Re: [PATCH v5 2/9] mm/demotion: Expose per node memory tier to sysfs

[Ying Huang]

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
>