_dev_type *memtype;
>> +
>> +       list_for_each_entry(memtype, &memtier->memory_types, tier_sibiling)
>> +               nodes_or(nodes, nodes, memtype->nodes);
>> +
>> +       return nodes;
>> +}
>> +
>> +static void memory_tier_device_release(struct device *dev)
>> +{
>> +       struct memory_tier *tier = to_memory_tier(dev);
>> +       /*
>> +        * synchronize_rcu in clear_node_memory_tier makes sure
>> +        * we don't have rcu access to this memory tier.
>> +        */
>> +       kfree(tier);
>> +}
>> +
>> +static ssize_t nodes_show(struct device *dev,
>> +                         struct device_attribute *attr, char *buf)
>> +{
>> +       int ret;
>> +       nodemask_t nmask;
>> +
>> +       mutex_lock(&memory_tier_lock);
>> +       nmask = get_memtier_nodemask(to_memory_tier(dev));
>> +       ret = sysfs_emit(buf, "%*pbl\n", nodemask_pr_args(&nmask));
>> +       mutex_unlock(&memory_tier_lock);
>> +       return ret;
>> +}
>> +static DEVICE_ATTR_RO(nodes);
>> +
>> +static ssize_t abstract_distance_show(struct device *dev,
>> +                                     struct device_attribute *attr, char *buf)
>> +{
>> +       int ret;
>> +       struct memory_tier *memtier = to_memory_tier(dev);
>> +
>> +       mutex_lock(&memory_tier_lock);
>> +       ret = sysfs_emit(buf, "%d - %d\n", memtier->adistance_start,
>> +                        memtier->adistance_start + MEMTIER_CHUNK_SIZE - 1);
>> +       mutex_unlock(&memory_tier_lock);
>> +       return ret;
>> +}
>> +static DEVICE_ATTR_RO(abstract_distance);
>> +
>> +static struct attribute *memtier_dev_attrs[] = {
>> +       &dev_attr_nodes.attr,
>> +       &dev_attr_abstract_distance.attr,
>> +       NULL
>> +};
>> +
>> +static const struct attribute_group memtier_dev_group = {
>> +       .attrs = memtier_dev_attrs,
>> +};
>> +
>> +static const struct attribute_group *memtier_dev_groups[] = {
>> +       &memtier_dev_group,
>> +       NULL
>> +};
>> +
>>  static struct memory_tier *find_create_memory_tier(struct memory_dev_type *memtype)
>>  {
>> +       int ret;
>>         bool found_slot = false;
>>         struct memory_tier *memtier, *new_memtier;
>>         int adistance = memtype->adistance;
>> @@ -123,15 +196,14 @@ static struct memory_tier *find_create_memory_tier(struct memory_dev_type *memty
>>
>>         list_for_each_entry(memtier, &memory_tiers, list) {
>>                 if (adistance == memtier->adistance_start) {
>> -                       list_add(&memtype->tier_sibiling, &memtier->memory_types);
>> -                       return memtier;
>> +                       goto link_memtype;
>>                 } else if (adistance < memtier->adistance_start) {
>>                         found_slot = true;
>>                         break;
>>                 }
>>         }
>>
>> -       new_memtier = kmalloc(sizeof(struct memory_tier), GFP_KERNEL);
>> +       new_memtier = kzalloc(sizeof(struct memory_tier), GFP_KERNEL);
>>         if (!new_memtier)
>>                 return ERR_PTR(-ENOMEM);
>>
>> @@ -142,8 +214,23 @@ static struct memory_tier *find_create_memory_tier(struct memory_dev_type *memty
>>                 list_add_tail(&new_memtier->list, &memtier->list);
>>         else
>>                 list_add_tail(&new_memtier->list, &memory_tiers);
>> -       list_add(&memtype->tier_sibiling, &new_memtier->memory_types);
>> -       return new_memtier;
>> +
>> +       new_memtier->dev.id = adistance >> MEMTIER_CHUNK_BITS;
>> +       new_memtier->dev.bus = &memory_tier_subsys;
>> +       new_memtier->dev.release = memory_tier_device_release;
>> +       new_memtier->dev.groups = memtier_dev_groups;
>> +
>> +       ret = device_register(&new_memtier->dev);
>> +       if (ret) {
>> +               list_del(&memtier->list);
>> +               put_device(&memtier->dev);
>> +               return ERR_PTR(ret);
>> +       }
>> +       memtier = new_memtier;
>> +
>> +link_memtype:
>> +       list_add(&memtype->tier_sibiling, &memtier->memory_types);
>> +       return memtier;
>>  }
>>
>>  static struct memory_tier *__node_get_memory_tier(int node)
>> @@ -275,17 +362,6 @@ static void disable_all_demotion_targets(void)
>>         synchronize_rcu();
>>  }
>>
>> -static __always_inline nodemask_t get_memtier_nodemask(struct memory_tier *memtier)
>> -{
>> -       nodemask_t nodes = NODE_MASK_NONE;
>> -       struct memory_dev_type *memtype;
>> -
>> -       list_for_each_entry(memtype, &memtier->memory_types, tier_sibiling)
>> -               nodes_or(nodes, nodes, memtype->nodes);
>> -
>> -       return nodes;
>> -}
>> -
>>  /*
>>   * Find an automatic demotion target for all memory
>>   * nodes. Failing here is OK.  It might just indicate
>> @@ -432,11 +508,7 @@ static struct memory_tier *set_node_memory_tier(int node)
>>  static void destroy_memory_tier(struct memory_tier *memtier)
>>  {
>>         list_del(&memtier->list);
>> -       /*
>> -        * synchronize_rcu in clear_node_memory_tier makes sure
>> -        * we don't have rcu access to this memory tier.
>> -        */
>> -       kfree(memtier);
>> +       device_unregister(&memtier->dev);
>>  }
>>
>>  static bool clear_node_memory_tier(int node)
>> @@ -563,11 +635,50 @@ static int __meminit memtier_hotplug_callback(struct notifier_block *self,
>>         return notifier_from_errno(0);
>>  }
>>
>> +static ssize_t toptier_nodes_show(struct device *dev,
>> +                                    struct device_attribute *attr, char *buf)
>> +{
>> +       int ret;
>> +       nodemask_t nmask, top_tier_mask = NODE_MASK_NONE;
>> +       struct memory_tier *memtier = to_memory_tier(dev);
>> +
>> +       mutex_lock(&memory_tier_lock);
>> +       list_for_each_entry(memtier, &memory_tiers, list) {
>> +               if (memtier->adistance_start >= top_tier_adistance)
> 
> It is kind of confusing that a tier with top_tier_adistance is not
> considered as a toptier.  Can we redefine top_tier_adistance to be the
> inclusive upper bound of toptiers?
> 

