Re: [RFC PATCH v4 7/7] mm/demotion: Demote pages according to allocation fallback order

Aneesh Kumar K V
2022-06-06T08:04:34Z

On 6/6/22 1:12 PM, Ying Huang wrote:
> On Mon, 2022-06-06 at 11:51 +0530, Aneesh Kumar K.V wrote:
>> Ying Huang <ying.huang@intel.com> writes:
>>
>> .....
>>
>>>>>>
>>>>>>> https://lore.kernel.org/lkml/69f2d063a15f8c4afb4688af7b7890f32af55391.camel@intel.com/
>>>>>>>
>>>>>>> That is, something like below,
>>>>>>>
>>>>>>> static struct page *alloc_demote_page(struct page *page, unsigned long node)
>>>>>>> {
>>>>>>> struct page *page;
>>>>>>> nodemask_t allowed_mask;
>>>>>>> struct migration_target_control mtc = {
>>>>>>> /*
>>>>>>> * Allocate from 'node', or fail quickly and quietly.
>>>>>>> * When this happens, 'page' will likely just be discarded
>>>>>>> * instead of migrated.
>>>>>>> */
>>>>>>> .gfp_mask = (GFP_HIGHUSER_MOVABLE & ~__GFP_RECLAIM) |
>>>>>>> __GFP_THISNODE | __GFP_NOWARN |
>>>>>>> __GFP_NOMEMALLOC | GFP_NOWAIT,
>>>>>>> .nid = node
>>>>>>> };
>>>>>>>
>>>>>>> page = alloc_migration_target(page, (unsigned long)&mtc);
>>>>>>> if (page)
>>>>>>> return page;
>>>>>>>
>>>>>>> mtc.gfp_mask &= ~__GFP_THISNODE;
>>>>>>> mtc.nmask = &allowed_mask;
>>>>>>>
>>>>>>> return alloc_migration_target(page, (unsigned long)&mtc);
>>>>>>> }
>>>>>>
>>>>>> I skipped doing this in v5 because I was not sure this is really what we
>>>>>> want.
>>>>>
>>>>> I think so. And this is the original behavior. We should keep the
>>>>> original behavior as much as possible, then make changes if necessary.
>>>>>
>>>>
>>>> That is the reason I split the new page allocation as a separate patch.
>>>> Previous discussion on this topic didn't conclude on whether we really
>>>> need to do the above or not
>>>> https://lore.kernel.org/lkml/CAAPL-u9endrWf_aOnPENDPdvT-2-YhCAeJ7ONGckGnXErTLOfQ@mail.gmail.com/
>>>
>>> Please check the later email in the thread you referenced. Both Wei and
>>> me agree that the use case needs to be supported. We just didn't reach
>>> concensus about how to implement it. If you think Wei's solution is
>>> better (referenced as below), you can try to do that too. Although I
>>> think my proposed implementation is much simpler.
