Re: [PATCH v5 1/9] mm/demotion: Add support for explicit memory tiers

[Aneesh Kumar K V]

On 6/8/22 12:13 AM, Tim Chen wrote:
> On Fri, 2022-06-03 at 19:12 +0530, Aneesh Kumar K.V wrote:
> > The nodes which are part of a specific memory tier can be listed
> > via
> > /sys/devices/system/memtier/memtierN/nodelist
> >
> > "Rank" is an opaque value. Its absolute value doesn't have any
> > special meaning. But the rank values of different memtiers can be
> > compared with each other to determine the memory tier order.
> >
> > For example, if we have 3 memtiers: memtier0, memtier1, memiter2, and
> > their rank values are 300, 200, 100, then the memory tier order is:
> > memtier0 -> memtier2 -> memtier1,
>
> Why is memtier2 (rank 100) higher than memtier1 (rank 200)?  Seems like
> the order should be memtier0 -> memtier1 -> memtier2?
>                      (rank 300)  (rank 200)  (rank 100)
>
> > where memtier0 is the highest tier
> > and memtier1 is the lowest tier.
>
> I think memtier2 is the lowest as it has the lowest rank value.


typo error. Will fix that in the next update

> > The rank value of each memtier should be unique.
> >
> >
> > +
> > +static void memory_tier_device_release(struct device *dev)
> > +{
> > +	struct memory_tier *tier = to_memory_tier(dev);
> > +
>
> Do we need some ref counts on memory_tier?
> If there is another device still using the same memtier,
> free below could cause problem.
>
> > +	kfree(tier);
> > +}
> > +
> ...
> > +static struct memory_tier *register_memory_tier(unsigned int tier)
> > +{
> > +	int error;
> > +	struct memory_tier *memtier;
> > +
> > +	if (tier >= MAX_MEMORY_TIERS)
> > +		return NULL;
> > +
> > +	memtier = kzalloc(sizeof(struct memory_tier), GFP_KERNEL);
> > +	if (!memtier)
> > +		return NULL;
> > +
> > +	memtier->dev.id = tier;
> > +	memtier->rank = get_rank_from_tier(tier);
> > +	memtier->dev.bus = &memory_tier_subsys;
> > +	memtier->dev.release = memory_tier_device_release;
> > +	memtier->dev.groups = memory_tier_dev_groups;
> > +
>
> Should you take the mem_tier_lock before you insert to
> memtier-list?


Both register_memory_tier and unregister_memory_tier get called with 
memory_tier_lock held.

> > +	insert_memory_tier(memtier);
> > +
> > +	error = device_register(&memtier->dev);
> > +	if (error) {
> > +		list_del(&memtier->list);
> > +		put_device(&memtier->dev);
> > +		return NULL;
> > +	}
> > +	return memtier;
> > +}
> > +
> > +__maybe_unused // temporay to prevent warnings during bisects
> > +static void unregister_memory_tier(struct memory_tier *memtier)
> > +{
>
> I think we should take mem_tier_lock before modifying memtier->list.

unregister_memory_tier get called with memory_tier_lock held.

> > +	list_del(&memtier->list);
> > +	device_unregister(&memtier->dev);
> > +}
> > +

-aneesh