Agreed. I will fix that up by doing 

top_tier_adistance = memtier->adistance_start + MEMTIER_CHUNK_SIZE - 1;
			

>> +                       break;
>> +               nmask = get_memtier_nodemask(memtier);
>> +               nodes_or(top_tier_mask, top_tier_mask, nmask);
>> +       }
>> +
>> +       ret = sysfs_emit(buf, "%*pbl\n", nodemask_pr_args(&top_tier_mask));
>> +       mutex_unlock(&memory_tier_lock);
>> +       return ret;
>> +}
>> +static DEVICE_ATTR_RO(toptier_nodes);
>> +
>> +static struct attribute *memtier_subsys_attrs[] = {
>> +       &dev_attr_toptier_nodes.attr,
>> +       NULL
>> +};
>> +
>> +static const struct attribute_group memtier_subsys_group = {
>> +       .attrs = memtier_subsys_attrs,
>> +};
>> +
>> +static const struct attribute_group *memtier_subsys_groups[] = {
>> +       &memtier_subsys_group,
>> +       NULL
>> +};
>> +
>>  static int __init memory_tier_init(void)
>>  {
>> -       int node;
>> +       int ret, node;
>>         struct memory_tier *memtier;
>>
>> +       ret = subsys_virtual_register(&memory_tier_subsys, memtier_subsys_groups);
>> +       if (ret)
>> +               panic("%s() failed to register memory tier subsystem\n", __func__);
>> +
>>  #ifdef CONFIG_MIGRATION
>>         node_demotion = kcalloc(nr_node_ids, sizeof(struct demotion_nodes),
>>                                 GFP_KERNEL);
>> --
>> 2.37.2
>>

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