>>
>> How about the below details
>>
>> diff --git a/include/linux/memory-tiers.h b/include/linux/memory-tiers.h
>> index 79bd8d26feb2..cd6e71f702ad 100644
>> --- a/include/linux/memory-tiers.h
>> +++ b/include/linux/memory-tiers.h
>> @@ -21,6 +21,7 @@ void node_remove_from_memory_tier(int node);
>>  int node_get_memory_tier_id(int node);
>>  int node_set_memory_tier(int node, int tier);
>>  int node_reset_memory_tier(int node, int tier);
>> +void node_get_allowed_targets(int node, nodemask_t *targets);
>>  #else
>>  #define numa_demotion_enabled false
>>  static inline int next_demotion_node(int node)
>> @@ -28,6 +29,10 @@ static inline int next_demotion_node(int node)
>>   return NUMA_NO_NODE;
>>  }
>>
>>
>>
>>
>> +static inline void node_get_allowed_targets(int node, nodemask_t *targets)
>> +{
>> + *targets = NODE_MASK_NONE;
>> +}
>>  #endif /* CONFIG_TIERED_MEMORY */
>>
>>
>>
>>
>>  #endif
>> diff --git a/mm/memory-tiers.c b/mm/memory-tiers.c
>> index b4e72b672d4d..592d939ec28d 100644
>> --- a/mm/memory-tiers.c
>> +++ b/mm/memory-tiers.c
>> @@ -18,6 +18,7 @@ struct memory_tier {
>>
>>
>>
>>
>>  struct demotion_nodes {
>>   nodemask_t preferred;
>> + nodemask_t allowed;
>>  };
>>
>>
>>
>>
>>  #define to_memory_tier(device) container_of(device, struct memory_tier, dev)
>> @@ -378,6 +379,25 @@ int node_set_memory_tier(int node, int tier)
>>  }
>>  EXPORT_SYMBOL_GPL(node_set_memory_tier);
>>
>>
>>
>>
>> +void node_get_allowed_targets(int node, nodemask_t *targets)
>> +{
>> + /*
>> + * node_demotion[] is updated without excluding this
>> + * function from running.
>> + *
>> + * If any node is moving to lower tiers then modifications
>> + * in node_demotion[] are still valid for this node, if any
>> + * node is moving to higher tier then moving node may be
>> + * used once for demotion which should be ok so rcu should
>> + * be enough here.
>> + */
>> + rcu_read_lock();
>> +
>> + *targets = node_demotion[node].allowed;
>> +
>> + rcu_read_unlock();
>> +}
>> +
>>  /**
>>   * next_demotion_node() - Get the next node in the demotion path
>>   * @node: The starting node to lookup the next node
>> @@ -437,8 +457,10 @@ static void __disable_all_migrate_targets(void)
>>  {
>>   int node;
>>
>>
>>
>>
>> - for_each_node_mask(node, node_states[N_MEMORY])
>> + for_each_node_mask(node, node_states[N_MEMORY]) {
>>   node_demotion[node].preferred = NODE_MASK_NONE;
>> + node_demotion[node].allowed = NODE_MASK_NONE;
>> + }
>>  }
>>
>>
>>
>>
>>  static void disable_all_migrate_targets(void)
>> @@ -465,7 +487,7 @@ static void establish_migration_targets(void)
>>   struct demotion_nodes *nd;
>>   int target = NUMA_NO_NODE, node;
>>   int distance, best_distance;
>> - nodemask_t used;
>> + nodemask_t used, allowed = NODE_MASK_NONE;
>>
>>
>>
>>
>>   if (!node_demotion)
>>   return;
>> @@ -511,6 +533,29 @@ static void establish_migration_targets(void)
>>   }
>>   } while (1);
>>   }
>> + /*
>> + * Now build the allowed mask for each node collecting node mask from
>> + * all memory tier below it. This allows us to fallback demotion page
>> + * allocation to a set of nodes that is closer the above selected
>> + * perferred node.
>> + */
>> + list_for_each_entry(memtier, &memory_tiers, list)
>> + nodes_or(allowed, allowed, memtier->nodelist);
>> + /*
>> + * Removes nodes not yet in N_MEMORY.
>> + */
>> + nodes_and(allowed, node_states[N_MEMORY], allowed);
>> +
>> + list_for_each_entry(memtier, &memory_tiers, list) {
>> + /*
>> + * Keep removing current tier from allowed nodes,
>> + * This will remove all nodes in current and above
>> + * memory tier from the allowed mask.
