Zi Yan <ziy@xxxxxxxxxx> writes:
> On 18 Jun 2021, at 2:15, Huang Ying wrote:
>
>> From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>>
>> When memory fills up on a node, memory contents can be
>> automatically migrated to another node. The biggest problems are
>> knowing when to migrate and to where the migration should be
>> targeted.
>>
>> The most straightforward way to generate the "to where" list would
>> be to follow the page allocator fallback lists. Those lists
>> already tell us if memory is full where to look next. It would
>> also be logical to move memory in that order.
>>
>> But, the allocator fallback lists have a fatal flaw: most nodes
>> appear in all the lists. This would potentially lead to migration
>> cycles (A->B, B->A, A->B, ...).
>>
>> Instead of using the allocator fallback lists directly, keep a
>> separate node migration ordering. But, reuse the same data used
>> to generate page allocator fallback in the first place:
>> find_next_best_node().
>>
>> This means that the firmware data used to populate node distances
>> essentially dictates the ordering for now. It should also be
>> architecture-neutral since all NUMA architectures have a working
>> find_next_best_node().
>>
>> The protocol for node_demotion[] access and writing is not
>> standard. It has no specific locking and is intended to be read
>> locklessly. Readers must take care to avoid observing changes
>> that appear incoherent. This was done so that node_demotion[]
>> locking has no chance of becoming a bottleneck on large systems
>> with lots of CPUs in direct reclaim.
>>
>> This code is unused for now. It will be called later in the
>> series.
>>
>> Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
>> Reviewed-by: Yang Shi <shy828301@xxxxxxxxx>
>> Cc: Michal Hocko <mhocko@xxxxxxxx>
>> Cc: Wei Xu <weixugc@xxxxxxxxxx>
>> Cc: David Rientjes <rientjes@xxxxxxxxxx>
>> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
>> Cc: David Hildenbrand <david@xxxxxxxxxx>
>> Cc: osalvador <osalvador@xxxxxxx>
>>
>> --
>>
>> Changes from 20200122:
>> * Add big node_demotion[] comment
>> Changes from 20210302:
>> * Fix typo in node_demotion[] comment
>> ---
>> mm/internal.h | 5 ++
>> mm/migrate.c | 175 +++++++++++++++++++++++++++++++++++++++++++++++-
>> mm/page_alloc.c | 2 +-
>> 3 files changed, 180 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/internal.h b/mm/internal.h
>> index 2f1182948aa6..0344cd78e170 100644
>> --- a/mm/internal.h
>> +++ b/mm/internal.h
>> @@ -522,12 +522,17 @@ static inline void mminit_validate_memmodel_limits(unsigned long *start_pfn,
>>
>> #ifdef CONFIG_NUMA
>> extern int node_reclaim(struct pglist_data *, gfp_t, unsigned int);
>> +extern int find_next_best_node(int node, nodemask_t *used_node_mask);
>> #else
>> static inline int node_reclaim(struct pglist_data *pgdat, gfp_t mask,
>> unsigned int order)
>> {
>> return NODE_RECLAIM_NOSCAN;
>> }
>> +static inline int find_next_best_node(int node, nodemask_t *used_node_mask)
>> +{
>> + return NUMA_NO_NODE;
>> +}
>> #endif
>>
>> extern int hwpoison_filter(struct page *p);
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 6cab668132f9..111f8565f75d 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1136,6 +1136,44 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
>> return rc;
>> }
>>
>> +
>> +/*
>> + * node_demotion[] example:
>> + *
>> + * Consider a system with two sockets. Each socket has
>> + * three classes of memory attached: fast, medium and slow.
>> + * Each memory class is placed in its own NUMA node. The
>> + * CPUs are placed in the node with the "fast" memory. The
>> + * 6 NUMA nodes (0-5) might be split among the sockets like
>> + * this:
>> + *
>> + * Socket A: 0, 1, 2
>> + * Socket B: 3, 4, 5
>> + *
>> + * When Node 0 fills up, its memory should be migrated to
>> + * Node 1. When Node 1 fills up, it should be migrated to
>> + * Node 2. The migration path start on the nodes with the
>> + * processors (since allocations default to this node) and
>> + * fast memory, progress through medium and end with the
>> + * slow memory:
>> + *
>> + * 0 -> 1 -> 2 -> stop
>> + * 3 -> 4 -> 5 -> stop
>> + *
>> + * This is represented in the node_demotion[] like this:
>> + *
>> + * { 1, // Node 0 migrates to 1
>> + * 2, // Node 1 migrates to 2
>> + * -1, // Node 2 does not migrate
>> + * 4, // Node 3 migrates to 4
>> + * 5, // Node 4 migrates to 5
>> + * -1} // Node 5 does not migrate
>> + */
>> +
>> +/*
>> + * Writes to this array occur without locking. READ_ONCE()
>> + * is recommended for readers to ensure consistent reads.
