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    SubjectRe: [PATCH v5 2/9] mm/demotion: Expose per node memory tier to sysfs
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    On Wed, 2022-06-08 at 10:25 +0530, Aneesh Kumar K V wrote:
    > On 6/8/22 1:45 AM, Tim Chen wrote:
    > > On Fri, 2022-06-03 at 19:12 +0530, Aneesh Kumar K.V wrote:
    > > >
    > > >   
    > > >
    > > >
    > > >
    > > > +static struct memory_tier *__node_get_memory_tier(int node)
    > > > +{
    > > > + struct memory_tier *memtier;
    > > > +
    > > > + list_for_each_entry(memtier, &memory_tiers, list) {
    > >
    > > We could need to map node to mem_tier quite often, if we need
    > > to account memory usage at tier level. It will be more efficient
    > > to have a pointer from node (pgdat) to memtier rather
    > > than doing a search through the list.
    > >
    > >
    >
    > That is something I was actively trying to avoid. Currently all struct
    > memory_tier references are with memory_tier_lock mutex held. That
    > simplify the locking and reference counting.
    >
    > As of now we are able to implement all the required interfaces without
    > pgdat having pointers to struct memory_tier. We can update pgdat with
    > memtier details when we are implementing changes requiring those. We
    > could keep additional memtier->dev reference to make sure memory tiers
    > are not destroyed while other part of the kernel is referencing the
    > same. But IMHO such changes should wait till we have users for the same.

    No. We need a convenient way to access memory tier information from
    inside the kernel. For example, from nid to memory tier rank, this is
    needed by migrate_misplaced_page() to do statistics too, iterate all
    nodes of a memory tier, etc.

    And, "allowed" field of struct demotion_nodes (introduced in [7/9] is
    per-memory tier instead of per-node. Please move it to struct
    memory_tier. And we just need a convenient way to access it.

    All these are not complex, unless you insist to use memory_tier_lock and
    device liftcycle to manage this in-kernel data structure.

    Best Regards,
    Huang, Ying

    > > > + if (node_isset(node, memtier->nodelist))
    > > > + return memtier;
    > > > + }
    > > > + return NULL;
    > > > +}
    > > > +
    > > >
    > >
    > > Tim
    > >
    >
    > -aneesh
    >


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