>> + */
>> + nodes_andnot(allowed, allowed, memtier->nodelist);
>> + for_each_node_mask(node, memtier->nodelist)
>> + node_demotion[node].allowed = allowed;
>> + }
>>  }
>>
>>
>>
>>
>>  /*
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 3a8f78277f99..b0792d838efb 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -1460,19 +1460,32 @@ static void folio_check_dirty_writeback(struct folio *folio,
>>   mapping->a_ops->is_dirty_writeback(folio, dirty, writeback);
>>  }
>>
>>
>>
>>
>> -static struct page *alloc_demote_page(struct page *page, unsigned long node)
>> +static struct page *alloc_demote_page(struct page *page, unsigned long private)
>>  {
>> - struct migration_target_control mtc = {
>> - /*
>> - * Allocate from 'node', or fail quickly and quietly.
>> - * When this happens, 'page' will likely just be discarded
>> - * instead of migrated.
>> - */
>> - .gfp_mask = (GFP_HIGHUSER_MOVABLE & ~__GFP_RECLAIM) |
>> - __GFP_THISNODE | __GFP_NOWARN |
>> - __GFP_NOMEMALLOC | GFP_NOWAIT,
>> - .nid = node
>> - };
>> + struct page *target_page;
>> + nodemask_t *allowed_mask;
>> + struct migration_target_control *mtc;
>> +
>> + mtc = (struct migration_target_control *)private;
>> +
>> + allowed_mask = mtc->nmask;
>> + /*
>> + * make sure we allocate from the target node first also trying to
>> + * reclaim pages from the target node via kswapd if we are low on
>> + * free memory on target node. If we don't do this and if we have low
>> + * free memory on the target memtier, we would start allocating pages
>> + * from higher memory tiers without even forcing a demotion of cold
>> + * pages from the target memtier. This can result in the kernel placing
>> + * hotpages in higher memory tiers.
>> + */
>> + mtc->nmask = NULL;
>> + mtc->gfp_mask |= __GFP_THISNODE;
>> + target_page = alloc_migration_target(page, (unsigned long)&mtc);
>> + if (target_page)
>> + return target_page;
>> +
>> + mtc->gfp_mask &= ~__GFP_THISNODE;
>> + mtc->nmask = allowed_mask;
>>
>>
>>
>>
>>   return alloc_migration_target(page, (unsigned long)&mtc);
>>  }
>> @@ -1487,6 +1500,19 @@ static unsigned int demote_page_list(struct list_head *demote_pages,
>>  {
>>   int target_nid = next_demotion_node(pgdat->node_id);
>>   unsigned int nr_succeeded;
>> + nodemask_t allowed_mask;
>> +
>> + struct migration_target_control mtc = {
>> + /*
>> + * Allocate from 'node', or fail quickly and quietly.
>> + * When this happens, 'page' will likely just be discarded
>> + * instead of migrated.
>> + */
>> + .gfp_mask = (GFP_HIGHUSER_MOVABLE & ~__GFP_RECLAIM) | __GFP_NOWARN |
>> + __GFP_NOMEMALLOC | GFP_NOWAIT,
>> + .nid = target_nid,
>> + .nmask = &allowed_mask
>> + };
>>
>>
>>
>>
>>   if (list_empty(demote_pages))
>>   return 0;
>> @@ -1494,10 +1520,12 @@ static unsigned int demote_page_list(struct list_head *demote_pages,
>>   if (target_nid == NUMA_NO_NODE)
>>   return 0;
>>
>>
>>
>>
>> + node_get_allowed_targets(pgdat->node_id, &allowed_mask);
>> +
>>   /* Demotion ignores all cpuset and mempolicy settings */
>>   migrate_pages(demote_pages, alloc_demote_page, NULL,
>> - target_nid, MIGRATE_ASYNC, MR_DEMOTION,
>> - &nr_succeeded);
>> + (unsigned long)&mtc, MIGRATE_ASYNC, MR_DEMOTION,
>> + &nr_succeeded);
>
> Firstly, it addressed my requirement, Thanks! And, I'd prefer to put
> mtc definition in alloc_demote_page(). Because that makes all page
> allocation logic in one function. Thus the readability of code is
> better.

The challenge is in allowed_mask computation. That is based on the
src_node and not target_node.

-aneesh