>> + */
>> static int node_demotion[MAX_NUMNODES] __read_mostly =
>> {[0 ... MAX_NUMNODES - 1] = NUMA_NO_NODE};
>>
>> @@ -1150,7 +1188,13 @@ static int node_demotion[MAX_NUMNODES] __read_mostly =
>> */
>> int next_demotion_node(int node)
>> {
>> - return node_demotion[node];
>> + /*
>> + * node_demotion[] is updated without excluding
>> + * this function from running. READ_ONCE() avoids
>> + * reading multiple, inconsistent 'node' values
>> + * during an update.
>> + */
>> + return READ_ONCE(node_demotion[node]);
>> }
>
> Is it necessary to have two separate patches to add node_demotion and
> next_demotion_node() then modify it immediately? Maybe merge Patch 1 into 2?
>
> Hmm, I just checked Patch 3 and it changes node_demotion again and uses RCU.
> I guess it might be much simpler to just introduce node_demotion with RCU
> in this patch and Patch 3 only takes care of hotplug events.
Hi, Dave,
What do you think about this?
>>
>> /*
>> @@ -3144,3 +3188,132 @@ void migrate_vma_finalize(struct migrate_vma *migrate)
>> }
>> EXPORT_SYMBOL(migrate_vma_finalize);
>> #endif /* CONFIG_DEVICE_PRIVATE */
>> +
>> +/* Disable reclaim-based migration. */
>> +static void disable_all_migrate_targets(void)
>> +{
>> + int node;
>> +
>> + for_each_online_node(node)
>> + node_demotion[node] = NUMA_NO_NODE;
>> +}
>> +
>> +/*
>> + * Find an automatic demotion target for 'node'.
>> + * Failing here is OK. It might just indicate
>> + * being at the end of a chain.
>> + */
>> +static int establish_migrate_target(int node, nodemask_t *used)
>> +{
>> + int migration_target;
>> +
>> + /*
>> + * Can not set a migration target on a
>> + * node with it already set.
>> + *
>> + * No need for READ_ONCE() here since this
>> + * in the write path for node_demotion[].
>> + * This should be the only thread writing.
>> + */
>> + if (node_demotion[node] != NUMA_NO_NODE)
>> + return NUMA_NO_NODE;
>> +
>> + migration_target = find_next_best_node(node, used);
>> + if (migration_target == NUMA_NO_NODE)
>> + return NUMA_NO_NODE;
>> +
>> + node_demotion[node] = migration_target;
>> +
>> + return migration_target;
>> +}
>> +
>> +/*
>> + * When memory fills up on a node, memory contents can be
>> + * automatically migrated to another node instead of
>> + * discarded at reclaim.
>> + *
>> + * Establish a "migration path" which will start at nodes
>> + * with CPUs and will follow the priorities used to build the
>> + * page allocator zonelists.
>> + *
>> + * The difference here is that cycles must be avoided. If
>> + * node0 migrates to node1, then neither node1, nor anything
>> + * node1 migrates to can migrate to node0.
>> + *
>> + * This function can run simultaneously with readers of
>> + * node_demotion[]. However, it can not run simultaneously
>> + * with itself. Exclusion is provided by memory hotplug events
>> + * being single-threaded.
>> + */
>> +static void __set_migration_target_nodes(void)
>> +{
>> + nodemask_t next_pass = NODE_MASK_NONE;
>> + nodemask_t this_pass = NODE_MASK_NONE;
>> + nodemask_t used_targets = NODE_MASK_NONE;
>> + int node;
>> +
>> + /*
>> + * Avoid any oddities like cycles that could occur
>> + * from changes in the topology. This will leave
>> + * a momentary gap when migration is disabled.
>> + */
>> + disable_all_migrate_targets();
>> +
>> + /*
>> + * Ensure that the "disable" is visible across the system.
>> + * Readers will see either a combination of before+disable
>> + * state or disable+after. They will never see before and
>> + * after state together.
>> + *
>> + * The before+after state together might have cycles and
>> + * could cause readers to do things like loop until this
>> + * function finishes. This ensures they can only see a
>> + * single "bad" read and would, for instance, only loop
>> + * once.
>> + */
>> + smp_wmb();
>> +
>> + /*
>> + * Allocations go close to CPUs, first. Assume that
>> + * the migration path starts at the nodes with CPUs.
>> + */
>> + next_pass = node_states[N_CPU];
>
> Is there a plan of allowing user to change where the migration
> path starts? Or maybe one step further providing an interface
> to allow user to specify the demotion path. Something like
> /sys/devices/system/node/node*/node_demotion.
I don't think that's necessary at least for now. Do you know any real
world use case for this?
Best Regards,
Huang, Ying
[snip]
